On Fri, Nov 23, 2012 at 9:24 PM, Jens Axboe [off-list ref] wrote:

On 2012-11-22 20:21, Linus Torvalds wrote:

quoted

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.

quoted

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 [off-list ref]
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 [off-list ref]
Date:   Mon Oct 22 19:37:47 2012 -0400

    percpu-rw-semaphores: use light/heavy barriers


commit 1bf11c53535ab87e3bf14ecdf6747bf46f601c5d
Author: Mikulas Patocka [off-list ref]
Date:   Mon Oct 22 19:39:16 2012 -0400

    percpu-rw-semaphores: use rcu_read_lock_sched


commit 1a25b1c4ce189e3926f2981f3302352a930086db
Author: Mikulas Patocka [off-list ref]
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