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Date:   Mon, 26 Aug 2019 11:25:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Liangyan <liangyan.peng@linux.alibaba.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Ben Segall <bsegall@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org, shanpeic@linux.alibaba.com,
        xlpang@linux.alibaba.com
Subject: Re: [PATCH v2] sched/fair: don't assign runtime for throttled cfs_rq
Message-ID: <20190826092506.GO2369@hirez.programming.kicks-ass.net>
References: <20190824091558.86296-1-liangyan.peng@linux.alibaba.com>
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On Sat, Aug 24, 2019 at 05:15:58PM +0800, Liangyan wrote:
> do_sched_cfs_period_timer() will refill cfs_b runtime and call
> distribute_cfs_runtime() to unthrottle cfs_rq, sometimes cfs_b->runtime
> will allocate all quota to one cfs_rq incorrectly.
> This will cause other cfs_rq can't get runtime and will be throttled.
> We find that one throttled cfs_rq has non-negative
> cfs_rq->runtime_remaining and cause an unexpetced cast from s64 to u64
> in snippet: distribute_cfs_runtime() {
> runtime = -cfs_rq->runtime_remaining + 1; }.
> This cast will cause that runtime will be a large number and
> cfs_b->runtime will be subtracted to be zero at last.
> According to Ben Segall, the throttled cfs_rq can have
> account_cfs_rq_runtime called on it because it is throttled before
> idle_balance, and the idle_balance calls update_rq_clock to add time
> that is accounted to the task.

That is distinctly unreadable. Please try again with a little bit of
whitespace added.

> This commit prevents cfs_rq to be assgined new runtime if it has been
> throttled to avoid the above incorrect type cast.
> 
> Signed-off-by: Liangyan <liangyan.peng@linux.alibaba.com>
> Reviewed-by: Ben Segall <bsegall@google.com>
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>