linux-kernel - Re: [PATCH 2/9] mm: workingset: tell cache transitions from workingset thrashing

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Date:   Thu, 2 Aug 2018 08:38:13 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     peter enderborg <peter.enderborg@...y.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Drake <drake@...lessm.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 2/9] mm: workingset: tell cache transitions from
 workingset thrashing

On Thu, Aug 02, 2018 at 08:57:31AM +0200, peter enderborg wrote:
> On 08/01/2018 05:13 PM, Johannes Weiner wrote:
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index e34a27727b9a..7af1c3c15d8e 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -69,13 +69,14 @@
> >   */
> >  enum pageflags {
> >  	PG_locked,		/* Page is locked. Don't touch. */
> > -	PG_error,
> >  	PG_referenced,
> >  	PG_uptodate,
> >  	PG_dirty,
> >  	PG_lru,
> >  	PG_active,
> > +	PG_workingset,
> >  	PG_waiters,		/* Page has waiters, check its waitqueue. Must be bit #7 and in the same byte as "PG_locked" */
> > +	PG_error,
> >  	PG_slab,
> >  	PG_owner_priv_1,	/* Owner use. If pagecache, fs may use*/
> >  	PG_arch_1,
> > @@ -280,6 +281,8 @@ PAGEFLAG(Dirty, dirty, PF_HEAD) TESTSCFLAG(Dirty, dirty, PF_HEAD)
> Any reason why the PG_error was moved? And dont you need to do some handling of this flag in proc/fs/page.c ?
> Some KFP_WORKINGSET ?

I wanted PG_workingset next to PG_active as they both describe how hot
the page is, but PG_waiters needs to remain with the same bit number.

As far as fs/proc/page.c and include/uapi/linux/kernel-page-flags.h
go, that's a good point and we'll probably want to make that available
to userspace eventually. But I'm not super eager to make a brandnew
page flag user ABI right away. Let's give the code that uses it some
wider exposure first and maybe publish it a few release cycles later.

Thanks