Re: [RFC PATCH 4/4] dm mpath: eliminate use of spinlock in IO fast-paths

From: Hannes Reinecke <hare@suse.de>
To: Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com
Cc: jmoyer@redhat.com, linux-block@vger.kernel.org,
	linux-scsi@vger.kernel.org, j-nomura@ce.jp.nec.com,
	sagig@dev.mellanox.co.il
Subject: Re: [RFC PATCH 4/4] dm mpath: eliminate use of spinlock in IO fast-paths
Date: Thu, 7 Apr 2016 17:03:50 +0200	[thread overview]
Message-ID: <570676D6.6090401@suse.de> (raw)
In-Reply-To: <1459454666-76428-5-git-send-email-snitzer@redhat.com>

On 03/31/2016 10:04 PM, Mike Snitzer wrote:
> The primary motivation of this commit is to improve the scalability of
> DM multipath on large NUMA systems where m->lock spinlock contention has
> been proven to be a serious bottleneck on really fast storage.
> 
> The ability to atomically read a pointer, using lockless_dereference(),
> is leveraged in this commit.  But all pointer writes are still protected
> by the m->lock spinlock (which is fine since these all now occur in the
> slow-path).
> 
> The following functions no longer require the m->lock spinlock in their
> fast-path: multipath_busy(), __multipath_map(), and do_end_io()
> 
> And choose_pgpath() is modified to _not_ update m->current_pgpath unless
> it also switches the path-group.  This is done to avoid needing to take
> the m->lock everytime __multipath_map() calls choose_pgpath().
> But m->current_pgpath will be reset if it is failed via fail_path().
> 
> Suggested-by: Jeff Moyer <jmoyer@redhat.com>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
>  drivers/md/dm-mpath.c | 170 +++++++++++++++++++++++++++-----------------------
>  1 file changed, 93 insertions(+), 77 deletions(-)
> 
I really like this :-)

Reviewed-by: Hannes Reinecke <hare@suse.com>
Tested-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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