include/linux/sbitmap.h | 5 +++++ lib/sbitmap.c | 45 ++++++++++++++++++++++++++++++++--------- 2 files changed, 41 insertions(+), 9 deletions(-)
Configuration for sbq:
depth=64, wake_batch=6, shift=6, map_nr=1
1. There are 64 requests in progress:
map->word = 0xFFFFFFFFFFFFFFFF
2. After all the 64 requests complete, and no more requests come:
map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
3. Now two tasks try to allocate requests:
T1: T2:
__blk_mq_get_tag .
__sbitmap_queue_get .
sbitmap_get .
sbitmap_find_bit .
sbitmap_find_bit_in_word .
__sbitmap_get_word -> nr=-1 __blk_mq_get_tag
sbitmap_deferred_clear __sbitmap_queue_get
/* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit
if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word
return false; __sbitmap_get_word -> nr=-1
mask = xchg(&map->cleared, 0) sbitmap_deferred_clear
atomic_long_andnot() /* map->cleared=0 */
if (!(map->cleared))
return false;
/*
* map->cleared is cleared by T1
* T2 fail to acquire the tag
*/
4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
up due to the wake_batch being set at 6. If no more requests come, T1
will wait here indefinitely.
This patch achieves two purposes:
1. Check on ->cleared and update on both ->cleared and ->word need to
be done atomically, and using spinlock could be the simplest solution.
So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
may cause potential race.
2. Add extra check in sbitmap_deferred_clear(), to identify whether
->word has free bits.
Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <yang.yang@vivo.com>
---
Changes from v3:
- Add more arguments to sbitmap_deferred_clear(), for those who
don't care about the return value, just pass 0
- Consider the situation when using sbitmap_get_shallow()
- Consider the situation when ->round_robin is true
- Modify commit message
Changes from v2:
- Modify commit message by suggestion
- Add extra check in sbitmap_deferred_clear() by suggestion
Changes from v1:
- simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
---
include/linux/sbitmap.h | 5 +++++
lib/sbitmap.c | 45 ++++++++++++++++++++++++++++++++---------
2 files changed, 41 insertions(+), 9 deletions(-)
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..ec0b0e73c906 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -36,6 +36,11 @@ struct sbitmap_word {
* @cleared: word holding cleared bits
*/
unsigned long cleared ____cacheline_aligned_in_smp;
+
+ /**
+ * @swap_lock: Held while swapping word <-> cleared
+ */
+ spinlock_t swap_lock;
} ____cacheline_aligned_in_smp;
/**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..9bd85a9b74b9 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -60,12 +60,32 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
/*
* See if we have deferred clears that we can batch move
*/
-static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
-{
- unsigned long mask;
+static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
+ unsigned int depth, unsigned int alloc_hint, bool wrap)
+{
+ unsigned long mask, flags, word_mask;
+ bool ret = false;
+
+ spin_lock_irqsave(&map->swap_lock, flags);
+
+ if (!map->cleared) {
+ if (depth > 0) {
+#if BITS_PER_LONG == 64
+ word_mask = U64_MAX >> (BITS_PER_LONG - depth);
+#else
+ word_mask = U32_MAX >> (BITS_PER_LONG - depth);
+#endif
+ if (!wrap && alloc_hint)
+ word_mask &= ~((1UL << alloc_hint) - 1);
+
+ if ((READ_ONCE(map->word) & word_mask) == word_mask)
+ ret = false;
+ else
+ ret = true;
+ }
- if (!READ_ONCE(map->cleared))
- return false;
+ goto out_unlock;
+ }
/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -77,7 +97,10 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
*/
atomic_long_andnot(mask, (atomic_long_t *)&map->word);
BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
- return true;
+ ret = true;
+out_unlock:
+ spin_unlock_irqrestore(&map->swap_lock, flags);
+ return ret;
}
int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@@ -85,6 +108,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
bool alloc_hint)
{
unsigned int bits_per_word;
+ int i;
if (shift < 0)
shift = sbitmap_calculate_shift(depth);
@@ -116,6 +140,9 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
return -ENOMEM;
}
+ for (i = 0; i < sb->map_nr; i++)
+ spin_lock_init(&sb->map[i].swap_lock);
+
return 0;
}
EXPORT_SYMBOL_GPL(sbitmap_init_node);
@@ -126,7 +153,7 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth)
unsigned int i;
for (i = 0; i < sb->map_nr; i++)
- sbitmap_deferred_clear(&sb->map[i]);
+ sbitmap_deferred_clear(&sb->map[i], 0, 0, 0);
sb->depth = depth;
sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
@@ -179,7 +206,7 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
alloc_hint, wrap);
if (nr != -1)
break;
- if (!sbitmap_deferred_clear(map))
+ if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
break;
} while (1);
@@ -496,7 +523,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
unsigned int map_depth = __map_depth(sb, index);
unsigned long val;
- sbitmap_deferred_clear(map);
+ sbitmap_deferred_clear(map, 0, 0, 0);
val = READ_ONCE(map->word);
if (val == (1UL << (map_depth - 1)) - 1)
goto next;
--
2.34.1On Mon, Jun 17, 2024 at 10:45:51AM +0800, Yang Yang wrote:
> Configuration for sbq:
> depth=64, wake_batch=6, shift=6, map_nr=1
>
> 1. There are 64 requests in progress:
> map->word = 0xFFFFFFFFFFFFFFFF
> 2. After all the 64 requests complete, and no more requests come:
> map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
> 3. Now two tasks try to allocate requests:
> T1: T2:
> __blk_mq_get_tag .
> __sbitmap_queue_get .
> sbitmap_get .
> sbitmap_find_bit .
> sbitmap_find_bit_in_word .
> __sbitmap_get_word -> nr=-1 __blk_mq_get_tag
> sbitmap_deferred_clear __sbitmap_queue_get
> /* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit
> if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word
> return false; __sbitmap_get_word -> nr=-1
> mask = xchg(&map->cleared, 0) sbitmap_deferred_clear
> atomic_long_andnot() /* map->cleared=0 */
> if (!(map->cleared))
> return false;
> /*
> * map->cleared is cleared by T1
> * T2 fail to acquire the tag
> */
>
> 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
> up due to the wake_batch being set at 6. If no more requests come, T1
> will wait here indefinitely.
>
> This patch achieves two purposes:
> 1. Check on ->cleared and update on both ->cleared and ->word need to
> be done atomically, and using spinlock could be the simplest solution.
> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
> may cause potential race.
>
> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> ->word has free bits.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <yang.yang@vivo.com>
>
> ---
> Changes from v3:
> - Add more arguments to sbitmap_deferred_clear(), for those who
> don't care about the return value, just pass 0
> - Consider the situation when using sbitmap_get_shallow()
> - Consider the situation when ->round_robin is true
> - Modify commit message
> Changes from v2:
> - Modify commit message by suggestion
> - Add extra check in sbitmap_deferred_clear() by suggestion
> Changes from v1:
> - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 45 ++++++++++++++++++++++++++++++++---------
> 2 files changed, 41 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index d662cf136021..ec0b0e73c906 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -36,6 +36,11 @@ struct sbitmap_word {
> * @cleared: word holding cleared bits
> */
> unsigned long cleared ____cacheline_aligned_in_smp;
> +
> + /**
> + * @swap_lock: Held while swapping word <-> cleared
> + */
> + spinlock_t swap_lock;
> } ____cacheline_aligned_in_smp;
>
> /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..9bd85a9b74b9 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -60,12 +60,32 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> /*
> * See if we have deferred clears that we can batch move
> */
> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> -{
> - unsigned long mask;
> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
> + unsigned int depth, unsigned int alloc_hint, bool wrap)
> +{
> + unsigned long mask, flags, word_mask;
> + bool ret = false;
> +
> + spin_lock_irqsave(&map->swap_lock, flags);
> +
> + if (!map->cleared) {
> + if (depth > 0) {
> +#if BITS_PER_LONG == 64
> + word_mask = U64_MAX >> (BITS_PER_LONG - depth);
> +#else
> + word_mask = U32_MAX >> (BITS_PER_LONG - depth);
> +#endif
Can we avoid the above conditional compiling by the following way?
word_mask = (~0UL) >> (BITS_PER_LONG - depth);
> + if (!wrap && alloc_hint)
> + word_mask &= ~((1UL << alloc_hint) - 1);
The current behavior is to always retry after moving ->cleared to
word, and we change it to retry in case of any free bits. So here
is it for avoiding dead loop by taking wrap & alloc_hint into
account? If yes, this way looks fine, but I'd suggest to document
this change.
Thanks,
Ming
On 2024/7/2 11:34, Ming Lei wrote:
> On Mon, Jun 17, 2024 at 10:45:51AM +0800, Yang Yang wrote:
>> Configuration for sbq:
>> depth=64, wake_batch=6, shift=6, map_nr=1
>>
>> 1. There are 64 requests in progress:
>> map->word = 0xFFFFFFFFFFFFFFFF
>> 2. After all the 64 requests complete, and no more requests come:
>> map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
>> 3. Now two tasks try to allocate requests:
>> T1: T2:
>> __blk_mq_get_tag .
>> __sbitmap_queue_get .
>> sbitmap_get .
>> sbitmap_find_bit .
>> sbitmap_find_bit_in_word .
>> __sbitmap_get_word -> nr=-1 __blk_mq_get_tag
>> sbitmap_deferred_clear __sbitmap_queue_get
>> /* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit
>> if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word
>> return false; __sbitmap_get_word -> nr=-1
>> mask = xchg(&map->cleared, 0) sbitmap_deferred_clear
>> atomic_long_andnot() /* map->cleared=0 */
>> if (!(map->cleared))
>> return false;
>> /*
>> * map->cleared is cleared by T1
>> * T2 fail to acquire the tag
>> */
>>
>> 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
>> up due to the wake_batch being set at 6. If no more requests come, T1
>> will wait here indefinitely.
>>
>> This patch achieves two purposes:
>> 1. Check on ->cleared and update on both ->cleared and ->word need to
>> be done atomically, and using spinlock could be the simplest solution.
>> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
>> may cause potential race.
>>
>> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
>> ->word has free bits.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>> Signed-off-by: Yang Yang <yang.yang@vivo.com>
>>
>> ---
>> Changes from v3:
>> - Add more arguments to sbitmap_deferred_clear(), for those who
>> don't care about the return value, just pass 0
>> - Consider the situation when using sbitmap_get_shallow()
>> - Consider the situation when ->round_robin is true
>> - Modify commit message
>> Changes from v2:
>> - Modify commit message by suggestion
>> - Add extra check in sbitmap_deferred_clear() by suggestion
>> Changes from v1:
>> - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
>> ---
>> include/linux/sbitmap.h | 5 +++++
>> lib/sbitmap.c | 45 ++++++++++++++++++++++++++++++++---------
>> 2 files changed, 41 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
>> index d662cf136021..ec0b0e73c906 100644
>> --- a/include/linux/sbitmap.h
>> +++ b/include/linux/sbitmap.h
>> @@ -36,6 +36,11 @@ struct sbitmap_word {
>> * @cleared: word holding cleared bits
>> */
>> unsigned long cleared ____cacheline_aligned_in_smp;
>> +
>> + /**
>> + * @swap_lock: Held while swapping word <-> cleared
>> + */
>> + spinlock_t swap_lock;
>> } ____cacheline_aligned_in_smp;
>>
>> /**
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 1e453f825c05..9bd85a9b74b9 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -60,12 +60,32 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>> /*
>> * See if we have deferred clears that we can batch move
>> */
>> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> -{
>> - unsigned long mask;
>> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
>> + unsigned int depth, unsigned int alloc_hint, bool wrap)
>> +{
>> + unsigned long mask, flags, word_mask;
>> + bool ret = false;
>> +
>> + spin_lock_irqsave(&map->swap_lock, flags);
>> +
>> + if (!map->cleared) {
>> + if (depth > 0) {
>> +#if BITS_PER_LONG == 64
>> + word_mask = U64_MAX >> (BITS_PER_LONG - depth);
>> +#else
>> + word_mask = U32_MAX >> (BITS_PER_LONG - depth);
>> +#endif
>
> Can we avoid the above conditional compiling by the following way?
>
> word_mask = (~0UL) >> (BITS_PER_LONG - depth);
Sure, this looks better.
>
>> + if (!wrap && alloc_hint)
>> + word_mask &= ~((1UL << alloc_hint) - 1);
>
> The current behavior is to always retry after moving ->cleared to
> word, and we change it to retry in case of any free bits. So here
> is it for avoiding dead loop by taking wrap & alloc_hint into
> account? If yes, this way looks fine, but I'd suggest to document
> this change.
Yes. Thank you for the suggestion. I will include the following
descriptions in the next version.
/*
* The current behavior is to always retry after moving
* ->cleared to word, and we change it to retry in case
* of any free bits. To avoid dead loop, we need to take
* wrap & alloc_hint into account. Without this, a soft
* lockup was detected in our test environment.
*/
Thanks.
>
> Thanks,
> Ming
>
>
On 2024/6/17 10:45, Yang Yang wrote:
> Configuration for sbq:
> depth=64, wake_batch=6, shift=6, map_nr=1
>
> 1. There are 64 requests in progress:
> map->word = 0xFFFFFFFFFFFFFFFF
> 2. After all the 64 requests complete, and no more requests come:
> map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
> 3. Now two tasks try to allocate requests:
> T1: T2:
> __blk_mq_get_tag .
> __sbitmap_queue_get .
> sbitmap_get .
> sbitmap_find_bit .
> sbitmap_find_bit_in_word .
> __sbitmap_get_word -> nr=-1 __blk_mq_get_tag
> sbitmap_deferred_clear __sbitmap_queue_get
> /* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit
> if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word
> return false; __sbitmap_get_word -> nr=-1
> mask = xchg(&map->cleared, 0) sbitmap_deferred_clear
> atomic_long_andnot() /* map->cleared=0 */
> if (!(map->cleared))
> return false;
> /*
> * map->cleared is cleared by T1
> * T2 fail to acquire the tag
> */
>
> 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
> up due to the wake_batch being set at 6. If no more requests come, T1
> will wait here indefinitely.
>
> This patch achieves two purposes:
> 1. Check on ->cleared and update on both ->cleared and ->word need to
> be done atomically, and using spinlock could be the simplest solution.
> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
> may cause potential race.
>
> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> ->word has free bits.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <yang.yang@vivo.com>
>
> ---
> Changes from v3:
> - Add more arguments to sbitmap_deferred_clear(), for those who
> don't care about the return value, just pass 0
> - Consider the situation when using sbitmap_get_shallow()
> - Consider the situation when ->round_robin is true
> - Modify commit message
> Changes from v2:
> - Modify commit message by suggestion
> - Add extra check in sbitmap_deferred_clear() by suggestion
> Changes from v1:
> - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
> ---
> include/linux/sbitmap.h | 5 +++++
> lib/sbitmap.c | 45 ++++++++++++++++++++++++++++++++---------
> 2 files changed, 41 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index d662cf136021..ec0b0e73c906 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -36,6 +36,11 @@ struct sbitmap_word {
> * @cleared: word holding cleared bits
> */
> unsigned long cleared ____cacheline_aligned_in_smp;
> +
> + /**
> + * @swap_lock: Held while swapping word <-> cleared
> + */
> + spinlock_t swap_lock;
> } ____cacheline_aligned_in_smp;
>
> /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..9bd85a9b74b9 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -60,12 +60,32 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> /*
> * See if we have deferred clears that we can batch move
> */
> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> -{
> - unsigned long mask;
> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
> + unsigned int depth, unsigned int alloc_hint, bool wrap)
> +{
> + unsigned long mask, flags, word_mask;
> + bool ret = false;
> +
> + spin_lock_irqsave(&map->swap_lock, flags);
> +
> + if (!map->cleared) {
> + if (depth > 0) {
> +#if BITS_PER_LONG == 64
> + word_mask = U64_MAX >> (BITS_PER_LONG - depth);
> +#else
> + word_mask = U32_MAX >> (BITS_PER_LONG - depth);
> +#endif
> + if (!wrap && alloc_hint)
> + word_mask &= ~((1UL << alloc_hint) - 1);
> +
> + if ((READ_ONCE(map->word) & word_mask) == word_mask)
> + ret = false;
> + else
> + ret = true;
> + }
>
> - if (!READ_ONCE(map->cleared))
> - return false;
> + goto out_unlock;
> + }
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,7 +97,10 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> */
> atomic_long_andnot(mask, (atomic_long_t *)&map->word);
> BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
> - return true;
> + ret = true;
> +out_unlock:
> + spin_unlock_irqrestore(&map->swap_lock, flags);
> + return ret;
> }
>
> int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
> @@ -85,6 +108,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
> bool alloc_hint)
> {
> unsigned int bits_per_word;
> + int i;
>
> if (shift < 0)
> shift = sbitmap_calculate_shift(depth);
> @@ -116,6 +140,9 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
> return -ENOMEM;
> }
>
> + for (i = 0; i < sb->map_nr; i++)
> + spin_lock_init(&sb->map[i].swap_lock);
> +
> return 0;
> }
> EXPORT_SYMBOL_GPL(sbitmap_init_node);
> @@ -126,7 +153,7 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth)
> unsigned int i;
>
> for (i = 0; i < sb->map_nr; i++)
> - sbitmap_deferred_clear(&sb->map[i]);
> + sbitmap_deferred_clear(&sb->map[i], 0, 0, 0);
>
> sb->depth = depth;
> sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
> @@ -179,7 +206,7 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
> alloc_hint, wrap);
> if (nr != -1)
> break;
> - if (!sbitmap_deferred_clear(map))
> + if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
> break;
> } while (1);
>
> @@ -496,7 +523,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
> unsigned int map_depth = __map_depth(sb, index);
> unsigned long val;
>
> - sbitmap_deferred_clear(map);
> + sbitmap_deferred_clear(map, 0, 0, 0);
> val = READ_ONCE(map->word);
> if (val == (1UL << (map_depth - 1)) - 1)
> goto next;
Gentle ping.
Thanks.
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