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Date: Fri, 24 Feb 2012 14:33:18 -0800
From: Venkatesh Pallipadi <venki@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>
Cc: Suresh Siddha <suresh.b.siddha@...el.com>,
Aaron Durbin <adurbin@...gle.com>,
Paul Turner <pjt@...gle.com>,
Yong Zhang <yong.zhang0@...il.com>,
Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
Venkatesh Pallipadi <venki@...gle.com>
Subject: [PATCH 2/4] x86: Mwait idle optimization to avoid CAL+RES IPIs -v2
smp_call_function_single and ttwu_queue_remote sends unconditional IPI
to target CPU. However, if the target CPU is in some form of poll based idle,
we can do IPI-less wakeups.
Doing this has certain advantages:
* Lower overhead on Async IPI send path. Measurements on Westmere based
systems show savings on "no wait" smp_call_function_single with idle
target CPU (as measured on the sender side).
local socket smp_call_func cost goes from ~1600 to ~1100 cycles
remote socket smp_call_func cost goes from ~2000 to ~1800 cycles
* Avoiding actual interrupts shows a measurable reduction (10%) in system
non-idle cycles and cache-references with micro-benchmark sending IPI from
one CPU to all the other mostly idle CPUs in the system.
* On a mostly idle system, turbostat shows a tiny decrease in C0(active) time
and a corresponding increase in C6 state (Each row being 10min avg)
%c0 %c1 %c6
Before
Run 1 1.51 2.93 95.55
Run 2 1.48 2.86 95.65
Run 3 1.46 2.78 95.74
After
Run 1 1.35 2.63 96.00
Run 2 1.46 2.78 95.74
Run 3 1.37 2.63 95.98
We started looking at this with one of our workloads where system is partially
busy and we noticed some kernel hotspots in find_next_bit and
default_send_IPI_mask_sequence_phys coming from sched wakeup (futex wakeups)
and networking call functions.
Thanks to Suresh for the suggestion of using TIF flags instead of
having a new percpu state variable and complicated update logic.
Notes:
* This only helps when target CPU is idle. When it is busy we will still send
IPI as before.
Signed-off-by: Venkatesh Pallipadi <venki@...gle.com>
---
arch/x86/include/asm/ipiless_wake.h | 84 +++++++++++++++++++++++++++++++++++
arch/x86/include/asm/thread_info.h | 3 +
arch/x86/kernel/acpi/cstate.c | 7 ++-
arch/x86/kernel/process_32.c | 2 +
arch/x86/kernel/process_64.c | 2 +
arch/x86/kernel/smp.c | 8 +++
6 files changed, 104 insertions(+), 2 deletions(-)
create mode 100644 arch/x86/include/asm/ipiless_wake.h
diff --git a/arch/x86/include/asm/ipiless_wake.h b/arch/x86/include/asm/ipiless_wake.h
new file mode 100644
index 0000000..a490dd3
--- /dev/null
+++ b/arch/x86/include/asm/ipiless_wake.h
@@ -0,0 +1,84 @@
+#ifndef _ASM_X86_IPILESS_WAKE_H
+#define _ASM_X86_IPILESS_WAKE_H
+
+#include <linux/hardirq.h>
+#include <linux/sched.h>
+#include <asm/thread_info.h>
+
+#ifdef CONFIG_SMP
+
+/*
+ * TIF_IN_IPILESS_IDLE CPU being in a idle state with ipiless wakeup
+ * capability, without any pending IPIs.
+ * It is conditionally reset by an IPI source CPU and the reset automatically
+ * brings the target CPU out of its idle state.
+ *
+ * TS_IPILESS_WAKEUP is only changed by local CPU and is a place to store
+ * the info that there is a pending IPI work needed after complete idle exit.
+ */
+
+static inline void enter_ipiless_idle(void)
+{
+ set_thread_flag(TIF_IN_IPILESS_IDLE);
+}
+
+static inline void exit_ipiless_idle(void)
+{
+ if (!test_and_clear_thread_flag(TIF_IN_IPILESS_IDLE)) {
+ /*
+ * Flag was already cleared, indicating that there is
+ * a pending IPIless wakeup.
+ * Save that info in status for later use.
+ */
+ current_thread_info()->status |= TS_IPILESS_WAKEUP;
+ }
+}
+
+static inline int is_ipiless_wakeup_pending(void)
+{
+ return need_resched() ||
+ unlikely(!test_thread_flag(TIF_IN_IPILESS_IDLE));
+}
+
+static inline void do_ipiless_pending_work(void)
+{
+ if (unlikely(current_thread_info()->status & TS_IPILESS_WAKEUP)) {
+ current_thread_info()->status &= ~TS_IPILESS_WAKEUP;
+
+ local_bh_disable();
+ local_irq_disable();
+
+ irq_enter();
+ generic_smp_call_function_single_interrupt();
+ irq_exit();
+
+ scheduler_ipi(); /* Does its own irq enter/exit */
+
+ local_irq_enable();
+ local_bh_enable(); /* Needed for bh handling */
+ }
+}
+
+static inline int try_ipiless_wakeup(int cpu)
+{
+ struct thread_info *idle_ti = task_thread_info(idle_task(cpu));
+
+ if (!(idle_ti->flags & _TIF_IN_IPILESS_IDLE))
+ return 0;
+
+ return test_and_clear_bit(TIF_IN_IPILESS_IDLE,
+ (unsigned long *)&idle_ti->flags);
+}
+
+#else
+static inline void do_ipiless_pending_work(void) { }
+static inline void enter_ipiless_idle(void) { }
+static inline void exit_ipiless_idle(void) { }
+
+static inline int is_ipiless_wakeup_pending(void)
+{
+ return need_resched();
+}
+#endif
+
+#endif /* _ASM_X86_IPILESS_WAKE_H */
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index a4d3888..3c5ae3b 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -96,6 +96,7 @@ struct thread_info {
#define TIF_LAZY_MMU_UPDATES 27 /* task is updating the mmu lazily */
#define TIF_SYSCALL_TRACEPOINT 28 /* syscall tracepoint instrumentation */
#define TIF_POLLING_NRFLAG 29 /* true if poll_idle() is polling TIF_NEED_RESCHED */
+#define TIF_IN_IPILESS_IDLE 30 /* Task in IPIless idle state */
#define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
@@ -118,6 +119,7 @@ struct thread_info {
#define _TIF_LAZY_MMU_UPDATES (1 << TIF_LAZY_MMU_UPDATES)
#define _TIF_SYSCALL_TRACEPOINT (1 << TIF_SYSCALL_TRACEPOINT)
#define _TIF_POLLING_NRFLAG (1 << TIF_POLLING_NRFLAG)
+#define _TIF_IN_IPILESS_IDLE (1 << TIF_IN_IPILESS_IDLE)
/* work to do in syscall_trace_enter() */
#define _TIF_WORK_SYSCALL_ENTRY \
@@ -253,6 +255,7 @@ static inline struct thread_info *current_thread_info(void)
this quantum (SMP) */
#define TS_COMPAT 0x0002 /* 32bit syscall active (64BIT)*/
#define TS_RESTORE_SIGMASK 0x0004 /* restore signal mask in do_signal() */
+#define TS_IPILESS_WAKEUP 0x0008 /* pending IPI-work on idle exit */
#ifndef __ASSEMBLY__
#define HAVE_SET_RESTORE_SIGMASK 1
diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
index f50e7fb..30ab435 100644
--- a/arch/x86/kernel/acpi/cstate.c
+++ b/arch/x86/kernel/acpi/cstate.c
@@ -12,6 +12,7 @@
#include <linux/sched.h>
#include <acpi/processor.h>
+#include <asm/ipiless_wake.h>
#include <asm/acpi.h>
#include <asm/mwait.h>
@@ -161,15 +162,17 @@ EXPORT_SYMBOL_GPL(acpi_processor_ffh_cstate_probe);
*/
void mwait_idle_with_hints(unsigned long ax, unsigned long cx)
{
- if (!need_resched()) {
+ enter_ipiless_idle();
+ if (!is_ipiless_wakeup_pending()) {
if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
clflush((void *)¤t_thread_info()->flags);
__monitor((void *)¤t_thread_info()->flags, 0, 0);
smp_mb();
- if (!need_resched())
+ if (!is_ipiless_wakeup_pending())
__mwait(ax, cx);
}
+ exit_ipiless_idle();
}
void acpi_processor_ffh_cstate_enter(struct acpi_processor_cx *cx)
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 5de6bb1..014e26d 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -44,6 +44,7 @@
#include <asm/system.h>
#include <asm/ldt.h>
#include <asm/processor.h>
+#include <asm/ipiless_wake.h>
#include <asm/i387.h>
#include <asm/desc.h>
#ifdef CONFIG_MATH_EMULATION
@@ -116,6 +117,7 @@ void cpu_idle(void)
if (cpuidle_idle_call())
pm_idle();
start_critical_timings();
+ do_ipiless_pending_work();
}
rcu_idle_exit();
tick_nohz_idle_exit();
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 98b1854..777bb7d 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -42,6 +42,7 @@
#include <asm/pgtable.h>
#include <asm/system.h>
#include <asm/processor.h>
+#include <asm/ipiless_wake.h>
#include <asm/i387.h>
#include <asm/mmu_context.h>
#include <asm/prctl.h>
@@ -148,6 +149,7 @@ void cpu_idle(void)
rcu_idle_exit();
start_critical_timings();
+ do_ipiless_pending_work();
/* In many cases the interrupt that ended idle
has already called exit_idle. But some idle
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 66c74f4..4b44bef 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -27,6 +27,7 @@
#include <asm/mtrr.h>
#include <asm/tlbflush.h>
#include <asm/mmu_context.h>
+#include <asm/ipiless_wake.h>
#include <asm/proto.h>
#include <asm/apic.h>
#include <asm/nmi.h>
@@ -120,11 +121,18 @@ static void native_smp_send_reschedule(int cpu)
WARN_ON(1);
return;
}
+
+ if (try_ipiless_wakeup(cpu))
+ return;
+
apic->send_IPI_mask(cpumask_of(cpu), RESCHEDULE_VECTOR);
}
void native_send_call_func_single_ipi(int cpu)
{
+ if (try_ipiless_wakeup(cpu))
+ return;
+
apic->send_IPI_mask(cpumask_of(cpu), CALL_FUNCTION_SINGLE_VECTOR);
}
--
1.7.7.3
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