Messages in this thread | | | Date | Mon, 11 Mar 2019 16:25:37 -0400 | From | Phil Auld <> | Subject | Re: [RFC] sched/fair: hard lockup in sched_cfs_period_timer |
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On Mon, Mar 11, 2019 at 10:44:25AM -0700 bsegall@google.com wrote: > 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.
Okay. I'll try to spin something up that does this. It may be a little trickier to keep the quota proportional to the new period. I think that's important since we'll be changing the user's setting.
Do you mean to have it break when it hits N and recalculates the period or reset the counter and keep going?
Cheers, Phil
> > > + > > 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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