Messages in this thread | | | Date | Tue, 19 Jun 2012 14:08:24 +0800 | From | Yong Zhang <> | Subject | Re: [PATCH] sched: Folding nohz load accounting more accurate |
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On Mon, Jun 18, 2012 at 06:03:37PM +0200, Peter Zijlstra wrote: > > Nohz exit is always caused > > by processes woken up--non-idle model. It's not fair here, idle > > calculated to non-idle. > > > > time-expect-sampling > > | time-do-sampling > > | | > > V V > > -|-------------------------|-- > > start_nohz stop_nohz > > I don't think the delay in sampling is the biggest problem, I think the > problem is the direct interaction between a cpu going idle and another > cpu taking a sample.
IIUC, you hook into tick_nohz_idle_exit() will cure Charles's problem.
And comments below.
> --- > kernel/sched/core.c | 290 ++++++++++++++++++++++++++++++++++------------ > kernel/sched/idle_task.c | 1 - > kernel/sched/sched.h | 2 - > kernel/time/tick-sched.c | 2 + > 4 files changed, 220 insertions(+), 75 deletions(-) > > + * - When we go NO_HZ idle during the window, we can negate our sample > + * contribution, causing under-accounting. > + * > + * We avoid this by keeping two idle-delta counters and flipping them > + * when the window starts, thus separating old and new NO_HZ load. > + * > + * The only trick is the slight shift in index flip for read vs write. > + * > + * 0 5 10 15 > + * +10 +10 +10 +10 > + * |-|-----------|-|-----------|-|-----------|-| > + * r:001 110 001 110 > + * w:011 100 011 100
I'm confused by this comments, looking at your code, index is increased by 1 for each samaple window.
> + * > + * This ensures we'll fold the old idle contribution in this window while > + * accumlating the new one. > + * > + * - When we wake up from NO_HZ idle during the window, we push up our > + * contribution, since we effectively move our sample point to a known > + * busy state. > + * > + * This is solved by pushing the window forward, and thus skipping the > + * sample, for this cpu (effectively using the idle-delta for this cpu which > + * was in effect at the time the window opened). This also solves the issue > + * of having to deal with a cpu having been in NOHZ idle for multiple > + * LOAD_FREQ intervals. > * > * When making the ILB scale, we should try to pull this in as well. > */ > -static long calc_load_fold_idle(void) > +void calc_load_exit_idle(void) > { > - long delta = 0; > + struct rq *this_rq = this_rq(); > > /* > - * Its got a race, we don't care... > + * If we're still outside the sample window, we're done. > */ > - if (atomic_long_read(&calc_load_tasks_idle)) > - delta = atomic_long_xchg(&calc_load_tasks_idle, 0); > + if (time_before(jiffies, this_rq->calc_load_update)) > + return; else if (time_before(jiffies, calc_load_update + 10) this_rq->calc_load_update = calc_load_update + LOAD_FREQ; else this_rq->calc_load_update = calc_load_update;
Otherwise if you woke after the sample window, we loose on sample? And maybe we need local variable to cache calc_load_update.
Thanks, Yong
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