linux-kernel - Re: [PATCH] sched/fair: Fix race in update_cfs_rq_h

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Date:	Thu, 16 Oct 2014 22:31:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched/fair: Fix race in update_cfs_rq_h_load()

On Thu, Oct 16, 2014 at 05:49:19PM +0400, Kirill Tkhai wrote:
> 
> The original idea of cfs_rq::last_h_load_update was to do not
> update cfs_rq::h_load more than once a jiffies.
> 
> Since task_numa_compare()->task_h_load()->update_cfs_rq_h_load()
> makes unlocked update, this may be a reason of race between two
> parallel updates, or with remote rq->curr changing.
> 
> The patch does not change idea, but it excludes the race. Now we
> update cfs_rq::h_load holding rq's lock. This happens in two
> places: every jiffie in task_tick_fair(), and in set_curr_task_fair()
> when task becomes fair.

But why is this a problem? Most of the load balance code is racy like
hell -- purposely so, added serialization is typically worse.
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