wbt: fix incorrect throttling due to flush latency

Commit Message

Jens Axboe Feb. 5, 2018, 7:22 p.m. UTC

On 2/5/18 12:11 PM, Mikulas Patocka wrote:
> I have a workload where one process sends many asynchronous write bios
> (without waiting for them) and another process sends synchronous flush
> bios. During this workload, writeback throttling throttles down to one
> outstanding bio, and this incorrect throttling causes performance
> degradation (all write bios sleep in __wbt_wait and they couldn't be sent
> in parallel).
> 
> The reason for this throttling is that wbt_data_dir counts flush requests
> in the read bucket. The flush requests (that take quite a long time)
> trigger this condition repeatedly:
> 	if (stat[READ].min > rwb->min_lat_nsec)
> and that condition causes scale down to one outstanding request, despite
> the fact that there are no read bios at all.
> 
> A similar problem could also show up with REQ_OP_ZONE_REPORT and
> REQ_OP_ZONE_RESET - they are also counted in the read bucket.
> 
> This patch fixes the function wbt_data_dir, so that only REQ_OP_READ 
> requests are counted in the read bucket. The patch improves SATA 
> write+flush throughput from 130MB/s to 350MB/s.

Good catch. But I do wonder if we should account them at all. It
might make sense to account flushes as writes, but we really
should not account anything that isn't regular read/write IO 
related.

Something like the below? Potentially including REQ_OP_FLUSH in the
write latencies as well.

Patch

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index ae8de9780085..55671e5f3b6e 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -697,7 +697,13 @@  u64 wbt_default_latency_nsec(struct request_queue *q)
 
 static int wbt_data_dir(const struct request *rq)
 {
-	return rq_data_dir(rq);
+	if (req_op(rq) == REQ_OP_READ)
+		return READ;
+	else if (req_op(rq) == REQ_OP_WRITE)
+		return WRITE;
+
+	/* don't account */
+	return -1;
 }
 
 int wbt_init(struct request_queue *q)