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

[邹明哲 <mingzhe.zou@xxxxxxxxxxxx>]

Original:
From：Julian Sun <sunjunchao2870@xxxxxxxxx>
Date：2025-02-12 13:51:26(中国 (GMT+08:00))
To：linux-bcache<linux-bcache@xxxxxxxxxxxxxxx>
Cc：colyli<colyli@xxxxxxxxxx> , kent.overstreet<kent.overstreet@xxxxxxxxx> , Julian Sun <sunjunchao2870@xxxxxxxxx>
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 <sunjunchao2870@xxxxxxxxx>
---
 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.

mingzhe
 
 			bch_ratelimit_reset(&amp;dc-&gt;writeback_rate);
 		}
-- 
2.39.5



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