blk-iolatency: Make initialization lazy

Other rq_qos policies such as wbt and iocost are lazy-initialized when they
are configured for the first time for the device but iolatency is
initialized unconditionally from blkcg_init_disk() during gendisk init. Lazy
init is beneficial because rq_qos policies add runtime overhead when
initialized as every IO has to walk all registered rq_qos callbacks.

This patch switches iolatency to lazy initialization too so that it only
registered its rq_qos policy when it is first configured.

Note that there is a known race condition between blkcg config file writes
and del_gendisk() and this patch makes iolatency susceptible to it by
exposing the init path to race against the deletion path. However, that
problem already exists in iocost and is being worked on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20230413000649.115785-5-tj@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index c154b08a7e9203126fd4f162942b2b02b00af91a..1c1ebeb51003025abc8ebbd02e2a23b4930265c7 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -33,7 +33,6 @@
 #include "blk-cgroup.h"
 #include "blk-ioprio.h"
 #include "blk-throttle.h"
-#include "blk-rq-qos.h"
 
 /*
  * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
@@ -1350,14 +1349,8 @@ int blkcg_init_disk(struct gendisk *disk)
        if (ret)
                goto err_ioprio_exit;
 
-       ret = blk_iolatency_init(disk);
-       if (ret)
-               goto err_throtl_exit;
-
        return 0;
 
-err_throtl_exit:
-       blk_throtl_exit(disk);
 err_ioprio_exit:
        blk_ioprio_exit(disk);
 err_destroy_all:
@@ -1373,7 +1366,6 @@ err_unlock:
 void blkcg_exit_disk(struct gendisk *disk)
 {
        blkg_destroy_all(disk);
-       rq_qos_exit(disk->queue);
        blk_throtl_exit(disk);
 }
 

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 2560708b91093bb23432c5048820fe4c763c3269..fd5fec989e39069b2ed5f436e21284409bb5c6c6 100644 (file)
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -755,7 +755,7 @@ static void blkiolatency_enable_work_fn(struct work_struct *work)
        }
 }
 
-int blk_iolatency_init(struct gendisk *disk)
+static int blk_iolatency_init(struct gendisk *disk)
 {
        struct blk_iolatency *blkiolat;
        int ret;
@@ -824,6 +824,29 @@ static void iolatency_clear_scaling(struct blkcg_gq *blkg)
        }
 }
 
+static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
+{
+       static DEFINE_MUTEX(init_mutex);
+       int ret;
+
+       ret = blkg_conf_open_bdev(ctx);
+       if (ret)
+               return ret;
+
+       /*
+        * blk_iolatency_init() may fail after rq_qos_add() succeeds which can
+        * confuse iolat_rq_qos() test. Make the test and init atomic.
+        */
+       mutex_lock(&init_mutex);
+
+       if (!iolat_rq_qos(ctx->bdev->bd_queue))
+               ret = blk_iolatency_init(ctx->bdev->bd_disk);
+
+       mutex_unlock(&init_mutex);
+
+       return ret;
+}
+
 static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
                             size_t nbytes, loff_t off)
 {
@@ -838,6 +861,10 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 
        blkg_conf_init(&ctx, buf);
 
+       ret = blk_iolatency_try_init(&ctx);
+       if (ret)
+               goto out;
+
        ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
        if (ret)
                goto out;

diff --git a/block/blk.h b/block/blk.h
index cc4e8873dfdea13b2fb9722b6718138768f75d03..2da83110347173bdaba2abf6e7aa102610f7760d 100644 (file)
--- a/block/blk.h
+++ b/block/blk.h
@@ -399,12 +399,6 @@ static inline struct bio *blk_queue_bounce(struct bio *bio,
        return bio;
 }
 
-#ifdef CONFIG_BLK_CGROUP_IOLATENCY
-int blk_iolatency_init(struct gendisk *disk);
-#else
-static inline int blk_iolatency_init(struct gendisk *disk) { return 0; };
-#endif
-
 #ifdef CONFIG_BLK_DEV_ZONED
 void disk_free_zone_bitmaps(struct gendisk *disk);
 void disk_clear_zone_settings(struct gendisk *disk);