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    Subject[PATCH 4.4 55/60] kernel/debug/debug_core.c: more properly delay for secondary CPUs
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Douglas Anderson <dianders@chromium.org>

    commit 2d13bb6494c807bcf3f78af0e96c0b8615a94385 upstream.

    We've got a delay loop waiting for secondary CPUs. That loop uses
    loops_per_jiffy. However, loops_per_jiffy doesn't actually mean how
    many tight loops make up a jiffy on all architectures. It is quite
    common to see things like this in the boot log:

    Calibrating delay loop (skipped), value calculated using timer
    frequency.. 48.00 BogoMIPS (lpj=24000)

    In my case I was seeing lots of cases where other CPUs timed out
    entering the debugger only to print their stack crawls shortly after the
    kdb> prompt was written.

    Elsewhere in kgdb we already use udelay(), so that should be safe enough
    to use to implement our timeout. We'll delay 1 ms for 1000 times, which
    should give us a full second of delay (just like the old code wanted)
    but allow us to notice that we're done every 1 ms.

    [akpm@linux-foundation.org: simplifications, per Daniel]
    Link: http://lkml.kernel.org/r/1477091361-2039-1-git-send-email-dianders@chromium.org
    Signed-off-by: Douglas Anderson <dianders@chromium.org>
    Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
    Cc: Jason Wessel <jason.wessel@windriver.com>
    Cc: Brian Norris <briannorris@chromium.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/debug/debug_core.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/kernel/debug/debug_core.c
    +++ b/kernel/debug/debug_core.c
    @@ -598,11 +598,11 @@ return_normal:
    /*
    * Wait for the other CPUs to be notified and be waiting for us:
    */
    - time_left = loops_per_jiffy * HZ;
    + time_left = MSEC_PER_SEC;
    while (kgdb_do_roundup && --time_left &&
    (atomic_read(&masters_in_kgdb) + atomic_read(&slaves_in_kgdb)) !=
    online_cpus)
    - cpu_relax();
    + udelay(1000);
    if (!time_left)
    pr_crit("Timed out waiting for secondary CPUs.\n");


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