Re: [PATCH] cfq: priority boost on meta/prio marked IO

At Facebook, we have a number of cases where people use ionice to set a
lower priority, then end up having tasks stuck for a long time because
eg meta data updates from an idle priority tasks is blocking out higher
priority processes. It's bad enough that it will trigger the softlockup
warning.

This patch adds code to CFQ that bumps the priority class and data for
an idle task, if is doing IO marked as PRIO or META. With this, we no
longer see the softlockups.

Signed-off-by: Jens Axboe <redacted>

diff --git a/block/blk-core.c b/block/blk-core.c
index 32a283eb7274..3cfd67d006fb 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -1781,6 +1781,11 @@ get_rq:
 		rw_flags |= REQ_SYNC;
 
 	/*
+	 * Add in META/PRIO flags, if set, before we get to the IO scheduler
+	 */
+	rw_flags |= (bio->bi_rw & (REQ_META | REQ_PRIO));
+
+	/*