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From: Michal Hocko <mhocko@suse.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Andi Kleen <ak@linux.intel.com>,
	Linux Memory Management List <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race
Date: Wed, 13 Jan 2021 12:22:01 +0100	[thread overview]
Message-ID: <20210113112201.GX22493@dhcp22.suse.cz> (raw)
In-Reply-To: <CAMZfGtUTZZyL6Pdop-SHt2vs2hLuYfB9dumhRHBm7QLzyRNzZA@mail.gmail.com>

On Wed 13-01-21 19:11:06, Muchun Song wrote:
> On Wed, Jan 13, 2021 at 6:38 PM Michal Hocko <mhocko@suse.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.
-- 
Michal Hocko
SUSE Labs

  parent reply	other threads:[~2021-01-13 11:23 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-13  5:22 [PATCH v4 0/6] Fix some bugs about HugeTLB code Muchun Song
2021-01-13  5:22 ` [PATCH v4 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one Muchun Song
2021-01-13 10:30   ` David Hildenbrand
2021-01-13 10:57   ` Oscar Salvador
2021-01-13 11:03     ` [External] " Muchun Song
2021-01-13  5:22 ` [PATCH v4 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page Muchun Song
2021-01-13 11:00   ` Oscar Salvador
2021-01-13  5:22 ` [PATCH v4 3/6] mm: hugetlb: fix a race between freeing and dissolving the page Muchun Song
2021-01-13  9:31   ` Michal Hocko
2021-01-13  5:22 ` [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race Muchun Song
2021-01-13  9:33   ` Michal Hocko
2021-01-13 10:14     ` [External] " Muchun Song
2021-01-13 10:38       ` Michal Hocko
2021-01-13 11:11         ` Muchun Song
2021-01-13 11:14           ` Oscar Salvador
2021-01-13 11:20             ` Muchun Song
2021-01-13 12:03               ` Michal Hocko
2021-01-13 11:22           ` Michal Hocko [this message]
2021-01-13 12:15             ` Muchun Song
2021-01-13  5:22 ` [PATCH v4 5/6] mm: hugetlb: fix a race between isolating and freeing page Muchun Song
2021-01-13  5:22 ` [PATCH v4 6/6] mm: hugetlb: remove VM_BUG_ON_PAGE from page_huge_active Muchun Song

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