Messages in this thread Patch in this message | | | Date | Wed, 19 May 2010 14:57:11 +0200 | From | Oleg Nesterov <> | Subject | [PATCH] schedule: simplify the reacquire_kernel_lock() logic |
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- Contrary to what 6d558c3a says, there is no need to reload prev = rq->curr after the context switch. You always schedule back to where you came from, prev must be equal to current even if cpu/rq was changed.
- This also means reacquire_kernel_lock() can use prev instead of current.
- No need to reassign switch_count if reacquire_kernel_lock() reports need_resched(), we can just move the initial assignment down, under the "need_resched_nonpreemptible:" label.
- Try to update the comment after context_switch().
Signed-off-by: Oleg Nesterov <oleg@redhat.com> ---
kernel/sched.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-)
--- 34-rc1/kernel/sched.c~SCHEDULE_PREV_EQ_TO_CURRENT 2010-05-18 23:32:50.000000000 +0200 +++ 34-rc1/kernel/sched.c 2010-05-19 14:32:57.000000000 +0200 @@ -3679,7 +3679,6 @@ need_resched: rq = cpu_rq(cpu); rcu_sched_qs(cpu); prev = rq->curr; - switch_count = &prev->nivcsw; release_kernel_lock(prev); need_resched_nonpreemptible: @@ -3693,6 +3692,7 @@ need_resched_nonpreemptible: update_rq_clock(rq); clear_tsk_need_resched(prev); + switch_count = &prev->nivcsw; if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) { if (unlikely(signal_pending_state(prev->state, prev))) prev->state = TASK_RUNNING; @@ -3719,8 +3719,10 @@ need_resched_nonpreemptible: context_switch(rq, prev, next); /* unlocks the rq */ /* - * the context switch might have flipped the stack from under - * us, hence refresh the local variables. + * The context switch have flipped the stack from under us + * and restored the local variables which were saved when + * this task called schedule() in the past. prev == current + * is still correct, but it can be moved to another cpu/rq. */ cpu = smp_processor_id(); rq = cpu_rq(cpu); @@ -3729,11 +3731,8 @@ need_resched_nonpreemptible: post_schedule(rq); - if (unlikely(reacquire_kernel_lock(current) < 0)) { - prev = rq->curr; - switch_count = &prev->nivcsw; + if (unlikely(reacquire_kernel_lock(prev))) goto need_resched_nonpreemptible; - } preempt_enable_no_resched(); if (need_resched())
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