Re: [PATCH] bcache: Use the lastest writeback_delay value when writeback thread is woken up

Hi, mingzhe

Thanks for your review and comments.

On Wed, Feb 12, 2025 at 3:06 PM 邹明哲 <[email protected]> wrote:
>
> Original:
> From：Julian Sun <[email protected]>
> Date：2025-02-12 13:51:26(中国 (GMT+08:00))
> To：linux-bcache<[email protected]>
> Cc：colyli<[email protected]> , kent.overstreet<[email protected]> , Julian Sun <[email protected]>
> Subject：[PATCH] bcache: Use the lastest writeback_delay value when writeback thread is woken up
> When users reset writeback_delay value and woke up writeback
> thread via sysfs interface, expect the writeback thread
> to do actual writeback work, but in reality, the writeback
> thread probably continue to sleep.
>
> For example the following script set writeback_delay to 0 and
> wake up writeback thread, but writeback thread just continue to
> sleep:
> echo 0 &gt; /sys/block/bcache0/bcache/writeback_delay
> echo 1 &gt; /sys/block/bcache0/bcache/writeback_running
>
> Using the lastest value when writeback thread is woken up can
> urge it to do actual writeback work.
>
> Signed-off-by: Julian Sun <[email protected]>
> ---
>  drivers/md/bcache/writeback.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index c1d28e365910..0d2d06aaacfe 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -825,8 +825,10 @@ static int bch_writeback_thread(void *arg)
>
> Hi, Julian Sun:
>
> We should first understand the role of writable_delay.
>
> The writeback thread only sleep when searched_full_index is True,
> which means that there are very few dirty keys at this time, all
> dirty keys are refilled at once.
>
>                         while (delay &amp;&amp;
>                                !kthread_should_stop() &amp;&amp;
>                                !test_bit(CACHE_SET_IO_DISABLE, &amp;c-&gt;flags) &amp;&amp;
> -                              !test_bit(BCACHE_DEV_DETACHING, &amp;dc-&gt;disk.flags))
> +                              !test_bit(BCACHE_DEV_DETACHING, &amp;dc-&gt;disk.flags)) {
>                                 delay = schedule_timeout_interruptible(delay);
> +                               delay = min(delay, dc-&gt;writeback_delay * HZ);
> +                       }
>
>
> > So, I don't think it is necessary to immediately adjust the sleep time
> > unless the writeback_delay is set very large. We need to set a reasonable
> > value for writable_delay at startup, rather than adjusting it at runtime.
> >

I understand your point, but I still believe this is important.
IMO, since /sys/block/bcacheX/bcache/writeback_delay allows adjusting
the writeback_delay value at runtime,  bcache should ideally support
this functionality. Otherwise, the current behavior may be confusing
for users: "I've adjusted it, but why does it seem ineffective?" :)

> mingzhe
>
>                         bch_ratelimit_reset(&amp;dc-&gt;writeback_rate);
>                 }
> --
> 2.39.5
>
>
>
> </[email protected]></[email protected]></[email protected]></[email protected]></[email protected]></[email protected]>
>

Thanks,
-- 
Julian Sun <[email protected]>