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

[David Hildenbrand]

On 18.02.21 09:17, 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.

Agreed. And dealing with PCP more reliably might also be of interest in 
context of more reliable alloc_contig_range().

--
Thanks,

David / dhildenb