linux-kernel - Re: [PATCH] sched: sync with the prev cfs when changing cgroup within a cpu

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Date:	Mon, 10 Aug 2015 13:47:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	byungchul.park@....com
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: sync with the prev cfs when changing cgroup
 within a cpu

On Mon, Aug 10, 2015 at 03:08:59PM +0900, byungchul.park@....com wrote:
> From: Byungchul Park <byungchul.park@....com>
> 
> current code seems to be wrong with cfs_rq->blocked_load_avg when changing
> a task's cgroup(=cfs_rq) to another. i tested with "echo pid > cgroup" and
> found that cfs_rq->blocked_load_avg became larger and larger whenever i
> changed a cgroup to another again and again.
> 
> it is possible to move between groups within a cpu, and each cfs_rq is
> tracking its own blocked load. so we have to sync se's average load with
> both *prev* cfs_rq and next cfs_rq when changing its group.
> 
> in addition, "#ifdef CONFIG_SMP" is removed becasuse we need to sync a
> se's load with its cfs_rq even in the case of !SMP. remember it is possible
> to move between groups in *a* cpu.
> 
> i also removed some comments mentioning migration_task_rq_fair().
> migration_task_rq_fair() can be called in three cases. and in each case,
> both decay counter and blocked load are already considered. so we
> don't need to consider these in task_move_group_fair() at all.
> 
> 1. the wake-up migration case
> enqueue_entity_load_avg() makes se->avg.decay_count zero after applying it.
> and it will be woken up soon so we don't need to add its load to
> cfs_rq->blocked_load_avg.
> 
> 2. the fork balancing case
> se->avg.decay_count is initialized in __sched_fork() to zero. and
> wake_up_new_task() calls activate_task() with flag = 0 so that
> enqueue_entity_load_avg() can omit adding its load to
> cfs_rq->blocked_load_avg, and it will be woken up soon.
> 
> 3. the rq migration case (not wake up case)
> the target task is already on rq, so we don't need to consider both its
> decay counter and blocked load in this case.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>

What code is this against? Please look at current code and try again.
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