linux-kernel - Re: [External] Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race

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Date:   Wed, 13 Jan 2021 20:15:47 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page
 when hitting race

On Wed, Jan 13, 2021 at 7:22 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Wed 13-01-21 19:11:06, Muchun Song wrote:
> > On Wed, Jan 13, 2021 at 6:38 PM Michal Hocko <mhocko@...e.com> wrote:
> [...]
> > > > I just want the fix patch to be small enough.
> > > > So I do the retry in this patch. If you do not agree with this. I
> > > > will fold this into the previous patch.
> > > >
> > > > Do you mean this?
> > > >
> > > > cpu_relax();
> > > > cond_resched();
> > > > cpu_relax();
> > >
> > > No, I am questiong the use of cpu_relax. What is the point?
> >
> > If there is no task to be scheduled. Here is just a while loop.
> > The cpu_relax is a good thing to insert into busy-wait loops,
> > right?
>
> Well in an ideal world we would simply have a way to block and wait for
> the particular page. This is probably an overkill for a rare event like
> this. And while you are right that theoretically there might be nobody
> else to run but I find it rather unlikely considering that this path is
> racing with somebody. Sure there is even less likely possibility that
> the race is actually waiting for worker context but really I would just
> make it simple retry loop. If we ever hit a real busy loop then this
> would be pretty straightforward to spot and fix up.
>
> It's not like I am against the patch with cpu_relax but I find it
> excessive for this purpose. If you feel strongly then just keep it.
>
> Once the two patches are squashed I will ack it.

OK. I will do this. Thanks.

> --
> Michal Hocko
> SUSE Labs