blk-throttle: set default latency baseline for harddisk

hard disk IO latency varies a lot depending on spindle move. The latency
range could be from several microseconds to several milliseconds. It's
pretty hard to get the baseline latency used by io.low.

We will use a different stragety here. The idea is only using IO with
spindle move to determine if cgroup IO is in good state. For HD, if io
latency is small (< 1ms), we ignore the IO. Such IO is likely from
sequential IO, and is helpless to help determine if a cgroup's IO is
impacted by other cgroups. With this, we only account IO with big
latency. Then we can choose a hardcoded baseline latency for HD (4ms,
which is typical IO latency with seek).  With all these settings, the
io.low latency works for both HD and SSD.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 17 insertions, 3 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 3b751f706c61..a7285bf2831c 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -27,6 +27,13 @@ static int throtl_quantum = 32;
 #define MIN_THROTL_IOPS (10)
 #define DFL_LATENCY_TARGET (-1L)
 #define DFL_IDLE_THRESHOLD (0)
+#define DFL_HD_BASELINE_LATENCY (4000L) /* 4ms */
+#define LATENCY_FILTERED_SSD (0)
+/*
+ * For HD, very small latency comes from sequential IO. Such IO is helpless to
+ * help determine if its IO is impacted by others, hence we ignore the IO
+ */
+#define LATENCY_FILTERED_HD (1000L) /* 1ms */
 
 #define SKIP_LATENCY (((u64)1) << BLK_STAT_RES_SHIFT)
 
@@ -212,6 +219,7 @@ struct throtl_data
         struct avg_latency_bucket avg_buckets[LATENCY_BUCKET_SIZE];
         struct latency_bucket __percpu *latency_buckets;
         unsigned long last_calculate_time;
+        unsigned long filtered_latency;
 
         bool track_bio_latency;
 };
@@ -2281,7 +2289,7 @@ void blk_throtl_bio_endio(struct bio *bio)
                 throtl_track_latency(tg->td, blk_stat_size(&bio->bi_issue_stat),
                         bio_op(bio), lat);
 
-        if (tg->latency_target) {
+        if (tg->latency_target && lat >= tg->td->filtered_latency) {
                 int bucket;
                 unsigned int threshold;
 
@@ -2417,14 +2425,20 @@ void blk_throtl_exit(struct request_queue *q)
 void blk_throtl_register_queue(struct request_queue *q)
 {
         struct throtl_data *td;
+        int i;
 
         td = q->td;
         BUG_ON(!td);
 
-        if (blk_queue_nonrot(q))
+        if (blk_queue_nonrot(q)) {
                 td->throtl_slice = DFL_THROTL_SLICE_SSD;
-        else
+                td->filtered_latency = LATENCY_FILTERED_SSD;
+        } else {
                 td->throtl_slice = DFL_THROTL_SLICE_HD;
+                td->filtered_latency = LATENCY_FILTERED_HD;
+                for (i = 0; i < LATENCY_BUCKET_SIZE; i++)
+                        td->avg_buckets[i].latency = DFL_HD_BASELINE_LATENCY;
+        }
 #ifndef CONFIG_BLK_DEV_THROTTLING_LOW
         /* if no low limit, use previous default */
         td->throtl_slice = DFL_THROTL_SLICE_HD;