linux-kernel - Re: [PATCH] Give kjournald a IOPRIO_CLASS

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Message-ID: <20081002164211.GK19428@kernel.dk>
Date:	Thu, 2 Oct 2008 18:42:11 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

On Thu, Oct 02 2008, Arjan van de Ven wrote:
> On Thu, 2 Oct 2008 15:47:37 +0200
> Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > Yes, it'll report '0' which means 'not set'. The kernel inteprets 'not
> > set' as the default values, BE/4. There's a big diffence, since '0'
> > means that we track CPU nice values where as if it returned be/4 then
> > that is a strict/fixed setting.
> 
> argh. "0" means both "not set" and "highest priority".

Arjan, please read the interface, it does not...

0 means not set, period. What matters is the class setting, if that is
non-zero then it is a valid setting. See

#define IOPRIO_PRIO_VALUE(class, data)  (((class) << IOPRIO_CLASS_SHIFT) | data)

So highest priority BE is IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 0), which
is of course valid.

-- 
Jens Axboe

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