Commit-ID: 06ec5d30e8d57b820d44df6340dcb25010d6d0fa Gitweb: https://git.kernel.org/tip/06ec5d30e8d57b820d44df6340dcb25010d6d0fa Author: Phil Auld pauld@redhat.com AuthorDate: Tue, 19 Mar 2019 09:00:05 -0400 Committer: Ingo Molnar mingo@kernel.org CommitDate: Wed, 3 Apr 2019 09:50:23 +0200

sched/fair: Limit sched_cfs_period_timer loop to avoid hard lockup

With extremely short cfs_period_us setting on a parent task group with a large number of children the for loop in sched_cfs_period_timer can run until the watchdog fires. There is no guarantee that the call to hrtimer_forward_now() will ever return 0. The large number of children can make do_sched_cfs_period_timer() take longer than the period.

NMI watchdog: Watchdog detected hard LOCKUP on cpu 24 RIP: 0010:tg_nop+0x0/0x10 <IRQ> walk_tg_tree_from+0x29/0xb0 unthrottle_cfs_rq+0xe0/0x1a0 distribute_cfs_runtime+0xd3/0xf0 sched_cfs_period_timer+0xcb/0x160 ? sched_cfs_slack_timer+0xd0/0xd0 __hrtimer_run_queues+0xfb/0x270 hrtimer_interrupt+0x122/0x270 smp_apic_timer_interrupt+0x6a/0x140 apic_timer_interrupt+0xf/0x20 </IRQ>

To prevent this we add protection to the loop that detects when the loop has run too many times and scales the period and quota up, proportionally, so that the timer can complete before then next period expires. This preserves the relative runtime quota while preventing the hard lockup.

A warning is issued reporting this state and the new values.

Signed-off-by: Phil Auld pauld@redhat.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Anton Blanchard anton@ozlabs.org Cc: Ben Segall bsegall@google.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Mike Galbraith efault@gmx.de Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20190319130005.25492-1-pauld@redhat.com Signed-off-by: Ingo Molnar mingo@kernel.org --- kernel/sched/fair.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 40bd1e27b1b7..d4cce633eac8 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)