linux-kernel - Re: [PATCH 2/3] sched,fair: Fix local starvation

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Message-ID: <20160520212405.GL21993@codeblueprint.co.uk>
Date:	Fri, 20 May 2016 22:24:05 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	Pavan Kondeti <pkondeti@...eaurora.org>,
	Ben Segall <bsegall@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Paul Turner <pjt@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>, byungchul.park@....com,
	Andrew Hunter <ahh@...gle.com>,
	Mike Galbraith <mgalbraith@...e.de>,
	Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 2/3] sched,fair: Fix local starvation

On Tue, 10 May, at 07:43:16PM, Peter Zijlstra wrote:
> Mike reported that the recent commit 3a47d5124a95 ("sched/fair: Fix
> fairness issue on migration") broke interactivity and the signal
> starve test.
> 
> The problem is that I assumed ENQUEUE_WAKING was only set when we do a
> cross-cpu wakeup (migration), which isn't true. This means we now
> destroy the vruntime history of tasks and wakeup-preemption suffers.
> 
> Cure this by making my assumption true, only call
> sched_class::task_waking() when we do a cross-cpu wakeup. This avoids
> the indirect call in the case we do a local wakeup.
> 
> Cc: Pavan Kondeti <pkondeti@...eaurora.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Matt Fleming <matt@...eblueprint.co.uk>
> Cc: Morten Rasmussen <morten.rasmussen@....com>
> Cc: Paul Turner <pjt@...gle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: byungchul.park@....com
> Cc: Andrew Hunter <ahh@...gle.com>
> Fixes: 3a47d5124a95 ("sched/fair: Fix fairness issue on migration")
> Reported-by: Mike Galbraith <mgalbraith@...e.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/sched/core.c |   29 +++++++++++++++++++++--------
>  kernel/sched/fair.c |   41 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 61 insertions(+), 9 deletions(-)
 
This patch appears to cause a regression for hackbench -pipe of
between ~8% and ~10% with groups >= NR_CPU.

I haven't probed much yet, but it looks like the vruntime of tasks has
gone nuts.