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    Subject[PATCH 4/4] scheduler: kill paranoia check in synchronize_sched_expedited()
    Date
    The paranoid check which verifies that the cpu_stop callback is
    actually called on all online cpus is completely superflous. It's
    guaranteed by cpu_stop facility and if it didn't work as advertised
    other things would go horribly wrong and trying to recover using
    synchronize_sched() wouldn't be very meaningful.

    Kill the paranoid check. Removal of this feature is done as a
    separate step so that it can serve as a bisection point if something
    actually goes wrong.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Dipankar Sarma <dipankar@in.ibm.com>
    Cc: Josh Triplett <josh@freedesktop.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Dimitri Sivanich <sivanich@sgi.com>
    ---
    kernel/sched.c | 40 +++-------------------------------------
    1 files changed, 3 insertions(+), 37 deletions(-)

    diff --git a/kernel/sched.c b/kernel/sched.c
    index 947b2bb..5b88521 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -8884,14 +8884,6 @@ static atomic_t synchronize_sched_expedited_count = ATOMIC_INIT(0);

    static int synchronize_sched_expedited_cpu_stop(void *data)
    {
    - static DEFINE_SPINLOCK(done_mask_lock);
    - struct cpumask *done_mask = data;
    -
    - if (done_mask) {
    - spin_lock(&done_mask_lock);
    - cpumask_set_cpu(smp_processor_id(), done_mask);
    - spin_unlock(&done_mask_lock);
    - }
    return 0;
    }

    @@ -8907,55 +8899,29 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
    */
    void synchronize_sched_expedited(void)
    {
    - cpumask_var_t done_mask_var;
    - struct cpumask *done_mask = NULL;
    int snap, trycount = 0;

    - /*
    - * done_mask is used to check that all cpus actually have
    - * finished running the stopper, which is guaranteed by
    - * stop_cpus() if it's called with cpu hotplug blocked. Keep
    - * the paranoia for now but it's best effort if cpumask is off
    - * stack.
    - */
    - if (zalloc_cpumask_var(&done_mask_var, GFP_ATOMIC))
    - done_mask = done_mask_var;
    -
    smp_mb(); /* ensure prior mod happens before capturing snap. */
    snap = atomic_read(&synchronize_sched_expedited_count) + 1;
    get_online_cpus();
    while (try_stop_cpus(cpu_online_mask,
    synchronize_sched_expedited_cpu_stop,
    - done_mask) == -EAGAIN) {
    + NULL) == -EAGAIN) {
    put_online_cpus();
    if (trycount++ < 10)
    udelay(trycount * num_online_cpus());
    else {
    synchronize_sched();
    - goto free_out;
    + return;
    }
    if (atomic_read(&synchronize_sched_expedited_count) - snap > 0) {
    smp_mb(); /* ensure test happens before caller kfree */
    - goto free_out;
    + return;
    }
    get_online_cpus();
    }
    atomic_inc(&synchronize_sched_expedited_count);
    - if (done_mask)
    - cpumask_xor(done_mask, done_mask, cpu_online_mask);
    put_online_cpus();
    -
    - /* paranoia - this can't happen */
    - if (done_mask && cpumask_weight(done_mask)) {
    - char buf[80];
    -
    - cpulist_scnprintf(buf, sizeof(buf), done_mask);
    - WARN_ONCE(1, "synchronize_sched_expedited: cpu online and done masks disagree on %d cpus: %s\n",
    - cpumask_weight(done_mask), buf);
    - synchronize_sched();
    - }
    -free_out:
    - free_cpumask_var(done_mask_var);
    }
    EXPORT_SYMBOL_GPL(synchronize_sched_expedited);

    --
    1.6.4.2


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