linux-kernel - Re: [RFC 1/2] mm: disable LRU pagevec during the migration temporarily

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Message-ID: <YC6UIEqNyPdRpmiq@google.com>
Date:   Thu, 18 Feb 2021 08:21:52 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, cgoldswo@...eaurora.org,
        linux-fsdevel@...r.kernel.org, david@...hat.com, vbabka@...e.cz,
        viro@...iv.linux.org.uk, joaodias@...gle.com
Subject: Re: [RFC 1/2] mm: disable LRU pagevec during the migration
 temporarily

On Thu, Feb 18, 2021 at 05:08:58PM +0100, Michal Hocko wrote:
> On Thu 18-02-21 07:52:25, Minchan Kim wrote:
> > On Thu, Feb 18, 2021 at 09:17:02AM +0100, Michal Hocko wrote:
> > > On Wed 17-02-21 13:32:05, Minchan Kim wrote:
> > > > On Wed, Feb 17, 2021 at 09:16:12PM +0000, Matthew Wilcox wrote:
> > > > > On Wed, Feb 17, 2021 at 12:46:19PM -0800, Minchan Kim wrote:
> > > > > > > I suspect you do not want to add atomic_read inside hot paths, right? Is
> > > > > > > this really something that we have to microoptimize for? atomic_read is
> > > > > > > a simple READ_ONCE on many archs.
> > > > > > 
> > > > > > It's also spin_lock_irq_save in some arch. If the new synchonization is
> > > > > > heavily compilcated, atomic would be better for simple start but I thought
> > > > > > this locking scheme is too simple so no need to add atomic operation in
> > > > > > readside.
> > > > > 
> > > > > What arch uses a spinlock for atomic_read()?  I just had a quick grep and
> > > > > didn't see any.
> > > > 
> > > > Ah, my bad. I was confused with update side.
> > > > Okay, let's use atomic op to make it simple.
> > > 
> > > Thanks. This should make the code much more simple. Before you send
> > > another version for the review I have another thing to consider. You are
> > > kind of wiring this into the migration code but control over lru pcp
> > > caches can be used in other paths as well. Memory offlining would be
> > > another user. We already disable page allocator pcp caches to prevent
> > > regular draining. We could do the same with lru pcp caches.
> > 
> > I didn't catch your point here. If memory offlining is interested on
> > disabling lru pcp, it could call migrate_prep and migrate_finish
> > like other places. Are you suggesting this one?
> 
> What I meant to say is that you can have a look at this not as an
> integral part of the migration code but rather a common functionality
> that migration and others can use. So instead of an implicit part of
> migrate_prep this would become disable_lru_cache and migrate_finish
> would become lruc_cache_enable. See my point? 
> 
> An advantage of that would be that this would match the pcp page
> allocator disabling and we could have it in place for the whole
> operation to make the page state more stable wrt. LRU state (PageLRU).

Understood. Thanks for the clarification.