Re: [PATCH v3] migrate_pages: Avoid blocking for IO in MIGRATE_SYNC_LIGHT

[Mel Gorman <mgorman@techsingularity.net>]

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@infradead.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Hillf Danton <hdanton@sina.com>
> Cc: Gao Xiang <hsiangkao@linux.alibaba.com>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs