linux-kernel - Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle tasks

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Date:	Wed, 13 Aug 2014 19:30:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	rafael@...nel.org
Subject: Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle
 tasks

On Wed, Aug 13, 2014 at 07:24:07PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 13, 2014 at 04:42:19PM +0200, Peter Zijlstra wrote:
> > Auditing all idle functions will be somewhat of a pain, but its entirely
> > doable. Looking at this stuff, it appears we can clean it up massively;
> > see how the generic cpuidle code already has the broadcast logic in, so
> > we can remove that from the drivers by setting the right flags.
> > 
> > We can similarly pull out the leave_mm() call by adding a
> > CPUIDLE_FLAG_TLB_FLUSH. At which point all we'd need to do is mark the
> > intel_idle (and all other cpuidle_state::enter functions with __notrace.
> 
> This removes the broadcast stuff from intel_idle.c; processor_idle.c hurts
> my brain, but something similar should be possible.
> 

And this moves the leave_mm() bit to generic code.

---
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -101,14 +101,6 @@ static int intel_idle_cpu_init(int cpu);
 static struct cpuidle_state *cpuidle_state_table;
 
 /*
- * Set this flag for states where the HW flushes the TLB for us
- * and so we don't need cross-calls to keep it consistent.
- * If this flag is set, SW flushes the TLB, so even if the
- * HW doesn't do the flushing, this flag is safe to use.
- */
-#define CPUIDLE_FLAG_TLB_FLUSHED	0x10000
-
-/*
  * MWAIT takes an 8-bit "hint" in EAX "suggesting"
  * the C-state (top nibble) and sub-state (bottom nibble)
  * 0x00 means "MWAIT(C1)", 0x10 means "MWAIT(C2)" etc.
@@ -508,14 +500,6 @@ static int intel_idle(struct cpuidle_dev
 	unsigned long ecx = 1; /* break on interrupt flag */
 	struct cpuidle_state *state = &drv->states[index];
 	unsigned long eax = flg2MWAIT(state->flags);
-	int cpu = smp_processor_id();
-
-	/*
-	 * leave_mm() to avoid costly and often unnecessary wakeups
-	 * for flushing the user TLB's associated with the active mm.
-	 */
-	if (state->flags & CPUIDLE_FLAG_TLB_FLUSHED)
-		leave_mm(cpu);
 
 	mwait_idle_with_hints(eax, ecx);
 
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -56,6 +56,7 @@ struct cpuidle_state {
 #define CPUIDLE_FLAG_TIME_VALID	(0x01) /* is residency time measurable? */
 #define CPUIDLE_FLAG_COUPLED	(0x02) /* state applies to multiple cpus */
 #define CPUIDLE_FLAG_TIMER_STOP (0x04)  /* timer is stopped on this state */
+#define CPUIDLE_FLAG_TLB_FLUSHED (0x08) /* TLBs are flushed on this state */
 
 #define CPUIDLE_DRIVER_FLAGS_MASK (0xFFFF0000)
 
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -79,7 +79,7 @@ static void cpuidle_idle_call(void)
 	struct cpuidle_device *dev = __this_cpu_read(cpuidle_devices);
 	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
 	int next_state, entered_state;
-	unsigned int broadcast;
+	unsigned int broadcast, flags;
 
 	/*
 	 * Check if the idle task must be rescheduled. If it is the
@@ -135,7 +135,16 @@ static void cpuidle_idle_call(void)
 		goto exit_idle;
 	}
 
-	broadcast = drv->states[next_state].flags & CPUIDLE_FLAG_TIMER_STOP;
+	flags = drv->states[next_state].flags;
+
+	/*
+	 * leave_mm() to avoid costly and often unnecessary wakeups
+	 * for flushing the user TLB's associated with the active mm.
+	 */
+	if (flags & CPUIDLE_FLAG_TLB_FLUSHED)
+		leave_mm(dev->cpu);
+
+	broadcast = flags & CPUIDLE_FLAG_TIMER_STOP;
 
 	/*
 	 * Tell the time framework to switch to a broadcast timer

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