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Date: Sun, 5 Dec 2010 13:28:19 +0800 From: Yong Zhang <yong.zhang0@...il.com> To: "Bjoern B. Brandenburg" <bbb.lst@...il.com> Cc: Mike Galbraith <efault@....de>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>, Andrea Bastoni <bastoni@...g.uniroma2.it>, "James H. Anderson" <anderson@...unc.edu>, linux-kernel@...r.kernel.org Subject: Re: Scheduler bug related to rq->skip_clock_update? On Mon, Nov 22, 2010 at 01:14:47PM -0500, Bjoern B. Brandenburg wrote: > On Mon, 22 Nov 2010, Mike Galbraith wrote: > > > On Sun, 2010-11-21 at 23:29 -0500, Bjoern B. Brandenburg wrote: > > > On Sun, 21 Nov 2010, Mike Galbraith wrote: > > > > > > > On Sat, 2010-11-20 at 23:22 -0500, Bjoern B. Brandenburg wrote: > > > > > > > > > I was under the impression that, as an invariant, tasks should not have > > > > > TIF_NEED_RESCHED set after they've blocked. In this case, the idle load > > > > > balancer should not mark the task that's on its way out with > > > > > set_tsk_need_resched(). > > > > > > > > Nice find. > > > > > > > > > In any case, check_preempt_curr() seems to assume that a resuming task cannot > > > > > have TIF_NEED_RESCHED already set. Setting skip_clock_update on a remote CPU > > > > > that hasn't even been notified via IPI seems wrong. > > > > > > > > Yes. Does the below fix it up for you? > > > > > > The patch definitely changes the behavior, but it doesn't seem to solve (all > > > of) the root cause(s). The failsafe kicks in and clears the flag the next > > > time that update_rq_clock() is called, but there can still be a significant > > > delay between setting and clearing the flag. Right after boot, I'm now seeing > > > values that go up to ~21ms. > > > > A pull isn't the only vulnerability. Since idle_balance() drops > > rq->lock, so another cpu can wake to this rq. > > > > > Please let me know if there is something else that I should test. > > > > Sched: clear_tsk_need_resched() after NEWIDLE balancing > > > > idle_balance() drops/retakes rq->lock, leaving the previous task > > vulnerable to set_tsk_need_resched() from another CPU. Clear it > > after NEWIDLE balancing to maintain the invariant that descheduled > > tasks are NOT marked for resched. > > > > This also confuses the skip_clock_update logic, which assumes that > > the next call to update_rq_clock() will come nearly ĩmmediately after > > being set. Make the optimization more robust by clearing before we > > balance and in update_rq_clock(). > > Unfortunately that doesn't seem to do it yet. > > After running five 'find /' instances to completion on the ARM platform, > I'm still seeing delays close to 10ms. > > bbb@...trict10:~$ egrep 'cpu#|skip' /proc/sched_debug > cpu#0 > .skip_clock_count : 89606 > .skip_clock_recent_max : 9817250 > .skip_clock_max : 21992375 > cpu#1 > .skip_clock_count : 81978 > .skip_clock_recent_max : 9582500 > .skip_clock_max : 17201750 > cpu#2 > .skip_clock_count : 74565 > .skip_clock_recent_max : 9678000 > .skip_clock_max : 9879250 > cpu#3 > .skip_clock_count : 81685 > .skip_clock_recent_max : 9300125 > .skip_clock_max : 14115750 > > On the x86_64 host, I've changed to HZ=100 and am now also seeing delays > close to 10ms after 'make clean && make -j8 bzImage'. > > bbb@...una:~$ egrep 'cpu#|skip' /proc/sched_debug > cpu#0, 2493.476 MHz > .skip_clock_count : 29703 > .skip_clock_recent_max : 9999858 > .skip_clock_max : 40645942 > cpu#1, 2493.476 MHz > .skip_clock_count : 32696 > .skip_clock_recent_max : 9959118 > .skip_clock_max : 35074771 > cpu#2, 2493.476 MHz > .skip_clock_count : 31742 > .skip_clock_recent_max : 9788654 > .skip_clock_max : 24821765 > cpu#3, 2493.476 MHz > .skip_clock_count : 31123 > .skip_clock_recent_max : 9858546 > .skip_clock_max : 44276033 > cpu#4, 2493.476 MHz > .skip_clock_count : 28346 > .skip_clock_recent_max : 10000775 > .skip_clock_max : 18681753 > cpu#5, 2493.476 MHz > .skip_clock_count : 29421 > .skip_clock_recent_max : 9997656 > .skip_clock_max : 138473407 > cpu#6, 2493.476 MHz > .skip_clock_count : 27721 > .skip_clock_recent_max : 9992074 > .skip_clock_max : 53436918 > cpu#7, 2493.476 MHz > .skip_clock_count : 29637 > .skip_clock_recent_max : 9994516 > .skip_clock_max : 566793528 > > These numbers were recorded with the below patch. > > Please let me know if I can help by testing or tracing something else. Please just ignore my previous email and sorry for those noise. I think I find the root cause after all-night sleep. :) when we init idle task, we doesn't mark it on_rq. My test show the concern is smoothed by below patch. Thanks, Yong --- diff --git a/kernel/sched.c b/kernel/sched.c index dc91a4d..21c76d9 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -5486,6 +5486,7 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu) __sched_fork(idle); idle->state = TASK_RUNNING; idle->se.exec_start = sched_clock(); + idle->se.on_rq = 1; cpumask_copy(&idle->cpus_allowed, cpumask_of(cpu)); /* -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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