Commit-ID: fd99f91aa007ba255aac44fe6cf21c1db398243a
Gitweb: http://git.kernel.org/tip/fd99f91aa007ba255aac44fe6cf21c1db398243a
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 9 Apr 2014 15:35:08 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 8 May 2014 09:16:58 +0200
sched/idle: Avoid spurious wakeup IPIs
Because mwait_idle_with_hints() gets called from !idle context it must
call current_clr_polling(). This however means that resched_task() is
very likely to send an IPI even when we were polling:
CPU0 CPU1
if (current_set_polling_and_test())
goto out;
__monitor(&ti->flags);
if (!need_resched())
__mwait(eax, ecx);
set_tsk_need_resched(p);
smp_mb();
out:
current_clr_polling();
if (!tsk_is_polling(p))
smp_send_reschedule(cpu);
So while it is correct (extra IPIs aren't a problem, whereas a missed
IPI would be) it is a performance problem (for some).
Avoid this issue by using fetch_or() to atomically set NEED_RESCHED
and test if POLLING_NRFLAG is set.
Since a CPU stuck in mwait is unlikely to modify the flags word,
contention on the cmpxchg is unlikely and thus we should mostly
succeed in a single go.
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-kf5suce6njh5xf5d3od13rr0@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/sched/core.c | 41 ++++++++++++++++++++++++++++++++++++-----
1 file changed, 36 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1c9c3b7..4b82622 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -506,6 +506,39 @@ static inline void init_hrtick(void)
#endif /* CONFIG_SCHED_HRTICK */
/*
+ * cmpxchg based fetch_or, macro so it works for different integer types
+ */
+#define fetch_or(ptr, val) \
+({ typeof(*(ptr)) __old, __val = *(ptr); \
+ for (;;) { \
+ __old = cmpxchg((ptr), __val, __val | (val)); \
+ if (__old == __val) \
+ break; \
+ __val = __old; \
+ } \
+ __old; \
+})
+
+#ifdef TIF_POLLING_NRFLAG
+/*
+ * Atomically set TIF_NEED_RESCHED and test for TIF_POLLING_NRFLAG,
+ * this avoids any races wrt polling state changes and thereby avoids
+ * spurious IPIs.
+ */
+static bool set_nr_and_not_polling(struct task_struct *p)
+{
+ struct thread_info *ti = task_thread_info(p);
+ return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
+}
+#else
+static bool set_nr_and_not_polling(struct task_struct *p)
+{
+ set_tsk_need_resched(p);
+ return true;
+}
+#endif
+
+/*
* resched_task - mark a task 'to be rescheduled now'.
*
* On UP this means the setting of the need_resched flag, on SMP it
@@ -521,17 +554,15 @@ void resched_task(struct task_struct *p)
if (test_tsk_need_resched(p))
return;
- set_tsk_need_resched(p);
-
cpu = task_cpu(p);
+
if (cpu == smp_processor_id()) {
+ set_tsk_need_resched(p);
set_preempt_need_resched();
return;
}
- /* NEED_RESCHED must be visible before we test polling */
- smp_mb();
- if (!tsk_is_polling(p))
+ if (set_nr_and_not_polling(p))
smp_send_reschedule(cpu);
}
--
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