linux-kernel - Re: [PATCH 0/6] autofs4: support RCU-walk

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Date:	Thu, 10 Jul 2014 15:43:40 +0800
From:	Ian Kent <raven@...maw.net>
To:	NeilBrown <neilb@...e.de>
Cc:	autofs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] autofs4: support RCU-walk

On Thu, 2014-07-10 at 09:41 +1000, NeilBrown wrote:
> autofs4 currently doesn't support RCU-walk - it immediately
> aborts any attempt at RCU-walk to force REF-walk for path name
> lookup.
> 
> This can cause a significant performance impact on multi-core
> systems.
> I have a client with a test case which spends >80% of its time
> waiting for spinlocks with a "make -j 40" on a 40 core system.

Right, sounds worth the effort.

> 
> This patchset aims to remove most of these spinlocks.  To be fully
> effective in the particular case it needs a second patch set which
> makes NFS RCU-walk friendly, but one thing at a time.
> 
> This has only been lightly tested so far so I'm really after feed-back
> rather than to have the patch set accepted, though the first two
> patches are trivial and could be taken immediately.

I've only scanned the patches so far, I'll need to spend a bit more time
on them before I can comment.

I'm going to be pressed for time for at least several days so I won't be
able to get to this right away.

I expect the submount_test I use to stress path walking and expire to
mount transitions will likely be a good test to use. I haven't used it
in my personal environment for quite a while now so I'll need to have a
look around and see if I can still find a suitable set of scripts.
Otherwise I'll need to decouple it from the RedHat automated test
environment.

> 
> The last two patches are the most interesting so review comments on
> those are particularly welcome.

Again I haven't looked closely at these but don't you mean the last
three patches or am I just fussing over an obviously straight forward
patch 3?

Thanks for your effort Bruce,
Ian

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