linux-kernel - Re: [PATCH RFC] nohz,blk-mq: do not create blk-mq workqueues on nohz dedicated CPUs

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Date:	Sun, 05 Apr 2015 07:31:59 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	axboe@...nel.org, fweisbec@...hat.com, mingo@...nel.org,
	linux-kernel@...r.kernel.org, lcapitulino@...hat.com,
	mtosatti@...hat.com
Subject: Re: [PATCH RFC] nohz,blk-mq: do not create blk-mq workqueues on
 nohz dedicated CPUs

On Tue, 2015-03-31 at 10:27 -0400, Rik van Riel wrote:
> CPUs with nohz_full do not want disruption from timer interrupts,
> or other random system things.  This includes block mq work.
> 
> There is another issue with block mq vs. realtime tasks that run
> 100% of the time, which is not uncommon on systems that have CPUs
> dedicated to real time use with isolcpus= and nohz_full=
> 
> Specifically, on systems like that, a block work item may never
> get to run, which could lead to filesystems getting stuck forever.

What reason is there to run a compute hog as RT if it's the only task 
on an isolated core?  In an RT kernel, PI may be a reason, but if a 
task is so critical that it needs bare metal, it had better not be.

        -Mike

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