linux-kernel - Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize

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Message-ID: <20120203002902.GQ2518@linux.vnet.ibm.com>
Date:	Thu, 2 Feb 2012 16:29:02 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mikulas Patocka <mpatocka@...hat.com>, axboe@...nel.dk,
	"Alasdair G. Kergon" <agk@...hat.com>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of
 synchronize_rcu

On Thu, Feb 02, 2012 at 10:59:04PM +0100, Peter Zijlstra wrote:
> On Thu, 2012-02-02 at 15:43 -0500, Mikulas Patocka wrote:
> > Do you have some measurable use case where the user is removing block 
> > devices so heavily that this causes a problem? 
> 
> Even one can be a problem, we're having people spend lots of time and
> effort to reduce machine wide jitter and interference. Adding it with
> such disregard isn't cool.
> 
> There's no reason a management cpu adding or removing block devices
> should perturb the high-freq trading or industrial laser control running
> on the other side of the machine.

Very true for real-time applications!

For the heavy trading apps, given Frederic's upcoming user-mode-idle work,
I can keep this stuff from perturbing the apps.  Still, batching would
be preferable.

							Thanx, Paul

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