Re: Recent kernel "mount" slow

Jeff Chua
2012-11-23T23:31:31Z

On Sat, Nov 24, 2012 at 6:21 AM, Jeff Chua <jeff.chua.linux@gmail.com> wrote:
> 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

I reverted these 3 patches and mount time is now 0.012s.

# time mount /dev/sda1 /mnt; sync; sync; umount /mnt

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

Looks like this one is not the problem. But I reverted it anyway
because it's part of the same chunk.

Happy Thanksgiving!

Jeff