linux-kernel - Re: [rfc] direct IO submission and completion scalability issues

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Date:	Mon, 4 Feb 2008 11:30:01 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	linux-kernel@...r.kernel.org, arjan@...ux.intel.com, mingo@...e.hu,
	ak@...e.de, jens.axboe@...cle.com, James.Bottomley@...elEye.com,
	andrea@...e.de, clameter@....com, akpm@...ux-foundation.org,
	andrew.vasquez@...gic.com, willy@...ux.intel.com,
	Zach Brown <zach.brown@...cle.com>
Subject: Re: [rfc] direct IO submission and completion scalability issues

> +	q = &__get_cpu_var(call_single_queue);
> +	spin_lock_irqsave(&q->lock, flags);
> +	list_replace_init(&q->list, &list);
> +	spin_unlock_irqrestore(&q->lock, flags);

I think you could do that lockless if you use a similar data structure
as netchannels (essentially a fixed size single buffer queue with atomic 
exchange of the first/last pointers) and not using a list. That would avoid 
at least one bounce for the lock and likely another one for the  list
manipulation.

Also the right way would be to not add a second mechanism for this,
but fix the standard smp_call_function_single() to support it.

-Andi
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