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

[Greg KH]

On Thu, May 24, 2018 at 10:27:59AM -0700, Hugh Dickins wrote:
> On Thu, May 24, 2018 at 4:28 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> > > 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.

And it's also what stable is for.  We have had backports of performance
improvements in the past, along with lots of other things over the
years.  This is a performance improvement.  A tiny one, yes, but getting
rid of a lock is a good thing, and I picked it up as part of my review
of what a distro decided was worth adding for their users, as that's a
huge signal that might be of value to others.

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 - perhaps that's just out of date?

Nope, that's the list I use to say "no" to.  You can't describe
everything in that file, it's a judgement call.

> 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.

This is a "a bunch of people I trust took it in their kernel, and it's
been running on zillion of machines for a while and causes no harm and a
slight benefit, so let's add it!" type of thing.  It's not the only
patch in this series that was like that, but for some reason this one is
the one the triggered the debate, which is funny to me as this does have
numbers in it showing that it is an improvement :)

thanks,

greg k-h