From: Ingo Molnar Date: Fri, 22 Dec 2006 09:11:56 +0000 (-0800) Subject: [PATCH] sched: fix bad missed wakeups in the i386, x86_64, ia64, ACPI and APM idle... X-Git-Tag: v2.6.20-rc2~14 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=0888f06ac99f993df2bb4c479f5b9306dafe154f;p=linux-2.6 [PATCH] sched: fix bad missed wakeups in the i386, x86_64, ia64, ACPI and APM idle code Fernando Lopez-Lezcano reported frequent scheduling latencies and audio xruns starting at the 2.6.18-rt kernel, and those problems persisted all until current -rt kernels. The latencies were serious and unjustified by system load, often in the milliseconds range. After a patient and heroic multi-month effort of Fernando, where he tested dozens of kernels, tried various configs, boot options, test-patches of mine and provided latency traces of those incidents, the following 'smoking gun' trace was captured by him: _------=> CPU# / _-----=> irqs-off | / _----=> need-resched || / _---=> hardirq/softirq ||| / _--=> preempt-depth |||| / ||||| delay cmd pid ||||| time | caller \ / ||||| \ | / IRQ_19-1479 1D..1 0us : __trace_start_sched_wakeup (try_to_wake_up) IRQ_19-1479 1D..1 0us : __trace_start_sched_wakeup <<...>-5856> (37 0) IRQ_19-1479 1D..1 0us : __trace_start_sched_wakeup (c01262ba 0 0) IRQ_19-1479 1D..1 0us : resched_task (try_to_wake_up) IRQ_19-1479 1D..1 0us : __spin_unlock_irqrestore (try_to_wake_up) ... -0 1...1 11us!: default_idle (cpu_idle) ... -0 0Dn.1 602us : smp_apic_timer_interrupt (c0103baf 1 0) ... <...>-5856 0D..2 618us : __switch_to (__schedule) <...>-5856 0D..2 618us : __schedule <-0> (20 162) <...>-5856 0D..2 619us : __spin_unlock_irq (__schedule) <...>-5856 0...1 619us : trace_stop_sched_switched (__schedule) <...>-5856 0D..1 619us : trace_stop_sched_switched <<...>-5856> (37 0) what is visible in this trace is that CPU#1 ran try_to_wake_up() for PID:5856, it placed PID:5856 on CPU#0's runqueue and ran resched_task() for CPU#0. But it decided to not send an IPI that no CPU - due to TS_POLLING. But CPU#0 never woke up after its NEED_RESCHED bit was set, and only rescheduled to PID:5856 upon the next lapic timer IRQ. The result was a 600+ usecs latency and a missed wakeup! the bug turned out to be an idle-wakeup bug introduced into the mainline kernel this summer via an optimization in the x86_64 tree: commit 495ab9c045e1b0e5c82951b762257fe1c9d81564 Author: Andi Kleen Date: Mon Jun 26 13:59:11 2006 +0200 [PATCH] i386/x86-64/ia64: Move polling flag into thread_info_status During some profiling I noticed that default_idle causes a lot of memory traffic. I think that is caused by the atomic operations to clear/set the polling flag in thread_info. There is actually no reason to make this atomic - only the idle thread does it to itself, other CPUs only read it. So I moved it into ti->status. the problem is this type of change: if (!hlt_counter && boot_cpu_data.hlt_works_ok) { - clear_thread_flag(TIF_POLLING_NRFLAG); + current_thread_info()->status &= ~TS_POLLING; smp_mb__after_clear_bit(); while (!need_resched()) { local_irq_disable(); this changes clear_thread_flag() to an explicit clearing of TS_POLLING. clear_thread_flag() is defined as: clear_bit(flag, &ti->flags); and clear_bit() is a LOCK-ed atomic instruction on all x86 platforms: static inline void clear_bit(int nr, volatile unsigned long * addr) { __asm__ __volatile__( LOCK_PREFIX "btrl %1,%0" hence smp_mb__after_clear_bit() is defined as a simple compile barrier: #define smp_mb__after_clear_bit() barrier() but the explicit TS_POLLING clearing introduced by the patch: + current_thread_info()->status &= ~TS_POLLING; is not an atomic op! So the clearing of the TS_POLLING bit is freely reorderable with the reading of the NEED_RESCHED bit - and both now reside in different memory addresses. CPU idle wakeup very much depends on ordered memory ops, the clearing of the TS_POLLING flag must always be done before we test need_resched() and hit the idle instruction(s). [Symmetrically, the wakeup code needs to set NEED_RESCHED before it tests the TS_POLLING flag, so memory ordering is paramount.] Fernando's dual-core Athlon64 system has a sufficiently advanced memory ordering model so that it triggered this scenario very often. ( And it also turned out that the reason why these latencies never triggered on my testsystems is that i routinely use idle=poll, which was the only idle variant not affected by this bug. ) The fix is to change the smp_mb__after_clear_bit() to an smp_mb(), to act as an absolute barrier between the TS_POLLING write and the NEED_RESCHED read. This affects almost all idling methods (default, ACPI, APM), on all 3 x86 architectures: i386, x86_64, ia64. Signed-off-by: Ingo Molnar Tested-by: Fernando Lopez-Lezcano Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/arch/i386/kernel/apm.c b/arch/i386/kernel/apm.c index b75cff25de..1990169275 100644 --- a/arch/i386/kernel/apm.c +++ b/arch/i386/kernel/apm.c @@ -785,7 +785,11 @@ static int apm_do_idle(void) polling = !!(current_thread_info()->status & TS_POLLING); if (polling) { current_thread_info()->status &= ~TS_POLLING; - smp_mb__after_clear_bit(); + /* + * TS_POLLING-cleared state must be visible before we + * test NEED_RESCHED: + */ + smp_mb(); } if (!need_resched()) { idled = 1; diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c index 99308510a1..c641056233 100644 --- a/arch/i386/kernel/process.c +++ b/arch/i386/kernel/process.c @@ -102,7 +102,12 @@ void default_idle(void) { if (!hlt_counter && boot_cpu_data.hlt_works_ok) { current_thread_info()->status &= ~TS_POLLING; - smp_mb__after_clear_bit(); + /* + * TS_POLLING-cleared state must be visible before we + * test NEED_RESCHED: + */ + smp_mb(); + local_irq_disable(); if (!need_resched()) safe_halt(); /* enables interrupts racelessly */ diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c index 51922b9808..17685abaf4 100644 --- a/arch/ia64/kernel/process.c +++ b/arch/ia64/kernel/process.c @@ -268,10 +268,16 @@ cpu_idle (void) /* endless idle loop with no priority at all */ while (1) { - if (can_do_pal_halt) + if (can_do_pal_halt) { current_thread_info()->status &= ~TS_POLLING; - else + /* + * TS_POLLING-cleared state must be visible before we + * test NEED_RESCHED: + */ + smp_mb(); + } else { current_thread_info()->status |= TS_POLLING; + } if (!need_resched()) { void (*idle)(void); diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c index a418ee4c8c..cbbc6adc1a 100644 --- a/arch/x86_64/kernel/process.c +++ b/arch/x86_64/kernel/process.c @@ -109,7 +109,11 @@ void exit_idle(void) static void default_idle(void) { current_thread_info()->status &= ~TS_POLLING; - smp_mb__after_clear_bit(); + /* + * TS_POLLING-cleared state must be visible before we + * test NEED_RESCHED: + */ + smp_mb(); local_irq_disable(); if (!need_resched()) { /* Enables interrupts one instruction before HLT. diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c index 65b3f056ad..6dac6050bb 100644 --- a/drivers/acpi/processor_idle.c +++ b/drivers/acpi/processor_idle.c @@ -211,7 +211,11 @@ acpi_processor_power_activate(struct acpi_processor *pr, static void acpi_safe_halt(void) { current_thread_info()->status &= ~TS_POLLING; - smp_mb__after_clear_bit(); + /* + * TS_POLLING-cleared state must be visible before we + * test NEED_RESCHED: + */ + smp_mb(); if (!need_resched()) safe_halt(); current_thread_info()->status |= TS_POLLING; @@ -345,7 +349,11 @@ static void acpi_processor_idle(void) */ if (cx->type == ACPI_STATE_C2 || cx->type == ACPI_STATE_C3) { current_thread_info()->status &= ~TS_POLLING; - smp_mb__after_clear_bit(); + /* + * TS_POLLING-cleared state must be visible before we + * test NEED_RESCHED: + */ + smp_mb(); if (need_resched()) { current_thread_info()->status |= TS_POLLING; local_irq_enable();