On Fri, 2009-07-10 at 14:57 +0200, Peter Zijlstra wrote:
> plain text document attachment (sched_opt_cond_resched.patch)
> Optimize cond_resched() by removing one conditional.
>
> Currently cond_resched() checks system_state ==
> SYSTEM_RUNNING in order to avoid scheduling before the
> scheduler is running.
>
> We can however, as per suggestion of Matt, use
> PREEMPT_ACTIVE to accomplish that very same.
Pedantically, introducing should_resched should be its own patch, but
other than that, these two patches look good.
Acked-by: Matt Mackall <mpm@xxxxxxxxxxx>
> Suggested-by: Matt Mackall <mpm@xxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
> include/linux/sched.h | 5 ++++-
> kernel/sched.c | 14 +++++++++-----
> 2 files changed, 13 insertions(+), 6 deletions(-)
>
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -503,8 +503,11 @@ struct task_cputime {
> /*
> * Disable preemption until the scheduler is running.
> * Reset by start_kernel()->sched_init()->init_idle().
> + *
> + * We include PREEMPT_ACTIVE to avoid cond_resched() from working
> + * before the scheduler is active -- see should_resched().
> */
> -#define INIT_PREEMPT_COUNT (1)
> +#define INIT_PREEMPT_COUNT (1 + PREEMPT_ACTIVE)
>
> /**
> * struct thread_group_cputimer - thread group interval timer counts
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -6580,6 +6580,11 @@ SYSCALL_DEFINE0(sched_yield)
> return 0;
> }
>
> +static inline int should_resched(void)
> +{
> + return need_resched() && !(preempt_count() & PREEMPT_ACTIVE);
> +}
> +
> static void __cond_resched(void)
> {
> #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
> @@ -6599,8 +6604,7 @@ static void __cond_resched(void)
>
> int __sched _cond_resched(void)
> {
> - if (need_resched() && !(preempt_count() & PREEMPT_ACTIVE) &&
> - system_state == SYSTEM_RUNNING) {
> + if (should_resched()) {
> __cond_resched();
> return 1;
> }
> @@ -6618,12 +6622,12 @@ EXPORT_SYMBOL(_cond_resched);
> */
> int cond_resched_lock(spinlock_t *lock)
> {
> - int resched = need_resched() && system_state == SYSTEM_RUNNING;
> + int resched = should_resched();
> int ret = 0;
>
> if (spin_needbreak(lock) || resched) {
> spin_unlock(lock);
> - if (resched && need_resched())
> + if (resched)
> __cond_resched();
> else
> cpu_relax();
> @@ -6638,7 +6642,7 @@ int __sched cond_resched_softirq(void)
> {
> BUG_ON(!in_softirq());
>
> - if (need_resched() && system_state == SYSTEM_RUNNING) {
> + if (should_resched()) {
> local_bh_enable();
> __cond_resched();
> local_bh_disable();
>
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