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Date: Fri, 18 Jul 2008 13:21:11 +0200
From: "Dmitry Adamushko" <dmitry.adamushko@...il.com>
To: "Ingo Molnar" <mingo@...e.hu>
Cc: LKML <linux-kernel@...r.kernel.org>,
"Jack Ren" <jack.ren@...vell.com>,
"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
"Thomas Gleixner" <tglx@...utronix.de>,
"eric miao" <eric.y.miao@...il.com>
Subject: Re: [PATCH] sched: do not stop ticks when cpu is not idle
2008/7/18 eric miao <eric.y.miao@...il.com>:
> Issue: the sched tick would be stopped in some race conditions.
>
> One of issues caused by that is:
>
> Since there is no timer ticks any more from then, the jiffies update will be
> up to other interrupt to happen. The jiffies will not be updated for a long
> time, until next interrupt happens. That will cause APIs like
> wait_for_completion_timeout(&complete, timeout) to return timeout by mistake,
> since it is using a old jiffies as start time.
>
> Please see comments (1)~(6) inline for how the ticks are stopped
> by mistake when cpu is not idle:
>
> void cpu_idle(void)
> {
> ...
> while (1) {
> void (*idle)(void) = pm_idle;
> if (!idle)
> idle = default_idle;
> leds_event(led_idle_start);
> tick_nohz_stop_sched_tick();
> while (!need_resched())
> idle();
> leds_event(led_idle_end);
> tick_nohz_restart_sched_tick();
> (1) ticks are retarted before switch to other tasks
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> }
> }
>
> asmlinkage void __sched schedule(void)
> {
> ...
> ...
> need_resched:
> (6) the idle task will be scheduled out again and switch to next task,
> with ticks stopped in (5). So the next task will be running with tick stopped.
> preempt_disable();
> cpu = smp_processor_id();
> rq = cpu_rq(cpu);
> rcu_qsctr_inc(cpu);
> prev = rq->curr;
> switch_count = &prev->nivcsw;
>
> release_kernel_lock(prev);
> need_resched_nonpreemptible:
>
> schedule_debug(prev);
>
> hrtick_clear(rq);
>
> /*
> * Do the rq-clock update outside the rq lock:
> */
> local_irq_disable();
> __update_rq_clock(rq);
> spin_lock(&rq->lock);
> clear_tsk_need_resched(prev); (2) resched flag is clear from idle task
>
> ....
>
> context_switch(rq, prev, next); /* unlocks the rq */
> (3) IRQ will be enabled at end of context_swtich( ).
> ...
> preempt_enable_no_resched();
> if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
> (4) the idle task is scheduled back. If an interrupt happen here,
> The irq_exit( ) will be called at end of the irq handler.
> goto need_resched;
(I've taken just a quick look so far, that's maybe why I'm a bit confused)
So what did set TIF_NEED_RESCHED flag here in (4)?
- at first, it was cleared in (2) - ok.
- An interrupt happens somewhere after context_switch() [ btw., what's
about archs that do ctx-switches with interrupts enabled... ]
irq_exit() calls tick_nohz_stop_sched_tick() _only_ when
!need_resched(), meaning TIF_NEED_RESCHED is _not_ set for rq->idle
(no new tasks were activated)
.
So do we have 2 interruts in a raw?
my (likely wrong) interpretation:
(1) schedule() some task - (switch to) -> idle
idle becomes active but is still running in schedule()
(2) an interrupt happens at (3), then irq_exit() calls
tick_nohz_stop_sched_tick()
so far, idle should still run -> this interrupt didn't lead to new
tasks having been activated
(3) another interrupt happens which actually wakes up a task;
TIF_NEED_RESCHED is set
(4) this fact is detected at (4) and --> goto need_resched() to pick
up a new task.
(5) we kind of have idle -> new task reschedule but cpu_idle() never
happened to be called _so_ sched-ticks were not resterted...
is it like this or I'm missing something?
> }
>
> void irq_exit(void)
> {
> ...
> /* Make sure that timer wheel updates are propagated */
> if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
> tick_nohz_stop_sched_tick();
> (5) The ticks will be stopped again since current
> task is idle task and its resched flag is clear in (2).
> rcu_irq_exit();
> preempt_enable_no_resched();
> }
>
> Signed-off-by: Jack Ren <jack.ren@...vell.com>
> ---
> kernel/sched.c | 3 ++-
> 1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index ff0a7e2..fd17d74 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4027,7 +4027,8 @@ need_resched_nonpreemptible:
> rq->nr_switches++;
> rq->curr = next;
> ++*switch_count;
> -
> + if (rq->curr != rq->idle)
> + tick_nohz_restart_sched_tick();
> context_switch(rq, prev, next); /* unlocks the rq */
> /*
> * the context switch might have flipped the stack from under
> --
> 1.5.4
> --
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--
Best regards,
Dmitry Adamushko
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