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SubjectRe: [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)
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