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From: Phil Auld <pauld@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>,
	Ming Lei <ming.lei@redhat.com>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-xfs@vger.kernel.org, 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>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: single aio thread is migrated crazily by scheduler
Date: Wed, 20 Nov 2019 17:03:13 -0500
Message-ID: <20191120220313.GC18056@pauld.bos.csb> (raw)
In-Reply-To: <20191120191636.GI4097@hirez.programming.kicks-ass.net>

Hi Peter,

On Wed, Nov 20, 2019 at 08:16:36PM +0100 Peter Zijlstra wrote:
> On Tue, Nov 19, 2019 at 07:40:54AM +1100, Dave Chinner wrote:
> > On Mon, Nov 18, 2019 at 10:21:21AM +0100, Peter Zijlstra wrote:
> 
> > > We typically only fall back to the active balancer when there is
> > > (persistent) imbalance and we fail to migrate anything else (of
> > > substance).
> > > 
> > > The tuning mentioned has the effect of less frequent scheduling, IOW,
> > > leaving (short) tasks on the runqueue longer. This obviously means the
> > > load-balancer will have a bigger chance of seeing them.
> > > 
> > > Now; it's been a while since I looked at the workqueue code but one
> > > possible explanation would be if the kworker that picks up the work item
> > > is pinned. That would make it runnable but not migratable, the exact
> > > situation in which we'll end up shooting the current task with active
> > > balance.
> > 
> > Yes, that's precisely the problem - work is queued, by default, on a
> > specific CPU and it will wait for a kworker that is pinned to that
> 
> I'm thinking the problem is that it doesn't wait. If it went and waited
> for it, active balance wouldn't be needed, that only works on active
> tasks.

Since this is AIO I wonder if it should queue_work on a nearby cpu by 
default instead of unbound.  

> 
> > specific CPU to dispatch it. We've already tested that queuing on a
> > different CPU (via queue_work_on()) makes the problem largely go
> > away as the work is not longer queued behind the long running fio
> > task.
> > 
> > This, however, is not at viable solution to the problem. The pattern
> > of a long running process queuing small pieces of individual work
> > for processing in a separate context is pretty common...
> 
> Right, but you're putting the scheduler in a bind. By overloading the
> CPU and only allowing the one task to migrate, it pretty much has no
> choice left.
> 
> Anyway, I'm still going to have try and reproduce -- I got side-tracked
> into a crashing bug, I'll hopefully get back to this tomorrow. Lastly,
> one other thing to try is -next. Vincent reworked the load-balancer
> quite a bit.
> 

I've tried it with the lb patch series. I get basically the same results.
With the high granularity settings I get 3700 migrations for the 30 
second run at 4k. Of those about 3200 are active balance on stock 5.4-rc7.
With the lb patches it's 3500 and 3000, a slight drop. 

Using the default granularity settings 50 and 22 for stock and 250 and 25.
So a few more total migrations with the lb patches but about the same active.


On this system I'm getting 100k migrations using 512 byte blocksize. Almost
all not active. I haven't looked into that closely yet but it's like 3000
per second looking like this:

...
64.19641 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19694 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19746 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19665 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.19718 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.19772 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.19800 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19828 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.19856 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19882 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.19909 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19937 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.19967 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.19995 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.20023 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.20053 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.20079 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.20107 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.20135 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.20163 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
64.20192 386     386     kworker/15:1    sched_migrate_task fio/2784 cpu 15->19 
64.20221 389     389     kworker/19:1    sched_migrate_task fio/2784 cpu 19->15 
...

Which is roughly equal to the number if iops it's doing. 

Cheers,
Phil

-- 


  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 [this message]
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
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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