linux-kernel - Re: very low IOPS due to "block: reduce kblockd_mod_delayed_work

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Date:   Sun, 19 Dec 2021 07:58:33 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>,
        linux-block@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>,
        ming.lei@...hat.com, hch@....de, Long Li <longli@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: Re: very low IOPS due to "block: reduce kblockd_mod_delayed_work_on()
 CPU consumption"

On 12/18/21 12:02 PM, Jens Axboe wrote:
> On 12/18/21 11:57 AM, Alex Xu (Hello71) wrote:
>> Hi,
>>
>> I recently noticed that between 6441998e2e and 9eaa88c703, I/O became 
>> much slower on my machine using ext4 on dm-crypt on NVMe with bfq 
>> scheduler. Checking iostat during heavy usage (find / -xdev and fstrim 
>> -v /), maximum IOPS had fallen from ~10000 to ~100. Reverting cb2ac2912a 
>> ("block: reduce kblockd_mod_delayed_work_on() CPU consumption") resolves 
>> the issue.
> 
> Hmm interesting. I'll try and see if I can reproduce this and come up
> with a fix.

I can reproduce this. Alex, can you see if this one helps? Trying to see
if we can hit a happy medium here that avoids hammering on that timer,
but it really depends on what the mix is here of delay with pending,
or no delay with no pending.

Dexuan, can you test this for your test case too? I'm going to queue
up a revert for -rc6 just in case.

diff --git a/block/blk-core.c b/block/blk-core.c
index c1833f95cb97..a3fbf4360ee9 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1481,12 +1481,15 @@ int kblockd_schedule_work(struct work_struct *work)
 }
 EXPORT_SYMBOL(kblockd_schedule_work);
 
-int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork,
-				unsigned long delay)
-{
-	if (!delay)
-		return queue_work_on(cpu, kblockd_workqueue, &dwork->work);
-	return mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay);
+void kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork,
+				 unsigned long delay)
+{
+	if (!delay) {
+		cancel_delayed_work(dwork);
+		queue_work_on(cpu, kblockd_workqueue, &dwork->work);
+	} else {
+		mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay);
+	}
 }
 EXPORT_SYMBOL(kblockd_mod_delayed_work_on);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index bd4370baccca..1c7ba45e8463 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1159,7 +1159,7 @@ static inline unsigned int block_size(struct block_device *bdev)
 }
 
 int kblockd_schedule_work(struct work_struct *work);
-int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork, unsigned long delay);
+void kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork, unsigned long delay);
 
 #define MODULE_ALIAS_BLOCKDEV(major,minor) \
 	MODULE_ALIAS("block-major-" __stringify(major) "-" __stringify(minor))


-- 
Jens Axboe