Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq

[bsegall@google.com]

Valentin Schneider <valentin.schneider@arm.com> writes:

> 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.

I'm not sure about stable policy in general, but it seems reasonable.
The WARN_ON might want to be WARN_ON_ONCE, and it seems fine to have it
or not.

>
>
> Other than that,
>
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
>
> [...]