From: Tiwei Bie <tiwei.btw@antgroup.com>
With SMP and NO_HZ enabled, the CPU may still need to sleep even
if the timer is disarmed. Switch to deciding whether to sleep based
on pending resched. This is a preparation for adding SMP support.
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
---
arch/um/include/shared/kern_util.h | 1 +
arch/um/kernel/process.c | 12 ++++++++++--
arch/um/os-Linux/time.c | 11 +++++------
3 files changed, 16 insertions(+), 8 deletions(-)
diff --git a/arch/um/include/shared/kern_util.h b/arch/um/include/shared/kern_util.h
index 00ca3e12fd9a..3daaa5c4b35d 100644
--- a/arch/um/include/shared/kern_util.h
+++ b/arch/um/include/shared/kern_util.h
@@ -55,6 +55,7 @@ extern int __uml_cant_sleep(void);
extern int get_current_pid(void);
extern int copy_from_user_proc(void *to, void *from, int size);
extern char *uml_strdup(const char *string);
+int uml_need_resched(void);
extern unsigned long to_irq_stack(unsigned long *mask_out);
extern unsigned long from_irq_stack(int nested);
diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 1be644de9e41..01b935c00454 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -209,10 +209,13 @@ int arch_dup_task_struct(struct task_struct *dst,
void um_idle_sleep(void)
{
- if (time_travel_mode != TT_MODE_OFF)
+ if (time_travel_mode != TT_MODE_OFF) {
time_travel_sleep();
- else
+ } else {
+ raw_local_irq_enable();
os_idle_sleep();
+ raw_local_irq_disable();
+ }
}
void arch_cpu_idle(void)
@@ -225,6 +228,11 @@ int __uml_cant_sleep(void) {
/* Is in_interrupt() really needed? */
}
+int uml_need_resched(void)
+{
+ return need_resched();
+}
+
extern exitcall_t __uml_exitcall_begin, __uml_exitcall_end;
void do_uml_exitcalls(void)
diff --git a/arch/um/os-Linux/time.c b/arch/um/os-Linux/time.c
index 4d5591d96d8c..f25a4196bab7 100644
--- a/arch/um/os-Linux/time.c
+++ b/arch/um/os-Linux/time.c
@@ -98,18 +98,17 @@ long long os_nsecs(void)
*/
void os_idle_sleep(void)
{
- struct itimerspec its;
sigset_t set, old;
- /* block SIGALRM while we analyze the timer state */
+ /* Block SIGALRM while performing the need_resched check. */
sigemptyset(&set);
sigaddset(&set, SIGALRM);
sigprocmask(SIG_BLOCK, &set, &old);
- /* check the timer, and if it'll fire then wait for it */
- timer_gettime(event_high_res_timer, &its);
- if (its.it_value.tv_sec || its.it_value.tv_nsec)
+ /* Sleep if no resched is pending. */
+ if (!uml_need_resched())
sigsuspend(&old);
- /* either way, restore the signal mask */
+
+ /* Restore the signal mask. */
sigprocmask(SIG_UNBLOCK, &set, NULL);
}
--
2.34.1On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote:
>
> void um_idle_sleep(void)
> {
> - if (time_travel_mode != TT_MODE_OFF)
> + if (time_travel_mode != TT_MODE_OFF) {
> time_travel_sleep();
> - else
> + } else {
> + raw_local_irq_enable();
> os_idle_sleep();
> + raw_local_irq_disable();
> + }
This seems wrong, with it, lockdep gets really unhappy, and if I remove
this change it seems to work OK?
I'll note that arch_cpu_idle() for x86 also doesn't change anything with
interrupts.
Was there something else that required this change that I'm missing?
johannes
On Thu, 11 Sep 2025 11:27:04 +0200, Johannes Berg wrote:
> On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote:
> >
> > void um_idle_sleep(void)
> > {
> > - if (time_travel_mode != TT_MODE_OFF)
> > + if (time_travel_mode != TT_MODE_OFF) {
> > time_travel_sleep();
> > - else
> > + } else {
> > + raw_local_irq_enable();
> > os_idle_sleep();
> > + raw_local_irq_disable();
> > + }
>
> This seems wrong, with it, lockdep gets really unhappy, and if I remove
> this change it seems to work OK?
>
> I'll note that arch_cpu_idle() for x86 also doesn't change anything with
> interrupts.
Thanks for catching that! I missed it. I'll take a closer look.
Regards,
Tiwei
On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote: > From: Tiwei Bie <tiwei.btw@antgroup.com> > > With SMP and NO_HZ enabled, the CPU may still need to sleep even > if the timer is disarmed. Switch to deciding whether to sleep based > on pending resched. This is a preparation for adding SMP support. What's the rationale for need_resched()? Is that somehow defined for this? Is it what other architectures use? I guess I'm just not entirely sure what it means. johannes
On Wed, 10 Sep 2025 14:10:37 +0200, Johannes Berg wrote: > On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote: > > From: Tiwei Bie <tiwei.btw@antgroup.com> > > > > With SMP and NO_HZ enabled, the CPU may still need to sleep even > > if the timer is disarmed. Switch to deciding whether to sleep based > > on pending resched. This is a preparation for adding SMP support. > > What's the rationale for need_resched()? Is that somehow defined for > this? Is it what other architectures use? I guess I'm just not entirely > sure what it means. Here is a relevant document: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/scheduler/sched-arch.rst?h=v6.17-rc5#n37 There is a similar check on x86: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/process.c?h=v6.17-rc5#n916 Regards, Tiwei
On Thu, 2025-09-11 at 12:39 +0800, Tiwei Bie wrote: > On Wed, 10 Sep 2025 14:10:37 +0200, Johannes Berg wrote: > > On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote: > > > From: Tiwei Bie <tiwei.btw@antgroup.com> > > > > > > With SMP and NO_HZ enabled, the CPU may still need to sleep even > > > if the timer is disarmed. Switch to deciding whether to sleep based > > > on pending resched. This is a preparation for adding SMP support. > > > > What's the rationale for need_resched()? Is that somehow defined for > > this? Is it what other architectures use? I guess I'm just not entirely > > sure what it means. > > Here is a relevant document: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/scheduler/sched-arch.rst?h=v6.17-rc5#n37 > > There is a similar check on x86: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/process.c?h=v6.17-rc5#n916 Ah cool, thanks for the pointers. Though "1." part there is a bit confusing, I guess I'll send a patch to reword it :) FWIW I already applied a few patches, but I'll take another look, hopefully today. johannes
On Thu, 11 Sep 2025 08:59:57 +0200, Johannes Berg wrote: > On Thu, 2025-09-11 at 12:39 +0800, Tiwei Bie wrote: > > On Wed, 10 Sep 2025 14:10:37 +0200, Johannes Berg wrote: > > > On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote: > > > > From: Tiwei Bie <tiwei.btw@antgroup.com> > > > > > > > > With SMP and NO_HZ enabled, the CPU may still need to sleep even > > > > if the timer is disarmed. Switch to deciding whether to sleep based > > > > on pending resched. This is a preparation for adding SMP support. > > > > > > What's the rationale for need_resched()? Is that somehow defined for > > > this? Is it what other architectures use? I guess I'm just not entirely > > > sure what it means. > > > > Here is a relevant document: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/scheduler/sched-arch.rst?h=v6.17-rc5#n37 > > > > There is a similar check on x86: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/process.c?h=v6.17-rc5#n916 > > Ah cool, thanks for the pointers. Though "1." part there is a bit > confusing, I guess I'll send a patch to reword it :) > > FWIW I already applied a few patches, but I'll take another look, > hopefully today. Thanks for the review! :) Regards, Tiwei
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