| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <20140923060812.GH11740@mtj.dyndns.org>
Date: Tue, 23 Sep 2014 02:08:12 -0400
From: Tejun Heo <tj@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
linux-scsi@...r.kernel.org
Subject: [PATCH block/for-3.18/core] blk-mq: start q->mq_usage_counter in
atomic mode
>From 83b06f4fc6ca2f7f3d706a168b71c248bdada668 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@...nel.org>
Date: Tue, 23 Sep 2014 01:58:34 -0400
blk-mq uses percpu_ref for its usage counter which tracks the number
of in-flight commands and used to synchronously drain the queue on
freeze. percpu_ref shutdown takes measureable wallclock time as it
involves a sched RCU grace period. This means that draining a blk-mq
takes measureable wallclock time. One would think that this shouldn't
matter as queue shutdown should be a rare event which takes place
asynchronously w.r.t. userland.
Unfortunately, SCSI probing involves synchronously setting up and then
tearing down a lot of request_queues back-to-back for non-existent
LUNs. This means that SCSI probing may take above ten seconds when
scsi-mq is used.
[ 0.949892] scsi host0: Virtio SCSI HBA
[ 1.007864] scsi 0:0:0:0: Direct-Access QEMU QEMU HARDDISK 1.1. PQ: 0 ANSI: 5
[ 1.021299] scsi 0:0:1:0: Direct-Access QEMU QEMU HARDDISK 1.1. PQ: 0 ANSI: 5
[ 1.520356] tsc: Refined TSC clocksource calibration: 2491.910 MHz
<stall>
[ 16.186549] sd 0:0:0:0: Attached scsi generic sg0 type 0
[ 16.190478] sd 0:0:1:0: Attached scsi generic sg1 type 0
[ 16.194099] osd: LOADED open-osd 0.2.1
[ 16.203202] sd 0:0:0:0: [sda] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB)
[ 16.208478] sd 0:0:0:0: [sda] Write Protect is off
[ 16.211439] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[ 16.218771] sd 0:0:1:0: [sdb] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB)
[ 16.223264] sd 0:0:1:0: [sdb] Write Protect is off
[ 16.225682] sd 0:0:1:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
This is also the reason why request_queues start in bypass mode which
is ended on blk_register_queue() as shutting down a fully functional
queue also involves a RCU grace period and the queues for non-existent
SCSI devices never reach registration.
blk-mq basically needs to do the same thing - start the mq in a
degraded mode which is faster to shut down and then make it fully
functional only after the queue reaches registration. percpu_ref
recently grew facilities to force atomic operation until explicitly
switched to percpu mode, which can be used for this purpose. This
patch makes blk-mq initialize q->mq_usage_counter in atomic mode and
switch it to percpu mode only once blk_register_queue() is reached.
Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Christoph Hellwig <hch@...radead.org>
Link: http://lkml.kernel.org/g/20140919113815.GA10791@lst.de
Fixes: add703fda981 ("blk-mq: use percpu_ref for mq usage count")
Cc: Kent Overstreet <kmo@...erainc.com>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Johannes Weiner <hannes@...xchg.org>
---
Hello,
This patch is on top of
block/for-3.18/core fe052529e465 ("scsi: move blk_mq_start_request call earlier")
+ [PATCHSET percpu/for-3.18] percpu_ref: implement switch_to_atomic/percpu()
http://lkml.kernel.org/g/1411451718-17807-1-git-send-email-tj@kernel.org
and available in the following git branch.
git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu.git review-blk_mq-latency-fix
Thanks.
block/blk-mq-sysfs.c | 6 ++++++
block/blk-mq.c | 6 +++++-
block/blk-sysfs.c | 11 +++++++++--
include/linux/blk-mq.h | 1 +
4 files changed, 21 insertions(+), 3 deletions(-)
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index ed52178..371d880 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -402,6 +402,12 @@ static void blk_mq_sysfs_init(struct request_queue *q)
}
}
+/* see blk_register_queue() */
+void blk_mq_finish_init(struct request_queue *q)
+{
+ percpu_ref_switch_to_percpu(&q->mq_usage_counter);
+}
+
int blk_mq_register_disk(struct gendisk *disk)
{
struct device *dev = disk_to_dev(disk);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 91c49b3..4ae58b1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1783,8 +1783,12 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
if (!q)
goto err_hctxs;
+ /*
+ * Init percpu_ref in atomic mode so that it's faster to shutdown.
+ * See blk_register_queue() for details.
+ */
if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
- 0, GFP_KERNEL))
+ PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
goto err_map;
setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 17f5c84..521ae90 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -551,12 +551,19 @@ int blk_register_queue(struct gendisk *disk)
return -ENXIO;
/*
- * Initialization must be complete by now. Finish the initial
- * bypass from queue allocation.
+ * SCSI probing may synchronously create and destroy a lot of
+ * request_queues for non-existent devices. Shutting down a fully
+ * functional queue takes measureable wallclock time as RCU grace
+ * periods are involved. To avoid excessive latency in these
+ * cases, a request_queue starts out in a degraded mode which is
+ * faster to shut down and is made fully functional here as
+ * request_queues for non-existent devices never get registered.
*/
if (!blk_queue_init_done(q)) {
queue_flag_set_unlocked(QUEUE_FLAG_INIT_DONE, q);
blk_queue_bypass_end(q);
+ if (q->mq_ops)
+ blk_mq_finish_init(q);
}
ret = blk_trace_init_sysfs(dev);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 3253495..4495dd4 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -144,6 +144,7 @@ enum {
};
struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
+void blk_mq_finish_init(struct request_queue *q);
int blk_mq_register_disk(struct gendisk *);
void blk_mq_unregister_disk(struct gendisk *);
--
1.9.3
--
tejun
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists