linux-kernel - Re: min_vruntime update when a task is sleeping/migrating

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Message-ID: <20170223095050.GA6515@twins.programming.kicks-ass.net>
Date:   Thu, 23 Feb 2017 10:50:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: min_vruntime update when a task is sleeping/migrating

On Thu, Feb 23, 2017 at 11:45:26AM +0530, Pavan Kondeti wrote:
> Hi Peter,
> 
> The comment and the code around 2nd update_min_vruntime() call in
> dequeue_entity() are not matching. If I understand commit b60205c7c558
> ("sched/fair: Fix min_vruntime tracking") correctly, the check is
> inverted there. We want to update min_vruntime when a task is
> sleeping/migrating. is my understanding right?

Hurm, yes that comment and the code are not in agreement :/

Having gone over the code again, I think the comment is right and the
code is wrong, but it would be good to double check that.

> static void
> dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> {
> 
>  <snip>
>   /*
>    * Now advance min_vruntime if @se was the entity holding it back,
>    * except when: DEQUEUE_SAVE && !DEQUEUE_MOVE, in this case we'll be
>    * put back on, and if we advance min_vruntime, we'll be placed back
>    * further than we started -- ie. we'll be penalized.
>    */
>    if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
>       update_min_vruntime(cfs_rq);
> }