Re: [External] Re: [PATCH v2 2/2] writeback: Add logging for slow writeback (exceeds sysctl_hung_task_timeout_secs)

[Julian Sun <sunjunchao@bytedance.com>]

On Mon, Sep 29, 2025 at 11:51 PM Jan Kara <jack@suse.cz> wrote:
>
> On Mon 29-09-25 20:28:50, Julian Sun wrote:
> > When a writeback work lasts for sysctl_hung_task_timeout_secs, we want
> > to identify that it's slow-this helps us pinpoint potential issues.
> >
> > Additionally, recording the starting jiffies is useful when debugging a
> > crashed vmcore.
> >
> > Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
>
> So this works but I'd rather do:
>
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 61785a9d6669..131d0d11672b 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -213,6 +213,7 @@ static void wb_queue_work(struct bdi_writeback *wb,
> >   */
> >  void wb_wait_for_completion(struct wb_completion *done)
> >  {
> > +     done->wait_stamp = jiffies;
> >       atomic_dec(&done->cnt);         /* put down the initial count */
> >       wait_event(*done->waitq,
> >                  ({ done->progress_stamp = jiffies; !atomic_read(&done->cnt); }));
>
> static bool wb_wait_for_completion_cb(struct wb_completion *done,
>                                       unsigned long wait_start)
> {
>         done->progress_stamp = jiffies;
>         if ((jiffies - wait_start) / HZ > sysctl_hung_task_timeout_secs)
>                 pr_info(dump here kind of hang check warning);
>
>         return !atomic_read(&done->cnt);
> }
>
> void wb_wait_for_completion(struct wb_completion *done)
> {
>         unsigned long wait_start = jiffies;
>
>         atomic_dec(&done->cnt);         /* put down the initial count */
>         wait_event(*done->waitq, wb_wait_for_completion_cb(done, wait_start));
> }
>
> This way we can properly dump info about blocked task which is IMO more
> interesting than info about BDI.

Indeed, this makes more sense. Thank you for your suggestion.
>
>                                                                 Honza
>
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR



-- 
Julian Sun <sunjunchao@bytedance.com>