Hi, On Sat, Apr 29, 2023 at 3:14 AM Hillf Danton <hdanton@xxxxxxxx> wrote: > > 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? I put some more traces in and reproduced it again. I saw something that looked like this: 1. balance_pgdat() called wakeup_kcompactd() with order=10 and that caused us to get all the way to the end and wakeup kcompactd (there were previous calls to wakeup_kcompactd() that returned early). 2. kcompactd started and completed kcompactd_do_work() without blocking. 3. kcompactd called proactive_compact_node() and there blocked for ~92ms in one case, ~120ms in another case, ~131ms in another case. > > 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? It was definitely in kcompactd. I can go back and trace through this too, if it's useful, but I suspect it's the same. > > 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/ That looks like an interesting series. Obviously it would need to be tested, but my hunch is that ${SUBJECT} patch would work well with that series. Specifically with Johannes's series it seems more important for the kcompactd thread to be working fruitfully. Having it blocked for a long time when there is other useful work it could be doing still seems wrong. With ${SUBJECT} patch it's not that we'll never come back and try again, but we'll just wait until a future iteration when (hopefully) the locks are easier to acquire. In the meantime, we're looking for other pages to migrate. -Doug
