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.
