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From: Vincent Guittot <vincent.guittot@linaro.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Dave Chinner <david@fromorbit.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Phil Auld <pauld@redhat.com>, Ming Lei <ming.lei@redhat.com>,
	linux-block <linux-block@vger.kernel.org>,
	linux-fs <linux-fsdevel@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Jeff Moyer <jmoyer@redhat.com>,
	Dave Chinner <dchinner@redhat.com>,
	Eric Sandeen <sandeen@redhat.com>, Christoph Hellwig <hch@lst.de>,
	Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@redhat.com>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v2] sched/core: Preempt current task in favour of bound kthread
Date: Tue, 10 Dec 2019 12:02:09 +0100
Message-ID: <CAKfTPtD1by06eQ=vJhh9SvfegRanSSwQrKPageLGo0OODu9bjg@mail.gmail.com> (raw)
In-Reply-To: <20191210101116.GA9139@linux.vnet.ibm.com>

On Tue, 10 Dec 2019 at 11:11, Srikar Dronamraju
<srikar@linux.vnet.ibm.com> wrote:
>
> * Vincent Guittot <vincent.guittot@linaro.org> [2019-12-10 10:43:46]:
>
> > On Tue, 10 Dec 2019 at 06:43, Srikar Dronamraju
> > <srikar@linux.vnet.ibm.com> wrote:
> > >
> > > This is more prone to happen if the current running task is CPU
> > > intensive and the sched_wake_up_granularity is set to larger value.
> > > When the sched_wake_up_granularity was relatively small, it was observed
> > > that the bound thread would complete before the load balancer would have
> > > chosen to move the cache hot task to a different CPU.
> > >
> > > To deal with this situation, the current running task would yield to a
> > > per CPU bound kthread, provided kthread is not CPU intensive.
> > >
> > > /pboffline/hwcct_prg_old/lib/fsperf -t overwrite --noclean -f 5g -b 4k /pboffline
> > >
> > > (With sched_wake_up_granularity set to 15ms)
> >
> > So you increase sched_wake_up_granularity to a high level to ensure
> > that current is no preempted by waking thread but then you add a way
> > to finally preempt it which is somewhat weird IMO
> >
>
> Yes, setting to a smaller value will help mitigate/solve the problem.
> There may be folks out who have traditionally set a high wake_up_granularity
> (and have seen better performance with it), who may miss out that when using
> blk-mq, such settings will cause more harm. And they may continue to see
> some performance regressions when they move to a lower wake_up_granularity.
>
> > Have you tried to increase the priority of workqueue thread  (decrease
> > nice priority) ? This is the right way to reduce the impact of the
> > sched_wake_up_granularity on the wakeup of your specific kthread.
> > Because what you want at the end is keeping a low wakeup granularity
> > for these io workqueues
> >
>
> Yes, people can tune the priority of workqueue threads and infact it may be
> easier to set wake_up_granularity to a lower value. However the point is how
> do we make everyone aware that they are running into a performance issue
> with a higher wakeup_granularity?

I did the test on my local setup to change the nice priority of io
workqueue and the active migrations are removed even with high
wakeup_granularity because IO workqueue can still preempt normal task
but let other workqueue behave normally.

>
> --
> Thanks and Regards
> Srikar Dronamraju
>

  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-14 11:31 single aio thread is migrated crazily by scheduler Ming Lei
2019-11-14 13:14 ` Peter Zijlstra
2019-11-15  0:09   ` Ming Lei
2019-11-15 14:16     ` Ming Lei
2019-11-14 23:54 ` Dave Chinner
2019-11-15  1:08   ` Ming Lei
2019-11-15  4:56     ` Dave Chinner
2019-11-15  7:08       ` Ming Lei
2019-11-15 23:40         ` Dave Chinner
2019-11-16  6:31           ` Ming Lei
2019-11-18  9:21           ` Peter Zijlstra
2019-11-18 14:54             ` Vincent Guittot
2019-11-18 20:40             ` Dave Chinner
2019-11-20 19:16               ` Peter Zijlstra
2019-11-20 22:03                 ` Phil Auld
2019-11-21  4:12                   ` Ming Lei
2019-11-21 14:12                     ` Phil Auld
2019-11-21 15:02                       ` Boaz Harrosh
2019-11-21 16:19                         ` Jens Axboe
2019-12-09 16:58                           ` Srikar Dronamraju
2019-11-21 22:10                       ` Dave Chinner
2019-11-21 13:29                   ` Peter Zijlstra
2019-11-21 14:21                     ` Phil Auld
2019-12-09 16:51                     ` Srikar Dronamraju
2019-12-09 23:17                       ` Dave Chinner
2019-12-10  3:27                         ` Srikar Dronamraju
2019-12-10  5:43                         ` [PATCH v2] sched/core: Preempt current task in favour of bound kthread Srikar Dronamraju
2019-12-10  9:26                           ` Peter Zijlstra
2019-12-10  9:33                             ` Peter Zijlstra
2019-12-10 10:18                               ` Srikar Dronamraju
2019-12-10 10:16                             ` Srikar Dronamraju
2019-12-10  9:43                           ` Vincent Guittot
2019-12-10 10:11                             ` Srikar Dronamraju
2019-12-10 11:02                               ` Vincent Guittot [this message]
2019-12-10 17:23                           ` [PATCH v3] " Srikar Dronamraju
2019-12-11 17:38                             ` [PATCH v4] " Srikar Dronamraju
2019-12-11 22:46                               ` Dave Chinner
2019-12-12 10:10                                 ` Peter Zijlstra
2019-12-12 10:14                                   ` Peter Zijlstra
2019-12-12 10:23                                     ` Peter Zijlstra
2019-12-12 11:20                                       ` Vincent Guittot
2019-12-12 13:12                                         ` Peter Zijlstra
2019-12-12 15:07                                   ` Srikar Dronamraju
2019-12-12 15:15                                     ` Peter Zijlstra
2019-12-13  5:32                                   ` Srikar Dronamraju
2019-11-18 16:26           ` single aio thread is migrated crazily by scheduler Srikar Dronamraju
2019-11-18 21:18             ` Dave Chinner
2019-11-19  8:54               ` Ming Lei
     [not found]         ` <20191128094003.752-1-hdanton@sina.com>
2019-11-28  9:53           ` Vincent Guittot
2019-12-02  2:46             ` Ming Lei
2019-12-02  4:02               ` Dave Chinner
2019-12-02  4:22                 ` Ming Lei
2019-12-02 13:45                 ` Vincent Guittot
2019-12-02 21:22                   ` Phil Auld
2019-12-03  9:45                     ` Vincent Guittot
2019-12-04 13:50                       ` Ming Lei
2019-12-02 23:53                   ` Dave Chinner
2019-12-03  0:18                     ` Ming Lei
2019-12-03 13:34                     ` Vincent Guittot
2019-12-02  7:39               ` Juri Lelli
2019-12-02  3:08           ` Dave Chinner
     [not found]           ` <20191202090158.15016-1-hdanton@sina.com>
2019-12-02 23:06             ` Dave Chinner
     [not found]             ` <20191203131514.5176-1-hdanton@sina.com>
2019-12-03 22:29               ` Dave Chinner
     [not found]               ` <20191204102903.896-1-hdanton@sina.com>
2019-12-04 22:59                 ` Dave Chinner

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