linux-kernel - Re: Timing performance regression 4.15 to 4.16-rc1

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Date:   Tue, 13 Mar 2018 04:03:54 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Zdenek Kabelac <zkabelac@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Mike Snitzer <snitzer@...hat.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Alasdair G Kergon <agk@...hat.com>
Subject: Re: Timing performance regression 4.15 to 4.16-rc1

On Sun, Mar 11, 2018 at 02:02:03PM +0100, Zdenek Kabelac wrote:
> Hi
> 
> I've noticed quite some drop of performance (around 15% in some cases) where
> execution of lvm2 tests took longer time - and while tests itself should not
> really load CPU system  - the overall running time just got bigger.
> 
> Running bisect game pointed clearly to this commit:
> 
> ---
> 
> commit 44c02a2c3dc55835e9f0d8ef73966406cd805001
> Author: Al Viro <viro@...iv.linux.org.uk>
> Date:   Thu Oct 5 12:59:44 2017 -0400
> 
>     dev_ioctl(): move copyin/copyout to callers
> 
> 
> ---
> 
> clear revert of this commit on top of 3266b5bd97eaa72793df0b6e5a106c69ccc166c4
> (recent  ~4.16-rc4)  restored timing of tests back.
> 
> 
> I'm not sure why - so at this moment this is just a patch causing 15% longer
> running times on lvm2 test suite.

I find it very hard to believe, TBH.  It doesn't go anywhere near drivers/md;
could you profile the damn thing and see where does it spend that extra time?

It really doesn't touch anything on non-sockets...