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Subject: Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq
To:     Liangyan <liangyan.peng@linux.alibaba.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, shanpeic@linux.alibaba.com,
        xlpang@linux.alibaba.com, pjt@google.com
References: <20190814180021.165389-1-liangyan.peng@linux.alibaba.com>
 <2994a6ee-9238-5285-3227-cb7084a834c8@arm.com>
 <7C1833A8-27A4-4755-9B1E-335C20207A66@linux.alibaba.com>
From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <39d1affb-9cfa-208d-8bf4-f4c802e8c7f9@arm.com>
Date:   Fri, 16 Aug 2019 15:02:04 +0100
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On 16/08/2019 08:08, Liangyan wrote:
> Please check below dmesg log with “WARN_ON(cfs_rq->runtime_remaining > 0)”. If apply my patch, the warning is gone.  Append the reproducing case in the end.
> 

[...]

Huh, thanks for the log & the reproducer. I'm still struggling to
understand how we could hit the condition you're adding, since
account_cfs_rq_runtime() shouldn't be called for throttled cfs_rqs (which
I guess is the bug). Also, if the cfs_rq is throttled, shouldn't we
prevent any further decrement of its ->runtime_remaining ?

I had a look at the callers of account_cfs_rq_runtime():

- update_curr(). Seems safe, but has a cfs_rq->curr check at the top. This
  won't catch throttled cfs_rq's because AFAICT their curr pointer isn't
  NULL'd on throttle.

- check_enqueue_throttle(). Already has a cfs_rq_throttled() check.

- set_next_task_fair(). Peter shuffled the whole set/put task thing
  recently but last I looked it seemed all sane.

I'll try to make sense of it, but have also Cc'd Paul since unlike me he
actually knows this stuff.