linux-ext4 - Re: [RFC] jbd2: add new "stats" proc file

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Date:   Mon, 3 Jun 2019 10:38:01 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Cc:     linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca
Subject: Re: [RFC] jbd2: add new "stats" proc file

On Mon, Jun 03, 2019 at 08:42:38PM +0800, Xiaoguang Wang wrote:
> /proc/fs/jbd2/${device}/info only shows whole average statistical
> info about jbd2's life cycle, but it can not show jbd2 info in
> specified time interval and sometimes this capability is very useful
> for trouble shooting. For example, we can not see how rs_locked and
> rs_flushing grows in specified time interval, but these two indexes
> can explain some reasons for app's behaviours.

We actually had something like this, but we removed it in commit
bf6993276f7: "jbd2: Use tracepoints for history file".  The idea was
that you can get the same information using the jbd2_run_tracepoints

# echo jbd2_run_stats > /sys/kernel/debug/tracing/set_event
# cat /sys/kernel/debug/tracing/trace_pipe 

... which will produce output like this:

      jbd2/vdg-8-293   [000] ...2   122.822487: jbd2_run_stats: dev 254,96 tid 4403 wait 0 request_delay 0 running 4 locked 0 flushing 0 logging 7 handle_count 98 blocks 3 blocks_logged 4
      jbd2/vdg-8-293   [000] ...2   122.833101: jbd2_run_stats: dev 254,96 tid 4404 wait 0 request_delay 0 running 14 locked 0 flushing 0 logging 4 handle_count 198 blocks 1 blocks_logged 2
      jbd2/vdg-8-293   [000] ...2   122.839325: jbd2_run_stats: dev 254,96 tid 4405 wait

With eBPF, we should be able to do something even more user friendly.

BTW, if you are looking to try to optimize jbd2, a good thing to do is
to take a look at jbd2_handle_stats, filtered on ones where the
interval is larger than some cut-off.  Ideally, the time between a
handle getting started and stopped should be as small as possible,
because if a transaction is trying to close, an open handle will get
in the way of that, and other CPU's will be stuck waiting for handle
to complete.  This means that pre-reading blocks before starting a
handle, etc., is a really good idea.  And monitoring jbd2_handle_stats
is a good way to find potential spots to topimize in ext4.

     	      	      		      	 	  - Ted