Hi, On Tue, Apr 18, 2023 at 5:34 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > >> >> >> TBH, the test case is too extreme for me. > >> >> > > >> >> > That's fair. That being said, I guess the point I was trying to make > >> >> > is that waiting for this lock could take an unbounded amount of time. > >> >> > Other parts of the system sometimes hold a page lock and then do a > >> >> > blocking operation. At least in the case of kcompactd there are better > >> >> > uses of its time than waiting for any given page. > >> >> > > >> >> >> And, we have multiple "sync" mode to deal with latency requirement, for > >> >> >> example, we use MIGRATE_SYNC_LIGHT for compaction to avoid too long > >> >> >> latency. If you have latency requirement for some users, you may > >> >> >> consider to add new "sync" mode. > >> >> > > >> >> > Sure. kcompactd_do_work() is currently using MIGRATE_SYNC_LIGHT. I > >> >> > guess my first thought would be to avoid adding a new mode and make > >> >> > MIGRATE_SYNC_LIGHT not block here. Then anyone that truly needs to > >> >> > wait for all the pages to be migrated can use the heavier sync modes. > >> >> > It seems to me like the current users of MIGRATE_SYNC_LIGHT would not > >> >> > want to block for an unbounded amount of time here. What do you think? > >> >> > >> >> It appears that you can just use MIGRATE_ASYNC if you think the correct > >> >> behavior is "NOT block at all". I found that there are more > >> >> fine-grained controls on this in compaction code, please take a look at > >> >> "enum compact_priority" and its comments. > >> > > >> > Actually, the more I think about it the more I think the right answer > >> > is to keep kcompactd as using MIGRATE_SYNC_LIGHT and make > >> > MIGRATE_SYNC_LIGHT not block on the folio lock. > >> > >> Then, what is the difference between MIGRATE_SYNC_LIGHT and > >> MIGRATE_ASYNC? > > > > Aren't there still some differences even if we remove blocking this > > one lock? ...or maybe your point is that maybe the other differences > > have similar properties? > > Sorry for confusing words. Here, I asked you to list the implementation > difference between MIGRATE_ASYNC and MIGRATE_SYNC_LIGHT after your > proposed changes. Which are waited in MIGRATE_SYNC_LIGHT but not in > MIGRATE_ASYNC? Ah, got it! It's not always the easiest to follow all the code paths, but let's see what I can find. I guess to start with, though, I will assert that someone seems to have believed that there was an important difference between MIGRATE_ASYNC and MIGRATE_SYNC_LIGHT besides waiting on the lock in migrate_folio_unmapt() since (as I found in my previous digging) the "direct reclaim" path never grabs this lock but explicitly sometimes chooses MIGRATE_ASYNC some times and MIGRATE_SYNC_LIGHT other times. OK, so looking at mainline Linux and comparing differences in behavior between SYNC_LIGHT and ASYNC and thoughts about which one should be used for kcompactd. Note that I won't go _too_ deep into all the differences... -- In nfs.c: 1. We will wait for the fscache if SYNC_LIGHT but not ASYNC. No idea what would be the most ideal for calls from kcompactd. In compaction.c: 2. We will update the non-async "compact_cached_migrate_pfn" for SYNC_LIGHT but not ASYNC since we keep track of sync and async progress separately. 3. compact_lock_irqsave() note contentions for ASYNC but not SYNC_LIGHT and cause an earlier abort. Seems like kcompactd would want the SYNC_LIGHT behavior since this isn't about things indefinitely blocking. 4. isolate_migratepages_block() will bail if too_many_isolated() for ASYNC but not SYNC_LIGHT. My hunch is that kcompactd wants the SYNC_LIGHT behavior for kcompact. 5. If in direct compaction, isolate_migratepages_block() sets "skip_on_failure" for ASYNC but not SYNC_LIGHT. My hunch is that kcompactd wants the SYNC_LIGHT behavior for kcompact. 6. suitable_migration_source() considers more things suitable migration sources when SYNC_LIGHT but not (ASYNC+direct_compaction). Doesn't matter since kcompactd isn't direct compaction and non-direct compaction is the same. 7. fast_isolate_around() does less scanning when SYNC_LIGHT but not (ASYNC+direct_compaction). Again, it doesn't matter for kcompactd. 8. isolate_freepages() uses a different stride with SYNC_LIGHT vs. ASYNC. My hunch is that kcompactd wants the SYNC_LIGHT behavior for kcompact. In migrate.c: 9. buffer_migrate_lock_buffers() will block waiting to lock buffers with SYNC_LIGHT but not ASYNC. I don't know for sure, but this feels like something we _wouldn't_ want to block on in kcompactd and instead should look for easier pickings. 10. migrate_folio_unmap() has the case we've already talked about 11. migrate_folio_unmap() does batch flushes for async because it doesn't need to worry about a class of deadlock. Somewhat recent code actually ends up running this code first even for sync modes to get the batch. 12. We'll retry a few more times for SYNC_LIGHT than ASYNC. Seems like the more retries won't really hurt for kcompactd. -- So from looking at all the above, I'll say that kcompactd should stick with SYNC_LIGHT and we should fix #10. In other words, like my original patch except that we keep blocking on the lock in the full SYNC modes. It's possible that we should also change case #9 I listed above. Do you know if locking buffers is likely to block on something as slow as page reading/writing? -Doug
