linux-kernel - Re: [patch 1/2]percpu

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Message-ID: <20140210122942.GB320@dhcp-26-207.brq.redhat.com>
Date:	Mon, 10 Feb 2014 13:29:43 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Kent Overstreet <kmo@...erainc.com>, Jens Axboe <axboe@...nel.dk>,
	Shaohua Li <shli@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2]percpu_ida: fix a live lock

On Mon, Feb 10, 2014 at 02:32:11AM -0800, Christoph Hellwig wrote:
> > May be we can walk off with a per-cpu timeout that flushes batch nr of tags
> > from local caches to the pool? Each local allocation would restart the timer,
> > but once allocation requests stopped coming on a CPU the tags would not gather
> > dust in local caches.
> 
> We'll defintively need a fix to be able to allow the whole tag space.
> For large numbers of tags per device the flush might work, but for
> devices with low number of tags we need something more efficient.  The
> case of less tags than CPUs isn't that unusual either and we probably
> want to switch to an allocator without per cpu allocations for them to
> avoid all this.  E.g. for many ATA devices we just have a single tag,
> and many scsi drivers also only want single digit outstanding commands
> per LUN.

Do we really always need the pool for these classes of devices?

Pulling tags from local caches to the pool just to (near to) dry it at
the very next iteration does not seem beneficial. Not to mention caches
vs pool locking complexities.

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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