dm cache policy mq: fix promotions to occur as expected
Micro benchmarks that repeatedly issued IO to a single block were
failing to cause a promotion from the origin device to the cache. Fix
this by not updating the stats during map() if -EWOULDBLOCK will be
returned.
The mq policy will only update stats, consider migration, etc, once per
tick period (a unit of time established between dm-cache core and the
policies).
When the IO thread calls the policy's map method, if it would like to
migrate the associated block it returns -EWOULDBLOCK, the IO then gets
handed over to a worker thread which handles the migration. The worker
thread calls map again, to check the migration is still needed (avoids a
race among other things). *BUT*, before this fix, if we were still in
the same tick period the stats were already updated by the previous map
call -- so the migration would no longer be requested.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/dm-cache-policy-mq.c b/drivers/md/dm-cache-policy-mq.c
index 416b7b7..64780ad 100644
--- a/drivers/md/dm-cache-policy-mq.c
+++ b/drivers/md/dm-cache-policy-mq.c
@@ -730,15 +730,18 @@
int r = 0;
bool updated = updated_this_tick(mq, e);
- requeue_and_update_tick(mq, e);
-
if ((!discarded_oblock && updated) ||
- !should_promote(mq, e, discarded_oblock, data_dir))
+ !should_promote(mq, e, discarded_oblock, data_dir)) {
+ requeue_and_update_tick(mq, e);
result->op = POLICY_MISS;
- else if (!can_migrate)
+
+ } else if (!can_migrate)
r = -EWOULDBLOCK;
- else
+
+ else {
+ requeue_and_update_tick(mq, e);
r = pre_cache_to_cache(mq, e, result);
+ }
return r;
}