On Tue, 10 May 2022 14:54:23 -0700 Minchan Kim <minchan@xxxxxxxxxx> 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? Something else?
