Messages in this thread | | | Date | Mon, 25 Sep 2023 13:47:42 -0400 | From | Sasha Levin <> | Subject | Re: [PATCH AUTOSEL 6.5 13/41] btrfs: do not block starts waiting on previous transaction commit |
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On Mon, Sep 25, 2023 at 03:01:12PM +0200, David Sterba wrote: >On Sun, Sep 24, 2023 at 09:15:01AM -0400, Sasha Levin wrote: >> From: Josef Bacik <josef@toxicpanda.com> >> >> [ Upstream commit 77d20c685b6baeb942606a93ed861c191381b73e ] >> >> Internally I got a report of very long stalls on normal operations like >> creating a new file when auto relocation was running. The reporter used >> the 'bpf offcputime' tracer to show that we would get stuck in >> start_transaction for 5 to 30 seconds, and were always being woken up by >> the transaction commit. >> >> Using my timing-everything script, which times how long a function takes >> and what percentage of that total time is taken up by its children, I >> saw several traces like this >> >> 1083 took 32812902424 ns >> 29929002926 ns 91.2110% wait_for_commit_duration >> 25568 ns 7.7920e-05% commit_fs_roots_duration >> 1007751 ns 0.00307% commit_cowonly_roots_duration >> 446855602 ns 1.36182% btrfs_run_delayed_refs_duration >> 271980 ns 0.00082% btrfs_run_delayed_items_duration >> 2008 ns 6.1195e-06% btrfs_apply_pending_changes_duration >> 9656 ns 2.9427e-05% switch_commit_roots_duration >> 1598 ns 4.8700e-06% btrfs_commit_device_sizes_duration >> 4314 ns 1.3147e-05% btrfs_free_log_root_tree_duration >> >> Here I was only tracing functions that happen where we are between >> START_COMMIT and UNBLOCKED in order to see what would be keeping us >> blocked for so long. The wait_for_commit() we do is where we wait for a >> previous transaction that hasn't completed it's commit. This can >> include all of the unpin work and other cleanups, which tends to be the >> longest part of our transaction commit. >> >> There is no reason we should be blocking new things from entering the >> transaction at this point, it just adds to random latency spikes for no >> reason. >> >> Fix this by adding a PREP stage. This allows us to properly deal with >> multiple committers coming in at the same time, we retain the behavior >> that the winner waits on the previous transaction and the losers all >> wait for this transaction commit to occur. Nothing else is blocked >> during the PREP stage, and then once the wait is complete we switch to >> COMMIT_START and all of the same behavior as before is maintained. >> >> Reviewed-by: Filipe Manana <fdmanana@suse.com> >> Signed-off-by: Josef Bacik <josef@toxicpanda.com> >> Reviewed-by: David Sterba <dsterba@suse.com> >> Signed-off-by: David Sterba <dsterba@suse.com> >> Signed-off-by: Sasha Levin <sashal@kernel.org> > >Please postpone adding this patch to stable trees until 6.6 is >released. Thanks.
Ack.
-- Thanks, Sasha
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