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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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