Doug Anderson <dianders@xxxxxxxxxxxx> writes: > Hi, > > On Mon, Apr 17, 2023 at 8:18 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Doug Anderson <dianders@xxxxxxxxxxxx> writes: >> >> > Hi, >> > >> > On Sun, Apr 16, 2023 at 6:15 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> Doug Anderson <dianders@xxxxxxxxxxxx> writes: >> >> >> >> > Hi, >> >> > >> >> > On Thu, Apr 13, 2023 at 8:10 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> >> >> Douglas Anderson <dianders@xxxxxxxxxxxx> writes: >> >> >> >> >> >> > Currently when we try to do page migration and we're in "synchronous" >> >> >> > mode (and not doing direct compaction) then we'll wait an infinite >> >> >> > amount of time for a page lock. This does not appear to be a great >> >> >> > idea. >> >> >> > >> >> >> > One issue can be seen when I put a device under extreme memory >> >> >> > pressure. I took a sc7180-trogdor Chromebook (4GB RAM, 8GB zram >> >> >> > swap). I ran the browser along with Android (which runs from a >> >> >> > loopback mounted 128K block-size squashfs "disk"). I then manually ran >> >> >> > the mmm_donut memory pressure tool [1]. The system is completely >> >> >> > unusable both with and without this patch since there are 8 processes >> >> >> > completely thrashing memory, but it was still interesting to look at >> >> >> > how migration was behaving. I put some timing code in and I could see >> >> >> > that we sometimes waited over 25 seconds (in the context of >> >> >> > kcompactd0) for a page lock to become available. Although the 25 >> >> >> > seconds was the high mark, it was easy to see tens, hundreds, or >> >> >> > thousands of milliseconds spent waiting on the lock. >> >> >> > >> >> >> > Instead of waiting, if I bailed out right away (as this patch does), I >> >> >> > could see kcompactd0 move forward to successfully to migrate other >> >> >> > pages instead. This seems like a better use of kcompactd's time. >> >> >> > >> >> >> > Thus, even though this didn't make the system any more usable in my >> >> >> > absurd test case, it still seemed to make migration behave better and >> >> >> > that feels like a win. It also makes the code simpler since we have >> >> >> > one fewer special case. >> >> >> >> >> >> 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? > OK, so let's think about just using MIGRATE_ASYNC and either leaving > MIGRATE_SYNC_LIGHT alone or deleting it (if there are no users left). > The nice thing is that the only users of MIGRATE_SYNC_LIGHT are in > "compaction.c" and there are only 3 places where it's specified. > > 1. kcompactd_do_work() - This is what I was analyzing and where I > argued that indefinite blocking is less useful than simply trying to > compact a different page. So sure, moving this to MIGRATE_ASYNC seems > like it would be OK? > > 2. proactive_compact_node() - Just like kcompactd_do_work(), this is > called from kcompactd and thus probably should have the same mode. > > 3. compact_zone_order() - This explicitly chooses between > MIGRATE_SYNC_LIGHT and MIGRATE_ASYNC, so I guess I'd keep > MIGRATE_SYNC_LIGHT just for this use case. It looks as if > compact_zone_order() is called for direct compaction and thus making > it synchronous can make sense, especially since it seems to go to the > synchronous case after it failed with the async case. Ironically, > though, the exact lock I was proposing to not wait on _isn't_ ever > waited on in direct reclaim (see the comment in migrate_folio_unmap() > about deadlock), but the other differences between SYNC_LIGHT and > ASYNC come into play. > > If the above sounds correct then I'm OK w/ moving #1 and #2 to > MIGRATE_ASYNC and leaving #3 as the sole user or MIGRATE_SYNC_LIGHT. > >> > kcompactd can accept some blocking but we don't want long / unbounded >> > blocking. Reading the comments for MIGRATE_SYNC_LIGHT, this also seems >> > like it fits pretty well. MIGRATE_SYNC_LIGHT says that the stall time >> > of writepage() is too much. It's entirely plausible that someone else >> > holding the lock is doing something as slow as writepage() and thus >> > waiting on the lock can be just as bad for latency. >> >> IIUC, during writepage(), the page/folio will be unlocked. >> >> But, during page reading, the page/folio will be locked. I don't really >> understand why we can wait for page reading but cannot wait for page >> writeback. > > I'm not sure I totally got your point here. It sorta sounds as if > you're making the same point that I was? Yes, kind of. It is a question, not conclusion. > IIUC by waiting on the lock > we may be implicitly waiting for someone to finish reading which seems > as bad as waiting for writing. That was why I was arguing that with > MIGRATE_SYNC_LIGHT (which says that waiting for the write was too > slow) that we shouldn't wait for the lock (which may be blocking on a > read). Best Regards, Huang, Ying
