Zhiyi reported an infinite loop issue in VFIO use case. The cause of that
was a separate discussion, however during that I found a regression of
dirty sync slowness when profiling.
Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's
statically allocated to be the max supported by the kernel. However after
Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
the max supported memslots reported now grows to some number large enough
so that it may not be wise to always statically allocate with the max
reported.
What's worse, QEMU kvm code still walks all the allocated memslots entries
to do any form of lookups. It can drastically slow down all memslot
operations because each of such loop can run over 32K times on the new
kernels.
Fix this issue by making the memslots to be allocated dynamically.
Here the initial size was set to 16 because it should cover the basic VM
usages, so that the hope is the majority VM use case may not even need to
grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default
it'll consume 9 memslots), however not too large to waste memory.
There can also be even better way to address this, but so far this is the
simplest and should be already better even than before we grow the max
supported memslots. For example, in the case of above issue when VFIO was
attached on a 32GB system, there are only ~10 memslots used. So it could
be good enough as of now.
In the above VFIO context, measurement shows that the precopy dirty sync
shrinked from ~86ms to ~3ms after this patch applied. It should also apply
to any KVM enabled VM even without VFIO.
Reported-by: Zhiyi Guo <zhguo@redhat.com>
Tested-by: Zhiyi Guo <zhguo@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
include/sysemu/kvm_int.h | 1 +
accel/kvm/kvm-all.c | 87 +++++++++++++++++++++++++++++++++-------
accel/kvm/trace-events | 1 +
3 files changed, 74 insertions(+), 15 deletions(-)
diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
index e5de43619e..e67b2e5a68 100644
--- a/include/sysemu/kvm_int.h
+++ b/include/sysemu/kvm_int.h
@@ -46,6 +46,7 @@ typedef struct KVMMemoryListener {
MemoryListener listener;
KVMSlot *slots;
unsigned int nr_used_slots;
+ unsigned int nr_slots_allocated;
int as_id;
QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_add;
QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_del;
diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index e408dbb753..0d379606e4 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -71,6 +71,8 @@
/* Default max allowed memslots if kernel reported nothing */
#define KVM_MEMSLOTS_NUM_MAX_DEFAULT 32
+/* Default num of memslots to be allocated when VM starts */
+#define KVM_MEMSLOTS_NUM_ALLOC_DEFAULT 16
struct KVMParkedVcpu {
unsigned long vcpu_id;
@@ -168,6 +170,52 @@ void kvm_resample_fd_notify(int gsi)
}
}
+/**
+ * kvm_slots_grow(): Grow the slots[] array in the KVMMemoryListener
+ *
+ * @kml: The KVMMemoryListener* to grow the slots[] array
+ * @nr_slots_new: The new size of slots[] array
+ *
+ * Returns: True if the array grows larger, false otherwise.
+ */
+static bool kvm_slots_grow(KVMMemoryListener *kml, unsigned int nr_slots_new)
+{
+ unsigned int i, cur = kml->nr_slots_allocated;
+ KVMSlot *slots;
+
+ if (nr_slots_new > kvm_state->nr_slots_max) {
+ nr_slots_new = kvm_state->nr_slots_max;
+ }
+
+ if (cur >= nr_slots_new) {
+ /* Big enough, no need to grow, or we reached max */
+ return false;
+ }
+
+ if (cur == 0) {
+ slots = g_new0(KVMSlot, nr_slots_new);
+ } else {
+ assert(kml->slots);
+ slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
+ /*
+ * g_renew() doesn't initialize extended buffers, however kvm
+ * memslots require fields to be zero-initialized. E.g. pointers,
+ * memory_size field, etc.
+ */
+ memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
+ }
+
+ for (i = cur; i < nr_slots_new; i++) {
+ slots[i].slot = i;
+ }
+
+ kml->slots = slots;
+ kml->nr_slots_allocated = nr_slots_new;
+ trace_kvm_slots_grow(cur, nr_slots_new);
+
+ return true;
+}
+
unsigned int kvm_get_max_memslots(void)
{
KVMState *s = KVM_STATE(current_accel());
@@ -196,15 +244,20 @@ unsigned int kvm_get_free_memslots(void)
/* Called with KVMMemoryListener.slots_lock held */
static KVMSlot *kvm_get_free_slot(KVMMemoryListener *kml)
{
- KVMState *s = kvm_state;
int i;
- for (i = 0; i < s->nr_slots_max; i++) {
+retry:
+ for (i = 0; i < kml->nr_slots_allocated; i++) {
if (kml->slots[i].memory_size == 0) {
return &kml->slots[i];
}
}
+ /* If no free slots, try to grow first by doubling */
+ if (kvm_slots_grow(kml, kml->nr_slots_allocated * 2)) {
+ goto retry;
+ }
+
return NULL;
}
@@ -225,10 +278,9 @@ static KVMSlot *kvm_lookup_matching_slot(KVMMemoryListener *kml,
hwaddr start_addr,
hwaddr size)
{
- KVMState *s = kvm_state;
int i;
- for (i = 0; i < s->nr_slots_max; i++) {
+ for (i = 0; i < kml->nr_slots_allocated; i++) {
KVMSlot *mem = &kml->slots[i];
if (start_addr == mem->start_addr && size == mem->memory_size) {
@@ -270,7 +322,7 @@ int kvm_physical_memory_addr_from_host(KVMState *s, void *ram,
int i, ret = 0;
kvm_slots_lock();
- for (i = 0; i < s->nr_slots_max; i++) {
+ for (i = 0; i < kml->nr_slots_allocated; i++) {
KVMSlot *mem = &kml->slots[i];
if (ram >= mem->ram && ram < mem->ram + mem->memory_size) {
@@ -1074,7 +1126,7 @@ static int kvm_physical_log_clear(KVMMemoryListener *kml,
kvm_slots_lock();
- for (i = 0; i < s->nr_slots_max; i++) {
+ for (i = 0; i < kml->nr_slots_allocated; i++) {
mem = &kml->slots[i];
/* Discard slots that are empty or do not overlap the section */
if (!mem->memory_size ||
@@ -1722,12 +1774,8 @@ static void kvm_log_sync_global(MemoryListener *l, bool last_stage)
/* Flush all kernel dirty addresses into KVMSlot dirty bitmap */
kvm_dirty_ring_flush();
- /*
- * TODO: make this faster when nr_slots_max is big while there are
- * only a few used slots (small VMs).
- */
kvm_slots_lock();
- for (i = 0; i < s->nr_slots_max; i++) {
+ for (i = 0; i < kml->nr_slots_allocated; i++) {
mem = &kml->slots[i];
if (mem->memory_size && mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
kvm_slot_sync_dirty_pages(mem);
@@ -1842,12 +1890,9 @@ void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
{
int i;
- kml->slots = g_new0(KVMSlot, s->nr_slots_max);
kml->as_id = as_id;
- for (i = 0; i < s->nr_slots_max; i++) {
- kml->slots[i].slot = i;
- }
+ kvm_slots_grow(kml, KVM_MEMSLOTS_NUM_ALLOC_DEFAULT);
QSIMPLEQ_INIT(&kml->transaction_add);
QSIMPLEQ_INIT(&kml->transaction_del);
@@ -2464,6 +2509,18 @@ static int kvm_init(MachineState *ms)
s->nr_slots_max = KVM_MEMSLOTS_NUM_MAX_DEFAULT;
}
+ /*
+ * A VM will at least require a few memslots to work, or it can even
+ * fail to boot. Make sure the supported value is always at least
+ * larger than what we will initially allocate.
+ */
+ if (s->nr_slots_max < KVM_MEMSLOTS_NUM_ALLOC_DEFAULT) {
+ ret = -EINVAL;
+ fprintf(stderr, "KVM max supported number of slots (%d) too small\n",
+ s->nr_slots_max);
+ goto err;
+ }
+
s->nr_as = kvm_check_extension(s, KVM_CAP_MULTI_ADDRESS_SPACE);
if (s->nr_as <= 1) {
s->nr_as = 1;
diff --git a/accel/kvm/trace-events b/accel/kvm/trace-events
index 37626c1ac5..ad2ae6fca5 100644
--- a/accel/kvm/trace-events
+++ b/accel/kvm/trace-events
@@ -36,3 +36,4 @@ kvm_io_window_exit(void) ""
kvm_run_exit_system_event(int cpu_index, uint32_t event_type) "cpu_index %d, system_even_type %"PRIu32
kvm_convert_memory(uint64_t start, uint64_t size, const char *msg) "start 0x%" PRIx64 " size 0x%" PRIx64 " %s"
kvm_memory_fault(uint64_t start, uint64_t size, uint64_t flags) "start 0x%" PRIx64 " size 0x%" PRIx64 " flags 0x%" PRIx64
+kvm_slots_grow(unsigned int old, unsigned int new) "%u -> %u"
--
2.45.0On 04.09.24 21:16, Peter Xu wrote:
> Zhiyi reported an infinite loop issue in VFIO use case. The cause of that
> was a separate discussion, however during that I found a regression of
> dirty sync slowness when profiling.
>
> Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's
> statically allocated to be the max supported by the kernel. However after
> Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
> the max supported memslots reported now grows to some number large enough
> so that it may not be wise to always statically allocate with the max
> reported.
>
> What's worse, QEMU kvm code still walks all the allocated memslots entries
> to do any form of lookups. It can drastically slow down all memslot
> operations because each of such loop can run over 32K times on the new
> kernels.
>
> Fix this issue by making the memslots to be allocated dynamically.
>
> Here the initial size was set to 16 because it should cover the basic VM
> usages, so that the hope is the majority VM use case may not even need to
> grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default
> it'll consume 9 memslots), however not too large to waste memory.
>
> There can also be even better way to address this, but so far this is the
> simplest and should be already better even than before we grow the max
> supported memslots. For example, in the case of above issue when VFIO was
> attached on a 32GB system, there are only ~10 memslots used. So it could
> be good enough as of now.
>
> In the above VFIO context, measurement shows that the precopy dirty sync
> shrinked from ~86ms to ~3ms after this patch applied. It should also apply
> to any KVM enabled VM even without VFIO.
>
> Reported-by: Zhiyi Guo <zhguo@redhat.com>
> Tested-by: Zhiyi Guo <zhguo@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> {
> int i;
>
> - kml->slots = g_new0(KVMSlot, s->nr_slots_max);
> kml->as_id = as_id;
>
> - for (i = 0; i < s->nr_slots_max; i++) {
> - kml->slots[i].slot = i;
> - }
> + kvm_slots_grow(kml, KVM_MEMSLOTS_NUM_ALLOC_DEFAULT);
I would just keep the static initialization here, and add the additional
kml->nr_slots_allocated = KVM_MEMSLOTS_NUM_ALLOC_DEFAULT;
here.
Then, you can remove the parameter from kvm_slots_grow() completely and just call it
kvm_slots_double() and simplify a bit:
static bool kvm_slots_double(KVMMemoryListener *kml)
{
unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
KVMSlot *slots;
nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
if (nr_slots_new == kvm_state->nr_slots_max) {
/* We reached the maximum */
return false;
}
assert(kml->slots);
slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
/*
* g_renew() doesn't initialize extended buffers, however kvm
* memslots require fields to be zero-initialized. E.g. pointers,
* memory_size field, etc.
*/
memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
for (i = cur; i < nr_slots_new; i++) {
slots[i].slot = i;
}
kml->slots = slots;
kml->nr_slots_allocated = nr_slots_new;
trace_kvm_slots_grow(cur, nr_slots_new);
return true;
}
Apart from that looks sane. On the slot freeing/allocation path, there is certainly
more optimization potential :)
I'm surprised this 32k loop wasn't found earlier.
--
Cheers,
David / dhildenb
On Wed, Sep 04, 2024 at 11:07:44PM +0200, David Hildenbrand wrote:
> On 04.09.24 21:16, Peter Xu wrote:
> > Zhiyi reported an infinite loop issue in VFIO use case. The cause of that
> > was a separate discussion, however during that I found a regression of
> > dirty sync slowness when profiling.
> >
> > Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's
> > statically allocated to be the max supported by the kernel. However after
> > Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
> > the max supported memslots reported now grows to some number large enough
> > so that it may not be wise to always statically allocate with the max
> > reported.
> >
> > What's worse, QEMU kvm code still walks all the allocated memslots entries
> > to do any form of lookups. It can drastically slow down all memslot
> > operations because each of such loop can run over 32K times on the new
> > kernels.
> >
> > Fix this issue by making the memslots to be allocated dynamically.
> >
> > Here the initial size was set to 16 because it should cover the basic VM
> > usages, so that the hope is the majority VM use case may not even need to
> > grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default
> > it'll consume 9 memslots), however not too large to waste memory.
> >
> > There can also be even better way to address this, but so far this is the
> > simplest and should be already better even than before we grow the max
> > supported memslots. For example, in the case of above issue when VFIO was
> > attached on a 32GB system, there are only ~10 memslots used. So it could
> > be good enough as of now.
> >
> > In the above VFIO context, measurement shows that the precopy dirty sync
> > shrinked from ~86ms to ~3ms after this patch applied. It should also apply
> > to any KVM enabled VM even without VFIO.
> >
> > Reported-by: Zhiyi Guo <zhguo@redhat.com>
> > Tested-by: Zhiyi Guo <zhguo@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
>
>
>
> > {
> > int i;
> > - kml->slots = g_new0(KVMSlot, s->nr_slots_max);
> > kml->as_id = as_id;
> > - for (i = 0; i < s->nr_slots_max; i++) {
> > - kml->slots[i].slot = i;
> > - }
> > + kvm_slots_grow(kml, KVM_MEMSLOTS_NUM_ALLOC_DEFAULT);
>
> I would just keep the static initialization here, and add the additional
>
> kml->nr_slots_allocated = KVM_MEMSLOTS_NUM_ALLOC_DEFAULT;
IMHO it'll be cleaner to always allocate in the grow() so as to avoid
details on e.g. initializations of kml->slots[].slot above.
>
> here.
>
> Then, you can remove the parameter from kvm_slots_grow() completely and just call it
> kvm_slots_double() and simplify a bit:
>
> static bool kvm_slots_double(KVMMemoryListener *kml)
> {
> unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
> KVMSlot *slots;
>
> nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
> if (nr_slots_new == kvm_state->nr_slots_max) {
> /* We reached the maximum */
> return false;
> }
>
> assert(kml->slots);
> slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
> /*
> * g_renew() doesn't initialize extended buffers, however kvm
> * memslots require fields to be zero-initialized. E.g. pointers,
> * memory_size field, etc.
> */
> memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
>
> for (i = cur; i < nr_slots_new; i++) {
> slots[i].slot = i;
> }
>
> kml->slots = slots;
> kml->nr_slots_allocated = nr_slots_new;
> trace_kvm_slots_grow(cur, nr_slots_new);
>
> return true;
> }
Personally I still think it cleaner to allow setting whatever size.
We only have one place growing so far, which is pretty trivial to double
there, IMO. I'll wait for a second opinion, or let me know if you have
strong feelings..
>
>
> Apart from that looks sane. On the slot freeing/allocation path, there is certainly
> more optimization potential :)
>
> I'm surprised this 32k loop wasn't found earlier.
Yes, it's in the range where it isn't too big to be discovered I guess, but
large enough to affect many things, so better fix it sooner than later.
This reminded me we should probably copy stable for this patch. I think it
means I'll try to move this patch to the 1st patch to make Michael's life
and downstream easier.
--
Peter Xu
>>
>> Then, you can remove the parameter from kvm_slots_grow() completely and just call it
>> kvm_slots_double() and simplify a bit:
>>
>> static bool kvm_slots_double(KVMMemoryListener *kml)
>> {
>> unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
>> KVMSlot *slots;
>>
>> nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
>> if (nr_slots_new == kvm_state->nr_slots_max) {
>> /* We reached the maximum */
>> return false;
>> }
>>
>> assert(kml->slots);
>> slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
>> /*
>> * g_renew() doesn't initialize extended buffers, however kvm
>> * memslots require fields to be zero-initialized. E.g. pointers,
>> * memory_size field, etc.
>> */
>> memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
>>
>> for (i = cur; i < nr_slots_new; i++) {
>> slots[i].slot = i;
>> }
>>
>> kml->slots = slots;
>> kml->nr_slots_allocated = nr_slots_new;
>> trace_kvm_slots_grow(cur, nr_slots_new);
>>
>> return true;
>> }
>
> Personally I still think it cleaner to allow setting whatever size.
Why would one need that? If any, at some point we would want to shrink
or rather "compact".
>
> We only have one place growing so far, which is pretty trivial to double
> there, IMO. I'll wait for a second opinion, or let me know if you have
> strong feelings..
I think the simplicity of kvm_slots_double() speaks for itself, but I
won't fight for it.
--
Cheers,
David / dhildenb
On Wed, Sep 04, 2024 at 11:23:33PM +0200, David Hildenbrand wrote:
>
> > >
> > > Then, you can remove the parameter from kvm_slots_grow() completely and just call it
> > > kvm_slots_double() and simplify a bit:
> > >
> > > static bool kvm_slots_double(KVMMemoryListener *kml)
> > > {
> > > unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
> > > KVMSlot *slots;
> > >
> > > nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
> > > if (nr_slots_new == kvm_state->nr_slots_max) {
> > > /* We reached the maximum */
> > > return false;
> > > }
> > >
> > > assert(kml->slots);
> > > slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
> > > /*
> > > * g_renew() doesn't initialize extended buffers, however kvm
> > > * memslots require fields to be zero-initialized. E.g. pointers,
> > > * memory_size field, etc.
> > > */
> > > memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
> > >
> > > for (i = cur; i < nr_slots_new; i++) {
> > > slots[i].slot = i;
> > > }
> > >
> > > kml->slots = slots;
> > > kml->nr_slots_allocated = nr_slots_new;
> > > trace_kvm_slots_grow(cur, nr_slots_new);
> > >
> > > return true;
> > > }
> >
> > Personally I still think it cleaner to allow setting whatever size.
>
> Why would one need that? If any, at some point we would want to shrink or
> rather "compact".
>
> >
> > We only have one place growing so far, which is pretty trivial to double
> > there, IMO. I'll wait for a second opinion, or let me know if you have
> > strong feelings..
>
> I think the simplicity of kvm_slots_double() speaks for itself, but I won't
> fight for it.
Using kvm_slots_double() won't be able to share the same code when
initialize (to e.g. avoid hard-coded initialize of "slots[i].slot").
Thanks,
--
Peter Xu
On 04.09.24 23:34, Peter Xu wrote:
> On Wed, Sep 04, 2024 at 11:23:33PM +0200, David Hildenbrand wrote:
>>
>>>>
>>>> Then, you can remove the parameter from kvm_slots_grow() completely and just call it
>>>> kvm_slots_double() and simplify a bit:
>>>>
>>>> static bool kvm_slots_double(KVMMemoryListener *kml)
>>>> {
>>>> unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
>>>> KVMSlot *slots;
>>>>
>>>> nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
>>>> if (nr_slots_new == kvm_state->nr_slots_max) {
>>>> /* We reached the maximum */
>>>> return false;
>>>> }
>>>>
>>>> assert(kml->slots);
>>>> slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
>>>> /*
>>>> * g_renew() doesn't initialize extended buffers, however kvm
>>>> * memslots require fields to be zero-initialized. E.g. pointers,
>>>> * memory_size field, etc.
>>>> */
>>>> memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
>>>>
>>>> for (i = cur; i < nr_slots_new; i++) {
>>>> slots[i].slot = i;
>>>> }
>>>>
>>>> kml->slots = slots;
>>>> kml->nr_slots_allocated = nr_slots_new;
>>>> trace_kvm_slots_grow(cur, nr_slots_new);
>>>>
>>>> return true;
>>>> }
>>>
>>> Personally I still think it cleaner to allow setting whatever size.
>>
>> Why would one need that? If any, at some point we would want to shrink or
>> rather "compact".
>>
>>>
>>> We only have one place growing so far, which is pretty trivial to double
>>> there, IMO. I'll wait for a second opinion, or let me know if you have
>>> strong feelings..
>>
>> I think the simplicity of kvm_slots_double() speaks for itself, but I won't
>> fight for it.
>
> Using kvm_slots_double() won't be able to share the same code when
> initialize (to e.g. avoid hard-coded initialize of "slots[i].slot").
I don't see that as any problem and if you really care you could factor
exactly that part out in a helper. Anyhow, I learned that I am not good
at convincing you, so do what you think is best. The code itself should
get the job done.
Acked-by: David Hildenbrand <david@redhat.com>
--
Cheers,
David / dhildenb
On Wed, Sep 04, 2024 at 11:38:28PM +0200, David Hildenbrand wrote:
> On 04.09.24 23:34, Peter Xu wrote:
> > On Wed, Sep 04, 2024 at 11:23:33PM +0200, David Hildenbrand wrote:
> > >
> > > > >
> > > > > Then, you can remove the parameter from kvm_slots_grow() completely and just call it
> > > > > kvm_slots_double() and simplify a bit:
> > > > >
> > > > > static bool kvm_slots_double(KVMMemoryListener *kml)
> > > > > {
> > > > > unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
> > > > > KVMSlot *slots;
> > > > >
> > > > > nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
> > > > > if (nr_slots_new == kvm_state->nr_slots_max) {
> > > > > /* We reached the maximum */
> > > > > return false;
> > > > > }
> > > > >
> > > > > assert(kml->slots);
> > > > > slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
> > > > > /*
> > > > > * g_renew() doesn't initialize extended buffers, however kvm
> > > > > * memslots require fields to be zero-initialized. E.g. pointers,
> > > > > * memory_size field, etc.
> > > > > */
> > > > > memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
> > > > >
> > > > > for (i = cur; i < nr_slots_new; i++) {
> > > > > slots[i].slot = i;
> > > > > }
> > > > >
> > > > > kml->slots = slots;
> > > > > kml->nr_slots_allocated = nr_slots_new;
> > > > > trace_kvm_slots_grow(cur, nr_slots_new);
> > > > >
> > > > > return true;
> > > > > }
> > > >
> > > > Personally I still think it cleaner to allow setting whatever size.
> > >
> > > Why would one need that? If any, at some point we would want to shrink or
> > > rather "compact".
> > >
> > > >
> > > > We only have one place growing so far, which is pretty trivial to double
> > > > there, IMO. I'll wait for a second opinion, or let me know if you have
> > > > strong feelings..
> > >
> > > I think the simplicity of kvm_slots_double() speaks for itself, but I won't
> > > fight for it.
> >
> > Using kvm_slots_double() won't be able to share the same code when
> > initialize (to e.g. avoid hard-coded initialize of "slots[i].slot").
>
> I don't see that as any problem and if you really care you could factor
> exactly that part out in a helper. Anyhow, I learned that I am not good at
> convincing you, so do what you think is best. The code itself should get the
> job done.
It's only about that's the simplest for all of us, and I noticed it only
because I already planned to switch to kvm_slots_double(); that's normally
what I do when I don't strongly insist something. So you succeeded already
making me go there. :)
It's just that as you said it either requires more changes, or I'll need to
duplicate some code which I want to avoid.
>
> Acked-by: David Hildenbrand <david@redhat.com>
Thanks a lot for the late night reviews, David. I'll attach all your tags
when repost, though just to mention there'll be slight touch ups here and
there due to reordering. Feel free to double check when it's there.
--
Peter Xu
On Wed, Sep 04, 2024 at 05:46:55PM -0400, Peter Xu wrote:
> On Wed, Sep 04, 2024 at 11:38:28PM +0200, David Hildenbrand wrote:
> > On 04.09.24 23:34, Peter Xu wrote:
> > > On Wed, Sep 04, 2024 at 11:23:33PM +0200, David Hildenbrand wrote:
> > > >
> > > > > >
> > > > > > Then, you can remove the parameter from kvm_slots_grow() completely and just call it
> > > > > > kvm_slots_double() and simplify a bit:
> > > > > >
> > > > > > static bool kvm_slots_double(KVMMemoryListener *kml)
> > > > > > {
> > > > > > unsigned int i, nr_slots_new, cur = kml->nr_slots_allocated;
> > > > > > KVMSlot *slots;
> > > > > >
> > > > > > nr_slots_new = MIN(cur * 2, kvm_state->nr_slots_max);
> > > > > > if (nr_slots_new == kvm_state->nr_slots_max) {
> > > > > > /* We reached the maximum */
> > > > > > return false;
> > > > > > }
> > > > > >
> > > > > > assert(kml->slots);
> > > > > > slots = g_renew(KVMSlot, kml->slots, nr_slots_new);
> > > > > > /*
> > > > > > * g_renew() doesn't initialize extended buffers, however kvm
> > > > > > * memslots require fields to be zero-initialized. E.g. pointers,
> > > > > > * memory_size field, etc.
> > > > > > */
> > > > > > memset(&slots[cur], 0x0, sizeof(slots[0]) * (nr_slots_new - cur));
> > > > > >
> > > > > > for (i = cur; i < nr_slots_new; i++) {
> > > > > > slots[i].slot = i;
> > > > > > }
> > > > > >
> > > > > > kml->slots = slots;
> > > > > > kml->nr_slots_allocated = nr_slots_new;
> > > > > > trace_kvm_slots_grow(cur, nr_slots_new);
> > > > > >
> > > > > > return true;
> > > > > > }
> > > > >
> > > > > Personally I still think it cleaner to allow setting whatever size.
> > > >
> > > > Why would one need that? If any, at some point we would want to shrink or
> > > > rather "compact".
> > > >
> > > > >
> > > > > We only have one place growing so far, which is pretty trivial to double
> > > > > there, IMO. I'll wait for a second opinion, or let me know if you have
> > > > > strong feelings..
> > > >
> > > > I think the simplicity of kvm_slots_double() speaks for itself, but I won't
> > > > fight for it.
> > >
> > > Using kvm_slots_double() won't be able to share the same code when
> > > initialize (to e.g. avoid hard-coded initialize of "slots[i].slot").
> >
> > I don't see that as any problem and if you really care you could factor
> > exactly that part out in a helper. Anyhow, I learned that I am not good at
> > convincing you, so do what you think is best. The code itself should get the
> > job done.
>
> It's only about that's the simplest for all of us, and I noticed it only
> because I already planned to switch to kvm_slots_double(); that's normally
> what I do when I don't strongly insist something. So you succeeded already
> making me go there. :)
>
> It's just that as you said it either requires more changes, or I'll need to
> duplicate some code which I want to avoid.
>
> >
> > Acked-by: David Hildenbrand <david@redhat.com>
>
> Thanks a lot for the late night reviews, David. I'll attach all your tags
> when repost, though just to mention there'll be slight touch ups here and
> there due to reordering. Feel free to double check when it's there.
So I plan to squash this in, assuming this looks better to you:
===8<===
diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 78f2d8b80f..020fd16ab8 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -216,6 +216,11 @@ static bool kvm_slots_grow(KVMMemoryListener *kml, unsigned int nr_slots_new)
return true;
}
+static bool kvm_slots_double(KVMMemoryListener *kml)
+{
+ return kvm_slots_grow(kml, kml->nr_slots_allocated * 2);
+}
+
unsigned int kvm_get_max_memslots(void)
{
KVMState *s = KVM_STATE(current_accel());
@@ -254,7 +259,7 @@ retry:
}
/* If no free slots, try to grow first by doubling */
- if (kvm_slots_grow(kml, kml->nr_slots_allocated * 2)) {
+ if (kvm_slots_double(kml)) {
goto retry;
}
===8<===
Please let me know if otherwise.
--
Peter Xu
On 04.09.24 21:16, Peter Xu wrote:
> Zhiyi reported an infinite loop issue in VFIO use case. The cause of that
> was a separate discussion, however during that I found a regression of
> dirty sync slowness when profiling.
>
> Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's
> statically allocated to be the max supported by the kernel. However after
> Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
> the max supported memslots reported now grows to some number large enough
> so that it may not be wise to always statically allocate with the max
> reported.
>
> What's worse, QEMU kvm code still walks all the allocated memslots entries
> to do any form of lookups. It can drastically slow down all memslot
> operations because each of such loop can run over 32K times on the new
> kernels.
>
> Fix this issue by making the memslots to be allocated dynamically.
Wouldn't it be sufficient to limit the walk to the actually used slots?
I know, the large allocation might sound scary at first, but memory
overcommit+populate-on-demand should handle that, assuming nobody
touches the yet-unused slots.
--
Cheers,
David / dhildenb
On Wed, Sep 04, 2024 at 10:43:23PM +0200, David Hildenbrand wrote:
> On 04.09.24 21:16, Peter Xu wrote:
> > Zhiyi reported an infinite loop issue in VFIO use case. The cause of that
> > was a separate discussion, however during that I found a regression of
> > dirty sync slowness when profiling.
> >
> > Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's
> > statically allocated to be the max supported by the kernel. However after
> > Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
> > the max supported memslots reported now grows to some number large enough
> > so that it may not be wise to always statically allocate with the max
> > reported.
> >
> > What's worse, QEMU kvm code still walks all the allocated memslots entries
> > to do any form of lookups. It can drastically slow down all memslot
> > operations because each of such loop can run over 32K times on the new
> > kernels.
> >
> > Fix this issue by making the memslots to be allocated dynamically.
>
> Wouldn't it be sufficient to limit the walk to the actually used slots?
>
> I know, the large allocation might sound scary at first, but memory
> overcommit+populate-on-demand should handle that, assuming nobody touches
> the yet-unused slots.
I thought we can have holes within the array?
I meant e.g. when 10 slots populated, but then one of them got removed,
then nr_slots_used would be 9 even if slots[9] is still in use?
Thanks,
--
Peter Xu
On 04.09.24 22:43, David Hildenbrand wrote:
> On 04.09.24 21:16, Peter Xu wrote:
>> Zhiyi reported an infinite loop issue in VFIO use case. The cause of that
>> was a separate discussion, however during that I found a regression of
>> dirty sync slowness when profiling.
>>
>> Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's
>> statically allocated to be the max supported by the kernel. However after
>> Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
>> the max supported memslots reported now grows to some number large enough
>> so that it may not be wise to always statically allocate with the max
>> reported.
>>
>> What's worse, QEMU kvm code still walks all the allocated memslots entries
>> to do any form of lookups. It can drastically slow down all memslot
>> operations because each of such loop can run over 32K times on the new
>> kernels.
>>
>> Fix this issue by making the memslots to be allocated dynamically.
>
> Wouldn't it be sufficient to limit the walk to the actually used slots?
Ah, I remember that kvm_get_free_slot() is also rather inefficient
because we don't "close holes" when removing slots. So we would have to
walk up to the "highest slot ever used". Let me take a look at the patch.
--
Cheers,
David / dhildenb
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