Messages in this thread Patch in this message | | | Date | Fri, 18 Jul 2008 12:24:46 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] sched: do not stop ticks when cpu is not idle |
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* eric miao <eric.y.miao@gmail.com> wrote:
> Issue: the sched tick would be stopped in some race conditions.
> --- 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 */
applied to tip/sched/urgent, thanks Eric.
Thomas, Peter, Dmitry, do you concur with the analysis? (commit below)
It looks a bit ugly to me in the middle of schedule() - is there no wait to solve this within kernel/time/*.c ?
Ingo
--------------> commit ca1b5a8a9abb3db57562a838f41cdba842f13fe8 Author: eric miao <eric.y.miao@gmail.com> Date: Fri Jul 18 14:41:29 2008 +0800
sched: do not stop ticks when cpu is not idle 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; } 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@marvell.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> --- kernel/sched.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c index 1ee18db..e0e0162 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -4446,7 +4446,8 @@ 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
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