linux-kernel - Re: [PATCH 08/17] blkcg: shoot down blkio

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Message-ID: <20120123155745.GD12652@google.com>
Date:	Mon, 23 Jan 2012 07:57:45 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/17] blkcg: shoot down blkio_groups on elevator switch

On Mon, Jan 23, 2012 at 10:52:16AM -0500, Vivek Goyal wrote:
> Atleast throttling rules should not disappear over elevator switch. They
> are per device and not per IO scheduler. Think of losing nr_requests
> settings just because elevator switch happened.
> 
> Elevator switch can be low frequency but how would a user space know
> that elevator switch failed that's why we lost our rules and now lets
> put the rules back.

It's simple - store all the policy rules before switching elevators
and restore them afterwards regardless of success / failure.

> I am not sure how would we justify this that because of ease of programming
> in kernel, now user space is supposed to make sure that any programmed
> rules are still there and reprogram the cgroup if rules are gone for
> some reason.

Sanity in trade off?

-- 
tejun
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