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From: Ming Lei <ming.lei@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
	Hillf Danton <hdanton@sina.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>,
	Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Peter Zijlstra <peterz@infradead.org>,
	Rong Chen <rong.a.chen@intel.com>, Tejun Heo <tj@kernel.org>
Subject: Re: single aio thread is migrated crazily by scheduler
Date: Tue, 3 Dec 2019 08:18:43 +0800
Message-ID: <20191203001843.GA25002@ming.t460p> (raw)
In-Reply-To: <20191202235321.GJ2695@dread.disaster.area>

On Tue, Dec 03, 2019 at 10:53:21AM +1100, Dave Chinner wrote:
> On Mon, Dec 02, 2019 at 02:45:42PM +0100, Vincent Guittot wrote:
> > On Mon, 2 Dec 2019 at 05:02, Dave Chinner <david@fromorbit.com> wrote:
> > >
> > > On Mon, Dec 02, 2019 at 10:46:25AM +0800, Ming Lei wrote:
> > > > On Thu, Nov 28, 2019 at 10:53:33AM +0100, Vincent Guittot wrote:
> > > > > On Thu, 28 Nov 2019 at 10:40, Hillf Danton <hdanton@sina.com> wrote:
> > > > > > --- a/fs/iomap/direct-io.c
> > > > > > +++ b/fs/iomap/direct-io.c
> > > > > > @@ -157,10 +157,8 @@ static void iomap_dio_bio_end_io(struct
> > > > > >                         WRITE_ONCE(dio->submit.waiter, NULL);
> > > > > >                         blk_wake_io_task(waiter);
> > > > > >                 } else if (dio->flags & IOMAP_DIO_WRITE) {
> > > > > > -                       struct inode *inode = file_inode(dio->iocb->ki_filp);
> > > > > > -
> > > > > >                         INIT_WORK(&dio->aio.work, iomap_dio_complete_work);
> > > > > > -                       queue_work(inode->i_sb->s_dio_done_wq, &dio->aio.work);
> > > > > > +                       schedule_work(&dio->aio.work);
> > > > >
> > > > > I'm not sure that this will make a real difference because it ends up
> > > > > to call queue_work(system_wq, ...) and system_wq is bounded as well so
> > > > > the work will still be pinned to a CPU
> > > > > Using system_unbound_wq should make a difference because it doesn't
> > > > > pin the work on a CPU
> > > > >  +                       queue_work(system_unbound_wq, &dio->aio.work);
> > > >
> > > > Indeed, just run a quick test on my KVM guest, looks the following patch
> > > > makes a difference:
> > > >
> > > > diff --git a/fs/direct-io.c b/fs/direct-io.c
> > > > index 9329ced91f1d..2f4488b0ecec 100644
> > > > --- a/fs/direct-io.c
> > > > +++ b/fs/direct-io.c
> > > > @@ -613,7 +613,8 @@ int sb_init_dio_done_wq(struct super_block *sb)
> > > >  {
> > > >         struct workqueue_struct *old;
> > > >         struct workqueue_struct *wq = alloc_workqueue("dio/%s",
> > > > -                                                     WQ_MEM_RECLAIM, 0,
> > > > +                                                     WQ_MEM_RECLAIM |
> > > > +                                                     WQ_UNBOUND, 0,
> > > >                                                       sb->s_id);
> > >
> > > That's not an answer to the user task migration issue.
> > >
> > > That is, all this patch does is trade user task migration when the
> > > CPU is busy for migrating all the queued work off the CPU so the
> > > user task does not get migrated. IOWs, this forces all the queued
> > > work to be migrated rather than the user task. IOWs, it does not
> > > address the issue we've exposed in the scheduler between tasks with
> > > competing CPU affinity scheduling requirements - it just hides the
> > > symptom.
> > >
> > > Maintaining CPU affinity across dispatch and completion work has
> > > been proven to be a significant performance win. Right throughout
> > > the IO stack we try to keep this submitter/completion affinity,
> > > and that's the whole point of using a bound wq in the first place:
> > > efficient delayed batch processing of work on the local CPU.
> > 
> > Do you really want to target the same CPU ? looks like what you really
> > want to target the same cache instead
> 
> Well, yes, ideally we want to target the same cache, but we can't do
> that with workqueues.
> 
> However, the block layer already does that same-cache steering for
> it's directed completions (see __blk_mq_complete_request()), so we
> are *already running in a "hot cache" CPU context* when we queue
> work. When we queue to the same CPU, we are simply maintaining the
> "cache-hot" context that we are already running in.

__blk_mq_complete_request() doesn't always complete the request on
the submission CPU, which is only done in case of 1:1 queue mapping
and N:1 mapping when nr_hw_queues < nr_nodes. Also, the default
completion flag is SAME_GROUP, which just requires the completion
CPU to share cache with submission CPU:

#define QUEUE_FLAG_SAME_COMP    4       /* complete on same CPU-group */



Thanks, 
Ming


  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-14 11:31 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
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 [this message]
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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