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

[Johannes Thumshirn]

On 2016-03-31 22:04, 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 at redhat.com>
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>

Looks good,
Reviewed-by: Johannes Thumshirn <jthumshirn at suse.de>