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From:   bsegall@google.com
To:     Dave Chiluk <chiluk+linux@indeed.com>
Cc:     Qian Cai <cai@lca.pw>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Phil Auld <pauld@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -next v2] sched/fair: fix -Wunused-but-set-variable warnings
References: <1566326455-8038-1-git-send-email-cai@lca.pw>
        <xm26tvaaifoy.fsf@bsegall-linux.svl.corp.google.com>
        <CAC=E7cWYSEAjUhgN5vBECmR5ATXWmt4M7n8sNN0xXStEsb4YjA@mail.gmail.com>
Date:   Fri, 23 Aug 2019 10:28:02 -0700
In-Reply-To: <CAC=E7cWYSEAjUhgN5vBECmR5ATXWmt4M7n8sNN0xXStEsb4YjA@mail.gmail.com>
        (Dave Chiluk's message of "Fri, 23 Aug 2019 09:48:08 -0500")
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Dave Chiluk <chiluk+linux@indeed.com> writes:

> On Wed, Aug 21, 2019 at 12:36 PM <bsegall@google.com> wrote:
>>
>> Qian Cai <cai@lca.pw> writes:
>>
>> > The linux-next commit "sched/fair: Fix low cpu usage with high
>> > throttling by removing expiration of cpu-local slices" [1] introduced a
>> > few compilation warnings,
>> >
>> > kernel/sched/fair.c: In function '__refill_cfs_bandwidth_runtime':
>> > kernel/sched/fair.c:4365:6: warning: variable 'now' set but not used
>> > [-Wunused-but-set-variable]
>> > kernel/sched/fair.c: In function 'start_cfs_bandwidth':
>> > kernel/sched/fair.c:4992:6: warning: variable 'overrun' set but not used
>> > [-Wunused-but-set-variable]
>> >
>> > Also, __refill_cfs_bandwidth_runtime() does no longer update the
>> > expiration time, so fix the comments accordingly.
>> >
>> > [1] https://lore.kernel.org/lkml/1558121424-2914-1-git-send-email-chiluk+linux@indeed.com/
>> >
>> > Signed-off-by: Qian Cai <cai@lca.pw>
>>
>> Reviewed-by: Ben Segall <bsegall@google.com>
>>
>> > ---
>> >
>> > v2: Keep hrtimer_forward_now() in start_cfs_bandwidth() per Ben.
>> >
>> >  kernel/sched/fair.c | 19 ++++++-------------
>> >  1 file changed, 6 insertions(+), 13 deletions(-)
>> >
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index 84959d3285d1..06782491691f 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -4354,21 +4354,16 @@ static inline u64 sched_cfs_bandwidth_slice(void)
>> >  }
>> >
>> >  /*
>> > - * Replenish runtime according to assigned quota and update expiration time.
>> > - * We use sched_clock_cpu directly instead of rq->clock to avoid adding
>> > - * additional synchronization around rq->lock.
>> > + * Replenish runtime according to assigned quota. We use sched_clock_cpu
>> > + * directly instead of rq->clock to avoid adding additional synchronization
>> > + * around rq->lock.
>> >   *
>> >   * requires cfs_b->lock
>> >   */
>> >  void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
>> >  {
>> > -     u64 now;
>> > -
>> > -     if (cfs_b->quota == RUNTIME_INF)
>> > -             return;
>> > -
>> > -     now = sched_clock_cpu(smp_processor_id());
>> > -     cfs_b->runtime = cfs_b->quota;
>> > +     if (cfs_b->quota != RUNTIME_INF)
>> > +             cfs_b->runtime = cfs_b->quota;
>> >  }
>> >
>> >  static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
>> > @@ -4989,15 +4984,13 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>> >
>> >  void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>> >  {
>> > -     u64 overrun;
>> > -
>> >       lockdep_assert_held(&cfs_b->lock);
>> >
>> >       if (cfs_b->period_active)
>> >               return;
>> >
>> >       cfs_b->period_active = 1;
>> > -     overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>> > +     hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
>> >       hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
>> >  }
>
> Looks good.
> Reviewed-by: Dave Chiluk <chiluk+linux@indeed.com>
>
> Sorry for the slow response, I was on vacation.
>
> @Ben do you think it would be useful to still capture overrun, and
> WARN on any overruns?  We wouldn't expect overruns, but their
> existence would indicate an over-loaded node or too short of a
> cfs_period.  Additionally, it would be interesting to see if we could
> capture the offset between when the bandwidth was refilled, and when
> the timer was supposed to fire.  I've always done all my calculations
> assuming that the timer fires and is handled exceedingly close to the
> time it was supposed to fire.  Although, if the node is running that
> overloaded you probably have many more problems than worrying about
> timer warnings.

That "overrun" there is not really an overrun - it's the number of
complete periods the timer has been inactive for. It was used so that a
given tg's period timer would keep the same
phase/offset/whatever-you-call-it, even if it goes idle for a while,
rather than having the next period start N ms after a task wakes up.

Also, poor choices by userspace is not generally something the kernel
generally WARNs on, as I understand it.