On 28 Apr 2023 13:54:38 -0700 Douglas Anderson <dianders@xxxxxxxxxxxx> > 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. Was kcompactd waken up for PAGE_ALLOC_COSTLY_ORDER? > > 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. In the same kcompactd code path above? > > 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. Well this is the upside after this change, but given the win, what is the lose/cost paid? For example the changes in compact fail and success [1]. [1] https://lore.kernel.org/lkml/20230418191313.268131-1-hannes@xxxxxxxxxxx/
