Re: [PATCH AUTOSEL 6.5 13/41] btrfs: do not block starts waiting on previous transaction commit

[Sasha Levin <sashal@kernel.org>]

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