在 2022-05-28 08:00，Jens Axboe 写道：
On 5/27/22 11:05 AM, colyli wrote:
? 2022-05-27 23:49?Jens Axboe ???
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).
The initial patch has 5 as the default, and there are no sysfs knobs.
If
you ever need a sysfs knob, by all means make it a variable and make it
configurable too. But don't do it upfront where the '5' suitabled named
would do the job.
Copied.
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.
Just put it higher up if you want, it doesn't really matter, or leave
it
where it is.
Copied.
__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!
I would do something similar to my change and drop the 'dc' addition
for
the max retries as it, by definition, can only be one value right now.
For all I know, you'll never need to change it again, and then you're
just wasting memory and making the code harder to read by putting it in
dc instead of just having this define.
Copied. I will do the change and repost it again. Thank you for the
review and comments.
Coly Li
