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    Subject[PATCH] [130/139] Sched: fix skip_clock_update optimization
    Date
    2.6.35-longterm review patch.  If anyone has any objections, please let me know.

    ------------------
    From: Mike Galbraith <efault@gmx.de>

    commit f26f9aff6aaf67e9a430d16c266f91b13a5bff64 upstream.

    idle_balance() drops/retakes rq->lock, leaving the previous task
    vulnerable to set_tsk_need_resched(). Clear it after we return
    from balancing instead, and in setup_thread_stack() as well, so
    no successfully descheduled or never scheduled task has it set.

    Need resched confused the skip_clock_update logic, which assumes
    that the next call to update_rq_clock() will come nearly immediately
    after being set. Make the optimization robust against the waking
    a sleeper before it sucessfully deschedules case by checking that
    the current task has not been dequeued before setting the flag,
    since it is that useless clock update we're trying to save, and
    clear unconditionally in schedule() proper instead of conditionally
    in put_prev_task().

    Signed-off-by: Mike Galbraith <efault@gmx.de>
    Signed-off-by: Andi Kleen <ak@linux.intel.com>
    Reported-by: Bjoern B. Brandenburg <bbb.lst@gmail.com>
    Tested-by: Yong Zhang <yong.zhang0@gmail.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <1291802742.1417.9.camel@marge.simson.net>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    kernel/fork.c | 1 +
    kernel/sched.c | 6 +++---
    2 files changed, 4 insertions(+), 3 deletions(-)

    Index: linux-2.6.35.y/kernel/fork.c
    ===================================================================
    --- linux-2.6.35.y.orig/kernel/fork.c
    +++ linux-2.6.35.y/kernel/fork.c
    @@ -272,6 +272,7 @@ static struct task_struct *dup_task_stru

    setup_thread_stack(tsk, orig);
    clear_user_return_notifier(tsk);
    + clear_tsk_need_resched(tsk);
    stackend = end_of_stack(tsk);
    *stackend = STACK_END_MAGIC; /* for overflow detection */

    Index: linux-2.6.35.y/kernel/sched.c
    ===================================================================
    --- linux-2.6.35.y.orig/kernel/sched.c
    +++ linux-2.6.35.y/kernel/sched.c
    @@ -564,7 +564,7 @@ void check_preempt_curr(struct rq *rq, s
    * A queue event has occurred, and we're going to schedule. In
    * this case, we can save a useless back to back clock update.
    */
    - if (test_tsk_need_resched(p))
    + if (rq->curr->se.on_rq && test_tsk_need_resched(rq->curr))
    rq->skip_clock_update = 1;
    }

    @@ -3657,7 +3657,6 @@ static void put_prev_task(struct rq *rq,
    {
    if (prev->se.on_rq)
    update_rq_clock(rq);
    - rq->skip_clock_update = 0;
    prev->sched_class->put_prev_task(rq, prev);
    }

    @@ -3720,7 +3719,6 @@ need_resched_nonpreemptible:
    hrtick_clear(rq);

    raw_spin_lock_irq(&rq->lock);
    - clear_tsk_need_resched(prev);

    if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
    if (unlikely(signal_pending_state(prev->state, prev)))
    @@ -3737,6 +3735,8 @@ need_resched_nonpreemptible:

    put_prev_task(rq, prev);
    next = pick_next_task(rq);
    + clear_tsk_need_resched(prev);
    + rq->skip_clock_update = 0;

    if (likely(prev != next)) {
    sched_info_switch(prev, next);

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