Re: 2.6.35-rc2-git2: Reported regressions from 2.6.34

Ingo Molnar
2010-06-09T05:34:27Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=16129
> > Subject : BUG: using smp_processor_id() in preemptible [00000000] code: jbd2/sda2
> > Submitter : Jan Kreuzer <kontrollator@gmx.de>
> > Date : 2010-06-05 06:15 (4 days old)
>
> This seems to have been introduced by
>
> commit 7cbaef9c83e58bbd4bdd534b09052b6c5ec457d5
> Author: Ingo Molnar <mingo@elte.hu>
> Date: Sat Nov 8 17:05:38 2008 +0100
>
> sched: optimize sched_clock() a bit
>
> sched_clock() uses cycles_2_ns() needlessly - which is an irq-disabling
> variant of __cycles_2_ns().
>
> Most of the time sched_clock() is called with irqs disabled already.
> The few places that call it with irqs enabled need to be updated.
>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
> and this seems to be one of those calling cases that need to be updated.

That's a commit from 2008.

> Ingo? The call trace is:
>
> BUG: using smp_processor_id() in preemptible [00000000] code: jbd2/sda2-8/337
> caller is native_sched_clock+0x3c/0x68
> Pid: 337, comm: jbd2/sda2-8 Not tainted 2.6.35-rc1jan+ #4
> Call Trace:
> [<ffffffff812362c5>] debug_smp_processor_id+0xc9/0xe4
> [<ffffffff8101059d>] native_sched_clock+0x3c/0x68
> [<ffffffff8101043d>] sched_clock+0x9/0xd
> [<ffffffff81212d7a>] blk_rq_init+0x97/0xa3
> [<ffffffff81214d71>] get_request+0x1c4/0x2d0
> [<ffffffff81214ea6>] get_request_wait+0x29/0x1a6
> [<ffffffff81215537>] __make_request+0x338/0x45b
> [<ffffffff812147c2>] generic_make_request+0x2bb/0x330
> [<ffffffff81214909>] submit_bio+0xd2/0xef
> [<ffffffff811413cb>] submit_bh+0xf4/0x116
> [<ffffffff81144853>] block_write_full_page_endio+0x89/0x96
> [<ffffffff81144875>] block_write_full_page+0x15/0x17
> [<ffffffff8119b00a>] ext4_writepage+0x356/0x36b
> [<ffffffff810e1f91>] __writepage+0x1a/0x39
> [<ffffffff810e32a6>] write_cache_pages+0x20d/0x346
> [<ffffffff810e3406>] generic_writepages+0x27/0x29
> [<ffffffff811ca279>] journal_submit_data_buffers+0x110/0x17d
> [<ffffffff811ca986>] jbd2_journal_commit_transaction+0x4cb/0x156d
> [<ffffffff811d0cba>] kjournald2+0x147/0x37a
>
> (from the bugzilla thing)

The warning was introduced by this fresh commit (and a followup commit) merged
in the .35 merge window:

| commit 9195291e5f05e01d67f9a09c756b8aca8f009089
| Author: Divyesh Shah <dpshah@google.com>
| Date: Thu Apr 1 15:01:41 2010 -0700
|
| blkio: Increment the blkio cgroup stats for real now

IIRC Jens posted a fix for the regression. Jens, what's the status of that?

As the code there started using a raw sched_clock() call for block statistics
purposes, which was a poorly thought out (and buggy) approach:

- it takes timestamps on different cpus and then compares then, but doesnt
consider that sched_clock() is not comparable between CPUs without extra
care

- it doesnt consider the possibility for the sched_clock() result going
backwards on certain platforms (such as x86)

- it doesnt consider preemptability

(There's work ongoing to add a clock variant that can be used for such
purposes, but that's .36 fodder.)

Thanks,

Ingo