linux-kernel - Re: [PATCH 4.4 50/92] mm: filemap: avoid unnecessary calls to lock_page when waiting for IO to complete during a read

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Message-ID: <CANsGZ6anzAW3dcEQqgfRGxnk+n4ih_PpKOCGdcHv_SF86x2-0g@mail.gmail.com>
Date:   Thu, 24 May 2018 10:27:59 -0700
From:   Hugh Dickins <hughd@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Jan Kara <jack@...e.cz>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 4.4 50/92] mm: filemap: avoid unnecessary calls to
 lock_page when waiting for IO to complete during a read

On Thu, May 24, 2018 at 4:28 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> On Thu, May 24, 2018 at 04:17:12AM -0700, Hugh Dickins wrote:
> >   Thu, May 24, 2018 at 4:06 AM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org>
> > wrote:
> >
> > > On Thu, May 24, 2018 at 12:50:11PM +0200, Jan Kara wrote:
> > > > On Thu 24-05-18 11:38:27, Greg Kroah-Hartman wrote:
> > > > > 4.4-stable review patch.  If anyone has any objections, please
let me
> > know.
> > > >
> > > > Just one objection: Why does stable care about this (and the
previous
> > > > patch)? I've checked the stable queue and I don't see anything that
> > would
> > > > have these patches as a prerequisite. And on their own, they are
only
> > > > cleanups without substantial gains.
> >
> > > There's a small gain here:
> >
> > > > > paralleldd
> > > > >                                     4.4.0                 4.4.0
> > > > >                                   vanilla             avoidlock
> > > > > Amean    Elapsd-1          5.28 (  0.00%)        5.15 (  2.50%)
> > > > > Amean    Elapsd-4          5.29 (  0.00%)        5.17 (  2.12%)
> > > > > Amean    Elapsd-7          5.28 (  0.00%)        5.18 (  1.78%)
> > > > > Amean    Elapsd-12         5.20 (  0.00%)        5.33 ( -2.50%)
> > > > > Amean    Elapsd-21         5.14 (  0.00%)        5.21 ( -1.41%)
> > > > > Amean    Elapsd-30         5.30 (  0.00%)        5.12 (  3.38%)
> > > > > Amean    Elapsd-48         5.78 (  0.00%)        5.42 (  6.21%)
> > > > > Amean    Elapsd-79         6.78 (  0.00%)        6.62 (  2.46%)
> > > > > Amean    Elapsd-110        9.09 (  0.00%)        8.99 (  1.15%)
> > > > > Amean    Elapsd-128       10.60 (  0.00%)       10.43 (  1.66%)
> > > > >
> > > > > The impact is small but intuitively, it makes sense to avoid
> > unnecessary
> > > > > calls to lock_page.
> >
> > > Yes, it's small, but it's marked in the SLES kernel as "needs to be
> > > merged into stable", so obviously it matters to someone :)
> >
> > Hmm. I had the same reaction to these two as Jan, but assumed that they
> > made applying later patches easier, and didn't take the trouble he did
to
> > find that's not so.
> >
> > I've no wish to be disputatious, but it does seem that the definition of
> > "stable" has changed, and not necessarily for the better, if it's now a
> > home for small gains: I thought we left those to upstream.

> This is in the SLES kernel for a reason, and again, it's in the section
> that says "this should be pushed to stable".  So if it's good enough for
> the SLES kernel, why isn't it good enough for all users of this kernel
> tree?

> If you all think it should be dropped in both places, that's fine with
> me :)

I think they are perfectly fine in SLES: folding in good work is a part of
what distros are about.

But I cannot find anything in stable-kernel-rules.rst that would admit them
- perhaps that's just out of date?

If -stable is to be a compendium of "this looks nice, you might like to
include it", so be it: but the rules should then be updated.

Hugh