From 1e185b97b4364063f1135604b87f8d8469944233 Mon Sep 17 00:00:00 2001 From: "Chen, Kenneth W" Date: Tue, 15 Nov 2005 14:37:05 -0800 Subject: [PATCH] [PATCH] ia64: cpu_idle performance bug fix Our performance validation on 2.6.15-rc1 caught a disastrous performance regression on ia64 with netperf (-98%) and volanomark (-58%) compares to previous kernel version 2.6.14-git7. See the following chart (result group 1 & 2). http://kernel-perf.sourceforge.net/results.machine_id=26.html We have root caused it to commit 64c7c8f88559624abdbe12b5da6502e8879f8d28 This changeset broke the ia64 task resched notification. In sched.c:resched_task(), a reschedule IPI is conditioned upon TIF_POLLING_NRFLAG. However, the above changeset unconditionally set the polling thread flag for idle tasks regardless whether pal_halt_light is in use or not. As a result, resched IPI is not sent from resched_task(). And since the default behavior on ia64 is to use pal_halt_light, we end up delaying the rescheduling task until next timer tick, and thus cause the performance regression. This fixes the performance bug. I'm glad our performance suite is turning up bad performance bug like this in time. Signed-off-by: Ken Chen Signed-off-by: Linus Torvalds --- arch/ia64/kernel/process.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c index e92ea64d8040..4305d2ba76f6 100644 --- a/arch/ia64/kernel/process.c +++ b/arch/ia64/kernel/process.c @@ -202,12 +202,9 @@ default_idle (void) { local_irq_enable(); while (!need_resched()) { - if (can_do_pal_halt) { - local_irq_disable(); - if (!need_resched()) - safe_halt(); - local_irq_enable(); - } else + if (can_do_pal_halt) + safe_halt(); + else cpu_relax(); } } @@ -272,10 +269,14 @@ cpu_idle (void) { void (*mark_idle)(int) = ia64_mark_idle; int cpu = smp_processor_id(); - set_thread_flag(TIF_POLLING_NRFLAG); /* endless idle loop with no priority at all */ while (1) { + if (can_do_pal_halt) + clear_thread_flag(TIF_POLLING_NRFLAG); + else + set_thread_flag(TIF_POLLING_NRFLAG); + if (!need_resched()) { void (*idle)(void); #ifdef CONFIG_SMP -- 2.39.5