linux-kernel - Re: [PATCH] sched/fair: Limit sched_cfs_period

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Date:   Mon, 18 Mar 2019 10:14:22 -0700
From:   bsegall@...gle.com
To:     Phil Auld <pauld@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched/fair: Limit sched_cfs_period_timer loop to avoid hard lockup

Phil Auld <pauld@...hat.com> writes:

> On Fri, Mar 15, 2019 at 05:03:47PM +0100 Peter Zijlstra wrote:
>> On Fri, Mar 15, 2019 at 11:30:42AM -0400, Phil Auld wrote:
>> 
>> >> I'll rework the maths in the averaged version and post v2 if that makes sense.
>> > 
>> > It may have the extra timer fetch, although maybe I could rework it so that it used the 
>> > nsstart time the first time and did not need to do it twice in a row. I had originally
>> > reverted the hrtimer_forward_now() to hrtimer_forward() but put that back. 
>> 
>> Sure; but remember, simpler is often better, esp. for code that
>> typically 'never' runs.
>
> I reworked it to the below. This settles a bit faster. The average is sort of squishy because
> it's 3 samples divided by 4.  And if we stay in a single call after updating the period the "average"
> will be even less accurate. 
>
> It settles at a larger value faster so produces fewer messages and none of the callback supressed ones.
> The added complexity may not be worth it, though.
>
> I think this or your version, either one, would work.  
>
> What needs to happen now to get one of them to land somewhere? Should I just repost one with my 
> signed-off and let you add whatever other tags?  And if so do you have a preference for which one?  
>
> Also, Ben, thoughts?

It would probably make sense to have it just be ++count > 4 then I
think? But otherwise yeah, I'm fine with either.

>
> Cheers,
> Phil
>
> --
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ea74d43924b2..297fd228fdb0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4885,6 +4885,8 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
>  	return HRTIMER_NORESTART;
>  }
>  
> +extern const u64 max_cfs_quota_period;
> +
>  static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  {
>  	struct cfs_bandwidth *cfs_b =
> @@ -4892,14 +4894,46 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  	unsigned long flags;
>  	int overrun;
>  	int idle = 0;
> +	int count = 0;
> +	u64 start, now;
>  
>  	raw_spin_lock_irqsave(&cfs_b->lock, flags);
> +	now = start = ktime_to_ns(hrtimer_cb_get_time(timer));
>  	for (;;) {
> -		overrun = hrtimer_forward_now(timer, cfs_b->period);
> +		overrun = hrtimer_forward(timer, now, cfs_b->period);
>  		if (!overrun)
>  			break;
>  
> +		if (++count > 3) {
> +			u64 new, old = ktime_to_ns(cfs_b->period);
> +
> +                        /* rough average of the time each loop is taking
> +			  * really should be (n-s)/3 but this is easier for the machine
> +			  */
> +			new = (now - start) >> 2; 
> +			if (new < old)
> +				new = old;
> +			new = (new * 147) / 128; /* ~115% */
> +			new = min(new, max_cfs_quota_period);
> +
> +			cfs_b->period = ns_to_ktime(new);
> +
> +			/* since max is 1s, this is limited to 1e9^2, which fits in u64 */
> +			cfs_b->quota *= new;
> +			cfs_b->quota /= old;
> +
> +			pr_warn_ratelimited(
> +	"cfs_period_timer[cpu%d]: period too short, scaling up (new cfs_period_us %lld, cfs_quota_us = %lld)\n",
> +				smp_processor_id(),
> +				new/NSEC_PER_USEC,
> +				cfs_b->quota/NSEC_PER_USEC);
> +
> +			/* reset count so we don't come right back in here */
> +			count = 0;
> +		}
> +
>  		idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
> +		now = ktime_to_ns(hrtimer_cb_get_time(timer));
>  	}
>  	if (idle)
>  		cfs_b->period_active = 0;