linux-kernel - Re: IO queueing and complete affinity w/ threads: Some results

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Message-ID: <20080220080824.GP23197@kernel.dk>
Date:	Wed, 20 Feb 2008 09:08:25 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mike Travis <travis@....com>
Cc:	Paul Jackson <pj@....com>, Alan.Brunelle@...com,
	linux-kernel@...r.kernel.org, npiggin@...e.de, dgc@....com,
	arjan@...ux.intel.com
Subject: Re: IO queueing and complete affinity w/ threads: Some results

On Tue, Feb 19 2008, Mike Travis wrote:
> Paul Jackson wrote:
> > Jens wrote:
> >> My main worry with the current code is the ->lock in the per-cpu
> >> completion structure.
> > 
> > Drive-by-comment here:  Does the patch posted later this same day by Mike Travis:
> > 
> >   [PATCH 0/2] percpu: Optimize percpu accesses v3
> > 
> > help with this lock issue any?  (I have no real clue here -- just connecting
> > up the pretty colored dots ;).
> > 
> 
> I'm not sure of the context here but a big motivation for doing the
> zero-based per_cpu variables was to optimize access to the local
> per cpu variables to one instruction, reducing the need for locks.

I'm afraid the two things aren't related, although faster access to
per-cpu is of course a benefit for this as well. My expressed concern
was the:

        spin_lock(&bc->lock);
        was_empty = list_empty(&bc->list);
        list_add_tail(&req->donelist, &bc->list);
        spin_unlock(&bc->lock);

where 'bc' may be per-cpu data of another CPU

-- 
Jens Axboe

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