Date: Mon, 4 Feb 2008 11:30:01 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Nick Piggin <npiggin@suse.de>
Cc: "Siddha, Suresh B" <suresh.b.siddha@intel.com>,
       linux-kernel@vger.kernel.org, arjan@linux.intel.com, mingo@elte.hu,
       ak@suse.de, jens.axboe@oracle.com, James.Bottomley@SteelEye.com,
       andrea@suse.de, clameter@sgi.com, akpm@linux-foundation.org,
       andrew.vasquez@qlogic.com, willy@linux.intel.com,
       Zach Brown <zach.brown@oracle.com>
Subject: Re: [rfc] direct IO submission and completion scalability issues
Message-ID: <20080204103001.GA2674@basil.nowhere.org>
References: <20070728012128.GB10033@linux-os.sc.intel.com> <20080203095252.GA11043@wotan.suse.de>
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> +	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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