Messages in this thread | | | From | bsegall@google ... | Subject | Re: [RFC] sched/fair: hard lockup in sched_cfs_period_timer | Date | Mon, 11 Mar 2019 10:44:25 -0700 |
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Phil Auld <pauld@redhat.com> writes:
> On Wed, Mar 06, 2019 at 11:25:02AM -0800 bsegall@google.com wrote: >> Phil Auld <pauld@redhat.com> writes: >> >> > On Tue, Mar 05, 2019 at 12:45:34PM -0800 bsegall@google.com wrote: >> >> Phil Auld <pauld@redhat.com> writes: >> >> >> >> > Interestingly, if I limit the number of child cgroups to the number of >> >> > them I'm actually putting processes into (16 down from 2500) the problem >> >> > does not reproduce. >> >> >> >> That is indeed interesting, and definitely not something we'd want to >> >> matter. (Particularly if it's not root->a->b->c...->throttled_cgroup or >> >> root->throttled->a->...->thread vs root->throttled_cgroup, which is what >> >> I was originally thinking of) >> >> >> > >> > The locking may be a red herring. >> > >> > The setup is root->throttled->a where a is 1-2500. There are 4 threads in >> > each of the first 16 a groups. The parent, throttled, is where the >> > cfs_period/quota_us are set. >> > >> > I wonder if the problem is the walk_tg_tree_from() call in unthrottle_cfs_rq(). >> > >> > The distribute_cfg_runtime looks to be O(n * m) where n is number of >> > throttled cfs_rqs and m is the number of child cgroups. But I'm not >> > completely clear on how the hierarchical cgroups play together here. >> > >> > I'll pull on this thread some. >> > >> > Thanks for your input. >> > >> > >> > Cheers, >> > Phil >> >> Yeah, that isn't under the cfs_b lock, but is still part of distribute >> (and under rq lock, which might also matter). I was thinking too much >> about just the cfs_b regions. I'm not sure there's any good general >> optimization there. >> > > It's really an edge case, but the watchdog NMI is pretty painful. > >> I suppose cfs_rqs (tgs/cfs_bs?) could have "nearest >> ancestor with a quota" pointer and ones with quota could have >> "descendants with quota" list, parallel to the children/parent lists of >> tgs. Then throttle/unthrottle would only have to visit these lists, and >> child cgroups/cfs_rqs without their own quotas would just check >> cfs_rq->nearest_quota_cfs_rq->throttle_count. throttled_clock_task_time >> can also probably be tracked there. > > That seems like it would add a lot of complexity for this edge case. Maybe > it would be acceptible to use the safety valve like my first example, or > something like the below which will tune the period up until it doesn't > overrun for ever. The down side of this one is it does change the user's > settings, but that could be preferable to an NMI crash.
Yeah, I'm not sure what solution is best here, but one of the solutions should be done.
> > Cheers, > Phil > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 310d0637fe4b..78f9e28adc7b 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4859,16 +4859,42 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer) > return HRTIMER_NORESTART; > } > > +extern const u64 max_cfs_quota_period; > +s64 cfs_quota_period_autotune_thresh = 100 * NSEC_PER_MSEC; > +int cfs_quota_period_autotune_shift = 4; /* 100 / 16 = 6.25% */
Letting it spin for 100ms and then only increasing by 6% seems extremely generous. If we went this route I'd probably say "after looping N times, set the period to time taken / N + X%" where N is like 8 or something. I think I'd probably perfer something like this to the previous "just abort and let it happen again next interrupt" one.
> + > 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 nsprev, nsnow, new_period; > + ktime_t now; > int overrun; > int idle = 0; > > raw_spin_lock(&cfs_b->lock); > + nsprev = ktime_to_ns(hrtimer_cb_get_time(timer)); > for (;;) { > - overrun = hrtimer_forward_now(timer, cfs_b->period); > + /* > + * Note this reverts the change to use hrtimer_forward_now, which avoids calling hrtimer_cb_get_time > + * for a value we already have > + */ > + now = hrtimer_cb_get_time(timer); > + nsnow = ktime_to_ns(now); > + if (nsnow - nsprev >= cfs_quota_period_autotune_thresh) { > + new_period = ktime_to_ns(cfs_b->period); > + new_period += new_period >> cfs_quota_period_autotune_shift; > + if (new_period <= max_cfs_quota_period) { > + cfs_b->period = ns_to_ktime(new_period); > + cfs_b->quota += cfs_b->quota >> cfs_quota_period_autotune_shift; > + pr_warn_ratelimited( > + "cfs_period_timer [cpu%d] : Running too long, scaling up (new period %lld, new quota = %lld)\n", > + smp_processor_id(), cfs_b->period/NSEC_PER_USEC, cfs_b->quota/NSEC_PER_USEC); > + } > + nsprev = nsnow; > + } > + > + overrun = hrtimer_forward(timer, now, cfs_b->period); > if (!overrun) > break;
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