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

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Date:   Fri, 15 Mar 2019 11:11:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched/fair: Limit sched_cfs_period_timer loop to avoid
 hard lockup

On Wed, Mar 13, 2019 at 11:08:26AM -0400, Phil Auld wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 310d0637fe4b..90cc67bbf592 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4859,19 +4859,51 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
>  	return HRTIMER_NORESTART;
>  }
>  
> +extern const u64 max_cfs_quota_period;
> +int cfs_period_autotune_loop_limit   = 8;
> +int cfs_period_autotune_cushion_pct  = 15; /* percentage added to period recalculation */

static const ?

> +
>  static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  {
>  	struct cfs_bandwidth *cfs_b =
>  		container_of(timer, struct cfs_bandwidth, period_timer);
> +	s64 nsstart, nsnow, new_period;

u64

>  	int overrun;
>  	int idle = 0;
> +	int count = 0;
>  
>  	raw_spin_lock(&cfs_b->lock);
> +	nsstart = ktime_to_ns(hrtimer_cb_get_time(timer));

we really should kill ktime :/ Anyway, you now do two indirect timer
calls back to back :/

And this is unconditional overhead.

>  	for (;;) {
>  		overrun = hrtimer_forward_now(timer, cfs_b->period);
>  		if (!overrun)
>  			break;
>  
> +		if (++count > cfs_period_autotune_loop_limit) {
> +			ktime_t old_period = ktime_to_ns(cfs_b->period);
> +
> +			nsnow = ktime_to_ns(hrtimer_cb_get_time(timer));
> +			new_period = (nsnow - nsstart)/cfs_period_autotune_loop_limit;
> +
> +			/*  Make sure new period will be larger than old. */
> +			if (new_period < old_period) {
> +				new_period = old_period;
> +			}
> +			new_period += (new_period *  cfs_period_autotune_cushion_pct) / 100;

Computers _suck_ at /100. And since you're free to pick the constant,
pick a power of two, computers love those.

> +
> +			if (new_period >  max_cfs_quota_period)
> +				new_period = max_cfs_quota_period;
> +
> +			cfs_b->period = ns_to_ktime(new_period);
> +			cfs_b->quota += (cfs_b->quota * ((new_period - old_period) * 100)/old_period)/100;

srsly!? Again, you can pick the constant to be anything, and you pick
such a horrid number?!

> +			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(), cfs_b->period/NSEC_PER_USEC, cfs_b->quota/NSEC_PER_USEC);

period was ktime_t, remember...

> +

And these here lines all all waaay too long.

Also, is that complexity really needed?

> +			/* reset count so we don't come right back in here */
> +			count = 0;
> +		}
> +
>  		idle = do_sched_cfs_period_timer(cfs_b, overrun);
>  	}
>  	if (idle)


Would not something simpler like the below also work?


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ea74d43924b2..b71557be6b42 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 =
@@ -4892,6 +4894,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 	unsigned long flags;
 	int overrun;
 	int idle = 0;
+	int count = 0;
 
 	raw_spin_lock_irqsave(&cfs_b->lock, flags);
 	for (;;) {
@@ -4899,6 +4902,28 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 		if (!overrun)
 			break;
 
+		if (++count > 3) {
+			u64 new, old = ktime_to_ns(cfs_b->period);
+
+			new = (old * 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);
 	}
 	if (idle)