linux-kernel - Re: serial percpu_ref draining in exit

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Date:	Thu, 19 Mar 2015 16:58:33 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Tejun Heo <tj@...nel.org>, kent.overstreet@...il.com,
	Benjamin LaHaise <bcrl@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: serial percpu_ref draining in exit_aio()

Jens Axboe <axboe@...nel.dk> writes:

> On 03/19/2015 11:34 AM, Tejun Heo wrote:
>> Hello,
>>
>> So, Jens noticed that fio process exiting takes seconds when there are
>> multiple aio contexts and the culprit seems to be the serial
>> percpu_ref draining in exit_aio().  It's generally a bad idea to
>> expose RCU latencies to userland because they add up really quickly
>> and are unrelated to other performance parameters.  Can you guys
>> please at least update the code so that it waits for all percpu_refs
>> to drain at the same time rather than one after another?  That should
>> resolve the worst part of the problem.
>
> This works for me. Before:
>
> real	0m5.872s
> user	0m0.020s
> sys	0m0.040s
>
> after
>
> real	0m0.246s
> user	0m0.020s
> sys	0m0.040s
>
> It solves the exit_aio() issue, but if the app calls io_destroy(),
> then we are back to square one...

Do you really want to do memory allocation in the exit path?  That
sounds like a bad idea to me.  (Of course, now you're going to point out
all the places that currently happens, right?  ;-)

-Jeff
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