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    Subject[PATCH 4.4 165/168] sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup
    Date
    [ Upstream commit 2e8e19226398db8265a8e675fcc0118b9e80c9e8 ]

    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: <stable@vger.kernel.org>
    Cc: Anton Blanchard <anton@ozlabs.org>
    Cc: Ben Segall <bsegall@google.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: https://lkml.kernel.org/r/20190319130005.25492-1-pauld@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <sashal@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 7671ae95753f..d706cf4fda99 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -4016,12 +4016,15 @@ 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 =
    container_of(timer, struct cfs_bandwidth, period_timer);
    int overrun;
    int idle = 0;
    + int count = 0;

    raw_spin_lock(&cfs_b->lock);
    for (;;) {
    @@ -4029,6 +4032,28 @@ 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 = div64_u64(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(),
    + div_u64(new, NSEC_PER_USEC),
    + div_u64(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);
    }
    if (idle)
    --
    2.19.1


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