Re: [PATCH] sched/loadavg: Avoid loadavg spikes caused by delayed NO_HZ accounting

[Frederic Weisbecker]

On Thu, Feb 09, 2017 at 04:07:53PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 08, 2017 at 01:29:24PM +0000, Matt Fleming wrote:
> > The calculation for the next sample window when exiting NOH_HZ idle
> > does not handle the fact that we may not have reached the next sample
> > window yet, i.e. that we came out of idle between sample windows.
> > 
> > If we wake from NO_HZ idle after the pending this_rq->calc_load_update
> > window time when we want idle but before the next sample window, we
> > will add an unnecessary LOAD_FREQ delay to the load average
> > accounting, delaying any update for potentially ~9seconds.
> > 
> > This can result in huge spikes in the load average values due to
> > per-cpu uninterruptible task counts being out of sync when accumulated
> > across all CPUs.
> > 
> > It's safe to update the per-cpu active count if we wake between sample
> > windows because any load that we left in 'calc_load_idle' will have
> > been zero'd when the idle load was folded in calc_global_load().
> 
> Right, so differently put; the problem is that we check against the
> 'stale' rq->calc_load_update, while the current and effective period
> boundary is 'calc_load_update'.
> 
> So, when rq->calc_load_update < jiffies < calc_load_update, we end up
> setting the next-update to calc_load_update+LOAD_FREQ, where it should
> have been calc_load_update.
> 
> > diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> > index a2d6eb71f06b..a7a6f3646970 100644
> > --- a/kernel/sched/loadavg.c
> > +++ b/kernel/sched/loadavg.c
> 
> > @@ -210,10 +211,16 @@ void calc_load_exit_idle(void)
> >  	 * We woke inside or after the sample window, this means we're already
> >  	 * accounted through the nohz accounting, so skip the entire deal and
> >  	 * sync up for the next window.
> > +	 *
> > +	 * The next window is 'calc_load_update' if we haven't reached it yet,
> > +	 * and 'calc_load_update + 10' if we're inside the current window.

Hmm, the comment doesn't seem to match the code.

> >  	 */
> > +	next_window = calc_load_update;
> > +
> > +	if (time_in_range_open(jiffies, next_window, next_window + 10)
> > +		next_window += LOAD_FREQ;
> > +
> > +	this_rq->calc_load_update = next_window;
> >  }
> 
> So I don't much like the time_in_range_open() thing. The simpler patch
> which you tested to also work was:
> 
> diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> index 7296b7308eca..cfb47bd0ee50 100644
> --- a/kernel/sched/loadavg.c
> +++ b/kernel/sched/loadavg.c
> @@ -201,6 +201,8 @@ void calc_load_exit_idle(void)
>  {
>  	struct rq *this_rq = this_rq();
>  
> +	this_rq->calc_load_update = calc_load_update;
> +
>  	/*
>  	 * If we're still before the sample window, we're done.
>  	 */
> @@ -212,7 +214,6 @@ void calc_load_exit_idle(void)
>  	 * accounted through the nohz accounting, so skip the entire deal and
>  	 * sync up for the next window.
>  	 */
> -	this_rq->calc_load_update = calc_load_update;
>  	if (time_before(jiffies, this_rq->calc_load_update + 10))
>  		this_rq->calc_load_update += LOAD_FREQ;
>  }
> 
> But the problem there is that we unconditionally issue that store. Now
> I've no idea how much of a problem that is, and it certainly is the
> simplest form (+- comments that need updating), so maybe that makes
> sense.

Well, that version looks fine to me.

Thanks.