Re: [PATCH v4] mm: don't be stuck to rmap lock on reclaim path

[Minchan Kim <minchan@kernel.org>]

On Wed, May 11, 2022 at 03:33:49PM -0700, Andrew Morton wrote:
> On Tue, 10 May 2022 14:54:23 -0700 Minchan Kim <minchan@kernel.org> wrote:
> 
> > The rmap locks(i_mmap_rwsem and anon_vma->root->rwsem) could be
> > contended under memory pressure if processes keep working on
> > their vmas(e.g., fork, mmap, munmap). It makes reclaim path
> > stuck. In our real workload traces, we see kswapd is waiting the
> > lock for 300ms+(worst case, a sec) and it makes other processes
> > entering direct reclaim, which were also stuck on the lock.
> > 
> > This patch makes lru aging path try_lock mode like shink_page_list
> > so the reclaim context will keep working with next lru pages
> > without being stuck. if it found the rmap lock contended, it rotates
> > the page back to head of lru in both active/inactive lrus to make
> > them consistent behavior, which is basic starting point rather than
> > adding more heristic.
> > 
> > Since this patch introduces a new "contended" field as out-param
> > along with try_lock in-param in rmap_walk_control, it's not
> > immutable any longer if the try_lock is set so remove const
> > keywords on rmap related functions. Since rmap walking is already
> > expensive operation, I doubt the const would help sizable benefit(
> > And we didn't have it until 5.17).
> > 
> > In a heavy app workload in Android, trace shows following statistics.
> > It almost removes rmap lock contention from reclaim path.
> 
> What might be the worst-case failure modes using this approach?
> 
> Could we burn much CPU time pointlessly churning though the LRU?  Could
> it mess up aging decisions enough to be performance-affecting in any
> workload?

Yes, correct. However, we are already churning LRUs by several
ways. For example, isolate and putback from LRU list for page
migration from several sources(typical example is compaction)
and trylock_page and sc->gfp_mask not allowing page to be
reclaimed in shrink_page_list.

> 
> Something else?

One thing I am worry about was the granularity of the churning.
Example above was page granuarity churning so might be execuse
but this one is address space's churning, especically for file LRU
(i_mmap_rwsem) which might cause too many rotating and live-lock
in the end(keey rotating in small LRU with heavy memory pressure).

If it could be a problem, maybe we use sc->priority to stop
the skipping on a certain level of memory pressure.

Any thought? Do we really need it?