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Subject: Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq
To:     bsegall@google.com, 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
References: <20190814180021.165389-1-liangyan.peng@linux.alibaba.com>
 <xm26d0gvirdg.fsf@bsegall-linux.svl.corp.google.com>
From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <942ae15c-ffa5-74da-208b-7e82df917e16@arm.com>
Date:   Sat, 24 Aug 2019 00:19:02 +0100
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On 23/08/2019 21:00, bsegall@google.com wrote:
[...]
> Could you mention in the message that this a 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.
> 

Mayhaps even a comment for the extra condition.

> I think this solution is less risky than unthrottling
> in this area, so other than that:
> 
> Reviewed-by: Ben Segall <bsegall@google.com>
> 

If you don't mind squashing this in:

-----8<-----
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b1d9cec9b1ed..b47b0bcf56bc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4630,6 +4630,10 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
 		if (!cfs_rq_throttled(cfs_rq))
 			goto next;
 
+		/* By the above check, this should never be true */
+		WARN_ON(cfs_rq->runtime_remaining > 0);
+
+		/* Pick the minimum amount to return to a positive quota state */
 		runtime = -cfs_rq->runtime_remaining + 1;
 		if (runtime > remaining)
 			runtime = remaining;
----->8-----

I'm not adamant about the extra comment, but the WARN_ON would be nice IMO.


@Ben, do you reckon we want to strap

Cc: <stable@vger.kernel.org>
Fixes: ec12cb7f31e2 ("sched: Accumulate per-cfs_rq cpu usage and charge against bandwidth")

to the thing? AFAICT the pick_next_task_fair() + idle_balance() dance you
described should still be possible on that commit.


Other than that,

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

[...]