linux-kernel - Re: [PATCH] sched/fair: don't restart enqueued cfs quota slack timer

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Message-ID: <xm26sgsqxvsc.fsf@bsegall-linux.svl.corp.google.com>
Date:   Mon, 03 Jun 2019 13:23:47 -0700
From:   bsegall@...gle.com
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Liangyan <liangyan.peng@...ux.alibaba.com>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, yun.wang@...ux.alibaba.com,
        shanpeic@...ux.alibaba.com, xlpang@...ux.alibaba.com,
        "liangyan.ply" <liangyan.ply@...ux.alibaba.com>
Subject: Re: [PATCH] sched/fair: don't restart enqueued cfs quota slack timer

Peter Zijlstra <peterz@...radead.org> writes:

> On Mon, Jun 03, 2019 at 12:43:09PM +0800, Liangyan wrote:
>> From: "liangyan.ply" <liangyan.ply@...ux.alibaba.com>
>> 
>> start_cfs_slack_bandwidth() will restart the quota slack timer,
>> if it is called frequently, this timer will be restarted continuously
>> and may have no chance to expire to unthrottle cfs tasks.
>> This will cause that the throttled tasks can't be unthrottled in time
>> although they have remaining quota.
>
> This looks very similar to the patch from Ben here:
>
>   https://lkml.kernel.org/r/xm26blzlyr9d.fsf@bsegall-linux.svl.corp.google.com

Yeah, it should do the same sort of thing, but testing hrtimer_active
is racy (we could miss restarting the timer if it's just dropped the
cfsb lock but hasn't finished yet), so the extra accounting flag is
needed.

>
>> 
>> Signed-off-by: Liangyan <liangyan.peng@...ux.alibaba.com>
>> ---
>>  kernel/sched/fair.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index d90a64620072..fdb03c752f97 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4411,9 +4411,11 @@ static void start_cfs_slack_bandwidth(struct cfs_bandwidth *cfs_b)
>>  	if (runtime_refresh_within(cfs_b, min_left))
>>  		return;
>>  
>> -	hrtimer_start(&cfs_b->slack_timer,
>> +	if (!hrtimer_active(&cfs_b->slack_timer)) {
>> +		hrtimer_start(&cfs_b->slack_timer,
>>  			ns_to_ktime(cfs_bandwidth_slack_period),
>>  			HRTIMER_MODE_REL);
>> +	}
>>  }
>>  
>>  /* we know any runtime found here is valid as update_curr() precedes return */
>> -- 
>> 2.14.4.44.g2045bb6
>>