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

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Message-ID: <20210113120320.GY22493@dhcp22.suse.cz>
Date:   Wed, 13 Jan 2021 13:03:20 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        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 13-01-21 19:20:17, Muchun Song wrote:
> On Wed, Jan 13, 2021 at 7:15 PM Oscar Salvador <osalvador@...e.de> wrote:
> >
> > On Wed, Jan 13, 2021 at 07:11:06PM +0800, Muchun Song wrote:
> > > 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?
> >
> > But if the race window is that small, does it make sense?
> 
> Actually, there is one exception. The race window could
> become larger. If the page is freed via a workqueue (see
> free_huge_page()). In this case, the cpu_relax() can
> make sense. Right?

The system would have to be under serious stress for WQ to clog. I do
not expect there would be nothing runable at that stage. Possible?
Maybe but if that matters than a short sleep would be more preferable
than cpu_relax.
-- 
Michal Hocko
SUSE Labs