linux-ext4 - Re: [PATCH 1/2] jbd2: add new tracepoint jbd2_sleep_on

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Date:   Sat, 7 Sep 2019 12:21:45 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2] jbd2: add new tracepoint jbd2_sleep_on_shadow

On Mon, Sep 02, 2019 at 10:54:41PM +0800, Xiaoguang Wang wrote:
> Sometimes process will be stalled in "wait_on_bit_io(&bh->b_state,
> BH_Shadow, TASK_UNINTERRUPTIBLE)" for a while, and in order to analyse
> app's latency thoroughly, add a new tracepoint to track this delay.
> 
> Trace info likes below:
> fsstress-5068  [008] .... 11007.757543: jbd2_sleep_on_shadow: dev 254,17 sleep 1
> fsstress-5070  [007] .... 11007.757544: jbd2_sleep_on_shadow: dev 254,17 sleep 2
> fsstress-5069  [009] .... 11007.757548: jbd2_sleep_on_shadow: dev 254,17 sleep 2
> fsstress-5067  [011] .... 11007.757569: jbd2_sleep_on_shadow: dev 254,17 sleep 1
> fsstress-5063  [007] .... 11007.757651: jbd2_sleep_on_shadow: dev 254,17 sleep 2
> fsstress-5070  [007] .... 11007.757792: jbd2_sleep_on_shadow: dev 254,17 sleep 0
> fsstress-5071  [011] .... 11007.763493: jbd2_sleep_on_shadow: dev 254,17 sleep 1
> 
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>

I think maybe it might be better to use units of microseconds and then
change sleep to usleep so the units are clear?  This is a spinlock, so
it should be quick.

For the other patch in this series, milliseconds seems fine, but if we
change the trace info to use "msleep" instead that would be clearer
--- or you could change it to use microseconds as well just for
consistency; I think either would be fine.

What do you think?

Cheers,

						- Ted