From 64c7c8f88559624abdbe12b5da6502e8879f8d28 Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Tue, 8 Nov 2005 21:39:04 -0800 Subject: [PATCH] sched: resched and cpu_idle rework Make some changes to the NEED_RESCHED and POLLING_NRFLAG to reduce confusion, and make their semantics rigid. Improves efficiency of resched_task and some cpu_idle routines. * In resched_task: - TIF_NEED_RESCHED is only cleared with the task's runqueue lock held, and as we hold it during resched_task, then there is no need for an atomic test and set there. The only other time this should be set is when the task's quantum expires, in the timer interrupt - this is protected against because the rq lock is irq-safe. - If TIF_NEED_RESCHED is set, then we don't need to do anything. It won't get unset until the task get's schedule()d off. - If we are running on the same CPU as the task we resched, then set TIF_NEED_RESCHED and no further action is required. - If we are running on another CPU, and TIF_POLLING_NRFLAG is *not* set after TIF_NEED_RESCHED has been set, then we need to send an IPI. Using these rules, we are able to remove the test and set operation in resched_task, and make clear the previously vague semantics of POLLING_NRFLAG. * In idle routines: - Enter cpu_idle with preempt disabled. When the need_resched() condition becomes true, explicitly call schedule(). This makes things a bit clearer (IMO), but haven't updated all architectures yet. - Many do a test and clear of TIF_NEED_RESCHED for some reason. According to the resched_task rules, this isn't needed (and actually breaks the assumption that TIF_NEED_RESCHED is only cleared with the runqueue lock held). So remove that. Generally one less locked memory op when switching to the idle thread. - Many idle routines clear TIF_POLLING_NRFLAG, and only set it in the inner most polling idle loops. The above resched_task semantics allow it to be set until before the last time need_resched() is checked before going into a halt requiring interrupt wakeup. Many idle routines simply never enter such a halt, and so POLLING_NRFLAG can be always left set, completely eliminating resched IPIs when rescheduling the idle task. POLLING_NRFLAG width can be increased, to reduce the chance of resched IPIs. Signed-off-by: Nick Piggin Cc: Ingo Molnar Cc: Con Kolivas Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- arch/i386/kernel/process.c | 64 ++++++++++++++++++++-------------------------- 1 file changed, 28 insertions(+), 36 deletions(-) (limited to 'arch/i386/kernel/process.c') diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c index 5296e284ea36..1cb261f225d5 100644 --- a/arch/i386/kernel/process.c +++ b/arch/i386/kernel/process.c @@ -99,14 +99,22 @@ EXPORT_SYMBOL(enable_hlt); */ void default_idle(void) { + local_irq_enable(); + if (!hlt_counter && boot_cpu_data.hlt_works_ok) { - local_irq_disable(); - if (!need_resched()) - safe_halt(); - else - local_irq_enable(); + clear_thread_flag(TIF_POLLING_NRFLAG); + smp_mb__after_clear_bit(); + while (!need_resched()) { + local_irq_disable(); + if (!need_resched()) + safe_halt(); + else + local_irq_enable(); + } + set_thread_flag(TIF_POLLING_NRFLAG); } else { - cpu_relax(); + while (!need_resched()) + cpu_relax(); } } #ifdef CONFIG_APM_MODULE @@ -120,29 +128,14 @@ EXPORT_SYMBOL(default_idle); */ static void poll_idle (void) { - int oldval; - local_irq_enable(); - /* - * Deal with another CPU just having chosen a thread to - * run here: - */ - oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED); - - if (!oldval) { - set_thread_flag(TIF_POLLING_NRFLAG); - asm volatile( - "2:" - "testl %0, %1;" - "rep; nop;" - "je 2b;" - : : "i"(_TIF_NEED_RESCHED), "m" (current_thread_info()->flags)); - - clear_thread_flag(TIF_POLLING_NRFLAG); - } else { - set_need_resched(); - } + asm volatile( + "2:" + "testl %0, %1;" + "rep; nop;" + "je 2b;" + : : "i"(_TIF_NEED_RESCHED), "m" (current_thread_info()->flags)); } #ifdef CONFIG_HOTPLUG_CPU @@ -181,6 +174,8 @@ void cpu_idle(void) { int cpu = smp_processor_id(); + set_thread_flag(TIF_POLLING_NRFLAG); + /* endless idle loop with no priority at all */ while (1) { while (!need_resched()) { @@ -246,15 +241,12 @@ static void mwait_idle(void) { local_irq_enable(); - if (!need_resched()) { - set_thread_flag(TIF_POLLING_NRFLAG); - do { - __monitor((void *)¤t_thread_info()->flags, 0, 0); - if (need_resched()) - break; - __mwait(0, 0); - } while (!need_resched()); - clear_thread_flag(TIF_POLLING_NRFLAG); + while (!need_resched()) { + __monitor((void *)¤t_thread_info()->flags, 0, 0); + smp_mb(); + if (need_resched()) + break; + __mwait(0, 0); } } -- cgit v1.2.1