On Fri, Apr 28, 2023 at 01:54:38PM -0700, Douglas Anderson wrote: > The MIGRATE_SYNC_LIGHT mode is intended to block for things that will > finish quickly but not for things that will take a long time. Exactly > how long is too long is not well defined, but waits of tens of > milliseconds is likely non-ideal. > > When putting a Chromebook under memory pressure (opening over 90 tabs > on a 4GB machine) it was fairly easy to see delays waiting for some > locks in the kcompactd code path of > 100 ms. While the laptop wasn't > amazingly usable in this state, it was still limping along and this > state isn't something artificial. Sometimes we simply end up with a > lot of memory pressure. > > Putting the same Chromebook under memory pressure while it was running > Android apps (though not stressing them) showed a much worse result > (NOTE: this was on a older kernel but the codepaths here are similar). > Android apps on ChromeOS currently run from a 128K-block, > zlib-compressed, loopback-mounted squashfs disk. If we get a page > fault from something backed by the squashfs filesystem we could end up > holding a folio lock while reading enough from disk to decompress 128K > (and then decompressing it using the somewhat slow zlib algorithms). > That reading goes through the ext4 subsystem (because it's a loopback > mount) before eventually ending up in the block subsystem. This extra > jaunt adds extra overhead. Without much work I could see cases where > we ended up blocked on a folio lock for over a second. With more > extreme memory pressure I could see up to 25 seconds. > > We considered adding a timeout in the case of MIGRATE_SYNC_LIGHT for > the two locks that were seen to be slow [1] and that generated much > discussion. After discussion, it was decided that we should avoid > waiting for the two locks during MIGRATE_SYNC_LIGHT if they were being > held for IO. We'll continue with the unbounded wait for the more full > SYNC modes. > > With this change, I couldn't see any slow waits on these locks with my > previous testcases. > > NOTE: The reason I stated digging into this originally isn't because > some benchmark had gone awry, but because we've received in-the-field > crash reports where we have a hung task waiting on the page lock > (which is the equivalent code path on old kernels). While the root > cause of those crashes is likely unrelated and won't be fixed by this > patch, analyzing those crash reports did point out these very long > waits seemed like something good to fix. With this patch we should no > longer hang waiting on these locks, but presumably the system will > still be in a bad shape and hang somewhere else. > > [1] https://lore.kernel.org/r/20230421151135.v2.1.I2b71e11264c5c214bc59744b9e13e4c353bc5714@changeid > > Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Cc: Hillf Danton <hdanton@xxxxxxxx> > Cc: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> -- Mel Gorman SUSE Labs
