Re: Recent kernel "mount" slow

Jeff Chua
2012-11-23T22:21:42Z

On Fri, Nov 23, 2012 at 9:24 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 2012-11-22 20:21, Linus Torvalds wrote:
>> Doesn't sound like a fsdevel issue since it seems to be independent of
>> filesystems. More like some generic block layer thing. Adding Jens
>> (and quoting the whole thing)
>>
>> Jens, any ideas? Most of your stuff came in after -rc2, which would
>> fit with the fact that most of the slowdown seems to be after -rc2
>> according to Jeff.
>
> No ideas. Looking at what went in from my side, only the rq plug sorting
> is a core change, and that should not cause any change in behaviour for
> a single device. That's commit 975927b9.
>
>> Jeff, more bisecting would be good, though.
>
> Probably required, yes...

This one slows mount from 0.012s to 0.168s.

commit 62ac665ff9fc07497ca524bd20d6a96893d11071
Author: Mikulas Patocka <mpatocka@redhat.com>
Date: Wed Sep 26 07:46:43 2012 +0200

blockdev: turn a rw semaphore into a percpu rw semaphore

There were couple of more changes to percpu-rw-semaphores after
3.7.0-rc2 and those slows mount further from 0.168s to 0.500s. I don't
really know, but I'm suspecting these. Still bisecting.

commit 5c1eabe68501d1e1b1586c7f4c46cc531828c4ab
Author: Mikulas Patocka <mpatocka@redhat.com>
Date: Mon Oct 22 19:37:47 2012 -0400

percpu-rw-semaphores: use light/heavy barriers

commit 1bf11c53535ab87e3bf14ecdf6747bf46f601c5d
Author: Mikulas Patocka <mpatocka@redhat.com>
Date: Mon Oct 22 19:39:16 2012 -0400

percpu-rw-semaphores: use rcu_read_lock_sched

commit 1a25b1c4ce189e3926f2981f3302352a930086db
Author: Mikulas Patocka <mpatocka@redhat.com>
Date: Mon Oct 15 17:20:17 2012 -0400

Lock splice_read and splice_write functions

I couldn't unpatch with the latest kernel, but still trying.

Jeff