On 4/17/23 16:28, Doug Anderson wrote: > 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. 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. +Cc Mel for potential insights. Sounds like a good compromise at first glance, but it's a tricky area. Also there are other callers of migration than compaction, and we should make sure we are not breaking them unexpectedly. > I'll try to send out a v2 with this approach today and we can see what > people think. Please Cc Mel and myself for further versions. > -Doug >
