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SubjectRe: BUG: using smp_processor_id() in preemptible [00000000] code: icedove-bin/5449

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, 2010-05-24 at 05:03 +0200, Piotr Hosowicz wrote:
> > [ 720.313607] BUG: using smp_processor_id() in preemptible [00000000] code: icedove-bin/5449
> > [ 720.313612] caller is native_sched_clock+0x3c/0x68
> > [ 720.313616] Pid: 5449, comm: icedove-bin Tainted: P 2.6.34-20100524-0407 #1
> > [ 720.313618] Call Trace:
> > [ 720.313624] [<ffffffff811a533b>] debug_smp_processor_id+0xc7/0xe0
> > [ 720.313629] [<ffffffff81009b87>] native_sched_clock+0x3c/0x68
> > [ 720.313634] [<ffffffff81009a4d>] sched_clock+0x9/0xd
> > [ 720.313637] [<ffffffff811823ec>] blk_rq_init+0x92/0x9d
> > [ 720.313641] [<ffffffff81184227>] get_request+0x1bf/0x2c7
> > [ 720.313646] [<ffffffff8118435c>] get_request_wait+0x2d/0x19d
>
> This comes from wreckage in the blk tree..
>
> ---
> 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
>
> We also add start_time_ns and io_start_time_ns fields to struct request
> here to record the time when a request is created and when it is
> dispatched to device. We use ns uints here as ms and jiffies are
> not very useful for non-rotational media.
>
> Signed-off-by: Divyesh Shah<dpshah@google.com>
> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> ---
>
> +#ifdef CONFIG_BLK_CGROUP
> +static inline void set_start_time_ns(struct request *req)
> +{
> + req->start_time_ns = sched_clock();
> +}
> +
> +static inline void set_io_start_time_ns(struct request *req)
> +{
> + req->io_start_time_ns = sched_clock();
> +}
> +#else
> +static inline void set_start_time_ns(struct request *req) {}
> +static inline void set_io_start_time_ns(struct request *req) {}
> +#endif
>
> ---
>
> Clearly nobody tested this, and its terribly broken to boot.
>
> I guess they want something like:
>
> ---
> Subject: sched_clock: Add local_clock()
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Tue May 25 10:48:51 CEST 2010
>
> For people who otherwise get to write: cpu_clock(smp_processor_id()),
> there is now: local_clock().

This doesnt fix the whole issue. cpu_clock() is local, while the measurements
done in the blk code are global ...

While the warning is fixed this way, the far more serious issue is still
there: time can go backwards if two points of time measurement are on
different CPUs and can mess up the statistics with negative values, etc...
Ingo


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