在 2022-05-27 23:49，Jens Axboe 写道：
On 5/27/22 9:28 AM, Coly Li wrote:
diff --git a/drivers/md/bcache/writeback.c
b/drivers/md/bcache/writeback.c
index d138a2d73240..c51671abe74e 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -214,6 +214,7 @@ static void update_writeback_rate(struct
work_struct *work)
struct cached_dev,
writeback_rate_update);
struct cache_set *c = dc->disk.c;
+ bool contention = false;
/*
* should check BCACHE_DEV_RATE_DW_RUNNING before calling
@@ -243,13 +244,41 @@ static void update_writeback_rate(struct
work_struct *work)
* in maximum writeback rate number(s).
*/
if (!set_at_max_writeback_rate(c, dc)) {
- down_read(&dc->writeback_lock);
- __update_writeback_rate(dc);
- update_gc_after_writeback(c);
- up_read(&dc->writeback_lock);
+ /*
+ * When contention happens on dc->writeback_lock with
+ * the writeback thread, this kwork may be blocked for
+ * very long time if there are too many dirty data to
+ * writeback, and kerne message will complain a (bogus)
+ * software lockup kernel message. To avoid potential
+ * starving, if down_read_trylock() fails, writeback
+ * rate updating will be skipped for dc->retry_max times
+ * at most while delay this worker a bit longer time.
+ * If dc->retry_max times are tried and the trylock
+ * still fails, then call down_read() to wait for
+ * dc->writeback_lock.
+ */
+ if (!down_read_trylock((&dc->writeback_lock))) {
+ contention = true;
+ dc->retry_nr++;
+ if (dc->retry_nr > dc->retry_max)
+ down_read(&dc->writeback_lock);
+ }
+
+ if (!contention || dc->retry_nr > dc->retry_max) {
+ __update_writeback_rate(dc);
+ update_gc_after_writeback(c);
+ up_read(&dc->writeback_lock);
+ dc->retry_nr = 0;
+ }
}
}
Hi Jens,
Thanks for looking into this :-)
This is really not very pretty. First of all, why bother with storing a
max retry value in there? Doesn't seem like it'd ever be different per
It is because the probability of the lock contention on
dc->writeback_lock
depends on the I/O speed backing device. From my observation during the
tests, for fast backing device with larger cache device, its writeback
thread may work harder to flush more dirty data to backing device, the
lock contention happens more and longer, so the writeback rate update
kworker has to wait longer time before acquires dc->writeback_lock. So
its dc->retry_max should be larger then slow backing device.
Therefore I'd like to have a tunable per-backing-device retry_max. And
the syses interface will be added when users/customers want it. The use
case is from SAP HANA users, I have report that they observe the soft
lockup warning for dc->writeback_lock contention and worry about whether
data is corrupted (indeed, of course not).
'dc' anyway. Secondly, something like the below would be a lot more
readable. Totally untested.
I response inline for the following suggestion.
diff --git a/drivers/md/bcache/writeback.c
b/drivers/md/bcache/writeback.c
index 9ee0005874cd..cbc01372c7a1 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -235,19 +235,27 @@ static void update_writeback_rate(struct
work_struct *work)
return;
}
- if (atomic_read(&dc->has_dirty) && dc->writeback_percent) {
+ if (atomic_read(&dc->has_dirty) && dc->writeback_percent &&
+ !set_at_max_writeback_rate(c, dc)) {
/*
* If the whole cache set is idle, set_at_max_writeback_rate()
* will set writeback rate to a max number. Then it is
* unncessary to update writeback rate for an idle cache set
* in maximum writeback rate number(s).
*/
- if (!set_at_max_writeback_rate(c, dc)) {
The reason I didn't place '!set_at_max_writeback_rate' with other items
in
previous if() was for the above code comment. If I moved it to previous
if() without other items, I was not comfortable to place the code
comments
neither before or after the if() check. So I used a separated if() check
for
'!set_at_max_writeback_rate'.
From your change, it seems placing the code comments behind is fine (or
better), can I understand in this way? I try to learn and follow your
way
to handle such code comments situation.
- down_read(&dc->writeback_lock);
+ do {
+ if (!down_read_trylock(&dc->writeback_lock)) {
+ dc->rate_update_retry++;
+ if (dc->rate_update_retry < MY_MAX)
+ break;
+ down_read(&dc->writeback_lock);
+ dc->rate_update_retry = 0;
The incremental reschedule delay was to avoid might-be-useless retry,
but
the above method works too. Just setting the default retry_max from 5 to
15, for 10 more retry with 5 seconds interval, it's fine. I can modify
the change in this way to recuse change size.
+ }
+
__update_writeback_rate(dc);
update_gc_after_writeback(c);
up_read(&dc->writeback_lock);
- }
+ } while (0);
Aha, this is cool! I never though of using do{}while(0) and break in
such a
genius way! Sure I will use this, thanks for the hint :-)
After you reply my defense of dc->retry_max, and the question of code
comments location, I will update and test the patch again, and re-sbumit
to
you.
Thanks for your constructive suggestion, especially the do{}while(0)
part!
Coly Li
