[PATCHSET v2 0/2] Split iowait into two states

[PATCHSET v2 0/2] Split iowait into two states

[Jens Axboe]

Hi,

This is v2 of the patch posted yesterday, where the current in_iowait
state is split into two parts:

1) The "task is sleeping waiting on IO", and would like cpufreq goodness
   in terms of sleep and wakeup latencies.
2) The above, and also accounted as such in the iowait stats.

The current ->in_iowait covers both, with this series we have ->in_iowait
for step 1 above, and ->in_iowait_acct for step 2. You cannot be
->in_iowait_acct without also having ->in_iowait set.

Patch 1 is a prep patch, that turns rq->nr_iowait into an unsigned int
rather than an atomic_t. Reasons given in that patch.

Patch 2 adds the ->in_iowait_acct stage inside the current ->in_iowait
setting.

I haven't been able to properly benchmark patch 1, as the atomics are
noise in any workloads that approximate normality. I can certainly
concoct a synthetic test case if folks are interested. My gut says that
we're trading 3 fast path atomics for none, and with the 4th case
_probably_ being way less likely. There we grab the rq lock.

Comments welcome! Peter, CC'ing you since I did in the previous, feel
free to ignore.

Since v1:
- Add prep patch 1, switching nr_iowait to an unsigned int
- Modify patch 2 to not use atomic_t as well, no changes otherwise

 arch/s390/appldata/appldata_base.c |  2 +-
 arch/s390/appldata/appldata_os.c   |  2 +-
 fs/proc/stat.c                     |  2 +-
 include/linux/sched.h              |  6 ++++
 include/linux/sched/stat.h         | 10 ++++--
 kernel/sched/core.c                | 55 +++++++++++++++++++++++-------
 kernel/sched/cputime.c             |  2 +-
 kernel/sched/sched.h               |  3 +-
 kernel/time/tick-sched.c           |  6 ++--
 9 files changed, 66 insertions(+), 22 deletions(-)

-- 
Jens Axboe

[PATCH 1/2] sched/core: switch struct rq->nr_iowait to an unsigned int

[Jens Axboe]

In 3 of the 4 spots where we modify rq->nr_iowait we already hold the
rq lock, and in the 4th one we can just grab it. This avoids an atomic
in the scheduler fast path if we're in iowait, with the tradeoff being
that we'll grab the rq lock for the case where a task is scheduled out
in iowait mode on one CPU, and scheduled in on another CPU.

This obviously leaves the reading side as potentially racy, but that
should be OK. iowait states change all of the time, and can change while
it's being read as well, or summed up.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/sched/core.c    | 15 ++++++++++-----
 kernel/sched/cputime.c |  2 +-
 kernel/sched/sched.h   |  2 +-
 3 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9116bcc90346..ecc6c26096e5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3789,7 +3789,7 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 #endif
 	if (p->in_iowait) {
 		delayacct_blkio_end(p);
-		atomic_dec(&task_rq(p)->nr_iowait);
+		task_rq(p)->nr_iowait--;
 	}
 
 	activate_task(rq, p, en_flags);
@@ -4354,8 +4354,13 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 		cpu = select_task_rq(p, p->wake_cpu, wake_flags | WF_TTWU);
 		if (task_cpu(p) != cpu) {
 			if (p->in_iowait) {
+				struct rq *rq = task_rq(p);
+				struct rq_flags rf;
+
+				rq_lock(rq, &rf);
+				task_rq(p)->nr_iowait--;
+				rq_unlock(rq, &rf);
 				delayacct_blkio_end(p);
-				atomic_dec(&task_rq(p)->nr_iowait);
 			}
 
 			wake_flags |= WF_MIGRATED;
@@ -5463,7 +5468,7 @@ unsigned long long nr_context_switches(void)
 
 unsigned int nr_iowait_cpu(int cpu)
 {
-	return atomic_read(&cpu_rq(cpu)->nr_iowait);
+	return cpu_rq(cpu)->nr_iowait;
 }
 
 /*
@@ -6681,7 +6686,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 			deactivate_task(rq, prev, DEQUEUE_SLEEP | DEQUEUE_NOCLOCK);
 
 			if (prev->in_iowait) {
-				atomic_inc(&rq->nr_iowait);
+				rq->nr_iowait++;
 				delayacct_blkio_start();
 			}
 		}
@@ -10029,7 +10034,7 @@ void __init sched_init(void)
 #endif
 #endif /* CONFIG_SMP */
 		hrtick_rq_init(rq);
-		atomic_set(&rq->nr_iowait, 0);
+		rq->nr_iowait = 0;
 
 #ifdef CONFIG_SCHED_CORE
 		rq->core = rq;
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index af7952f12e6c..b970b6c6151e 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -224,7 +224,7 @@ void account_idle_time(u64 cputime)
 	u64 *cpustat = kcpustat_this_cpu->cpustat;
 	struct rq *rq = this_rq();
 
-	if (atomic_read(&rq->nr_iowait) > 0)
+	if (rq->nr_iowait)
 		cpustat[CPUTIME_IOWAIT] += cputime;
 	else
 		cpustat[CPUTIME_IDLE] += cputime;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 001fe047bd5d..a1222a4bdc7b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1049,7 +1049,7 @@ struct rq {
 	u64			clock_idle_copy;
 #endif
 
-	atomic_t		nr_iowait;
+	unsigned int		nr_iowait;
 
 #ifdef CONFIG_SCHED_DEBUG
 	u64 last_seen_need_resched_ns;
-- 
2.43.0

Re: [PATCH 1/2] sched/core: switch struct rq->nr_iowait to an unsigned int

[David Wei]

On 2024-02-27 21:06, Jens Axboe wrote:
> In 3 of the 4 spots where we modify rq->nr_iowait we already hold the
> rq lock, and in the 4th one we can just grab it. This avoids an atomic
> in the scheduler fast path if we're in iowait, with the tradeoff being
> that we'll grab the rq lock for the case where a task is scheduled out
> in iowait mode on one CPU, and scheduled in on another CPU.
> 
> This obviously leaves the reading side as potentially racy, but that
> should be OK. iowait states change all of the time, and can change while
> it's being read as well, or summed up.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  kernel/sched/core.c    | 15 ++++++++++-----
>  kernel/sched/cputime.c |  2 +-
>  kernel/sched/sched.h   |  2 +-
>  3 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9116bcc90346..ecc6c26096e5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3789,7 +3789,7 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
>  #endif
>  	if (p->in_iowait) {
>  		delayacct_blkio_end(p);
> -		atomic_dec(&task_rq(p)->nr_iowait);
> +		task_rq(p)->nr_iowait--;
>  	}
>  
>  	activate_task(rq, p, en_flags);
> @@ -4354,8 +4354,13 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  		cpu = select_task_rq(p, p->wake_cpu, wake_flags | WF_TTWU);
>  		if (task_cpu(p) != cpu) {
>  			if (p->in_iowait) {
> +				struct rq *rq = task_rq(p);
> +				struct rq_flags rf;
> +
> +				rq_lock(rq, &rf);
> +				task_rq(p)->nr_iowait--;

Could this use rq directly, or does it not matter?

> +				rq_unlock(rq, &rf);
>  				delayacct_blkio_end(p);
> -				atomic_dec(&task_rq(p)->nr_iowait);
>  			}
>  
>  			wake_flags |= WF_MIGRATED;
> @@ -5463,7 +5468,7 @@ unsigned long long nr_context_switches(void)
>  
>  unsigned int nr_iowait_cpu(int cpu)
>  {
> -	return atomic_read(&cpu_rq(cpu)->nr_iowait);
> +	return cpu_rq(cpu)->nr_iowait;
>  }
>  
>  /*
> @@ -6681,7 +6686,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>  			deactivate_task(rq, prev, DEQUEUE_SLEEP | DEQUEUE_NOCLOCK);
>  
>  			if (prev->in_iowait) {
> -				atomic_inc(&rq->nr_iowait);
> +				rq->nr_iowait++;
>  				delayacct_blkio_start();
>  			}
>  		}
> @@ -10029,7 +10034,7 @@ void __init sched_init(void)
>  #endif
>  #endif /* CONFIG_SMP */
>  		hrtick_rq_init(rq);
> -		atomic_set(&rq->nr_iowait, 0);
> +		rq->nr_iowait = 0;

I checked that both ttwu_do_activate() and __schedule() have the rq lock
held, but I couldn't find it for this. Is it under the assumption that
the rq is in a pre-init state (maybe because scheduler_running = 0?) so
no lock is needed?

>  
>  #ifdef CONFIG_SCHED_CORE
>  		rq->core = rq;
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index af7952f12e6c..b970b6c6151e 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -224,7 +224,7 @@ void account_idle_time(u64 cputime)
>  	u64 *cpustat = kcpustat_this_cpu->cpustat;
>  	struct rq *rq = this_rq();
>  
> -	if (atomic_read(&rq->nr_iowait) > 0)
> +	if (rq->nr_iowait)
>  		cpustat[CPUTIME_IOWAIT] += cputime;
>  	else
>  		cpustat[CPUTIME_IDLE] += cputime;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 001fe047bd5d..a1222a4bdc7b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1049,7 +1049,7 @@ struct rq {
>  	u64			clock_idle_copy;
>  #endif
>  
> -	atomic_t		nr_iowait;
> +	unsigned int		nr_iowait;
>  
>  #ifdef CONFIG_SCHED_DEBUG
>  	u64 last_seen_need_resched_ns;

Re: [PATCH 1/2] sched/core: switch struct rq->nr_iowait to an unsigned int

[Jens Axboe]

On 2/27/24 4:05 PM, David Wei wrote:
>> @@ -4354,8 +4354,13 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>>  		cpu = select_task_rq(p, p->wake_cpu, wake_flags | WF_TTWU);
>>  		if (task_cpu(p) != cpu) {
>>  			if (p->in_iowait) {
>> +				struct rq *rq = task_rq(p);
>> +				struct rq_flags rf;
>> +
>> +				rq_lock(rq, &rf);
>> +				task_rq(p)->nr_iowait--;
> 
> Could this use rq directly, or does it not matter?

It certainly could, I'll make that edit. Same thing, but may as well use
the variable as defined. Also makes it clear we're modifying the one
we've locked.

>> @@ -10029,7 +10034,7 @@ void __init sched_init(void)
>>  #endif
>>  #endif /* CONFIG_SMP */
>>  		hrtick_rq_init(rq);
>> -		atomic_set(&rq->nr_iowait, 0);
>> +		rq->nr_iowait = 0;
> 
> I checked that both ttwu_do_activate() and __schedule() have the rq lock
> held, but I couldn't find it for this. Is it under the assumption that
> the rq is in a pre-init state (maybe because scheduler_running = 0?) so
> no lock is needed?

This is run at boot time (it's __init), so it's before anything is
running.

-- 
Jens Axboe

[PATCH 2/2] sched/core: split iowait state into two states

[Jens Axboe]

iowait is a bogus metric, but it's helpful in the sense that it allows
short waits to not enter sleep states that have a higher exit latency
than we would've picked for iowait'ing tasks. However, it's harmless in
that lots of applications and monitoring assumes that iowait is busy
time, or otherwise use it as a health metric. Particularly for async
IO it's entirely nonsensical.

Split the iowait part into two parts - one that tracks whether we need
boosting for short waits, and one that says we need to account the task
as such. ->in_iowait_acct nests inside of ->in_iowait, both for
efficiency reasons, but also so that the relationship between the two
is clear. A waiter may set ->in_wait alone and not care about the
accounting.

Existing users of nr_iowait() for accounting purposes are switched to
use nr_iowait_acct(), which leaves the governor using nr_iowait() as it
only cares about iowaiters, not the accounting side.

io_schedule_prepare() and io_schedule_finish() are changed to return
a simple mask of two state bits, as we now have more than one state to
manage. Outside of that, no further changes are needed to suppor this
generically.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 arch/s390/appldata/appldata_base.c |  2 +-
 arch/s390/appldata/appldata_os.c   |  2 +-
 fs/proc/stat.c                     |  2 +-
 include/linux/sched.h              |  6 +++++
 include/linux/sched/stat.h         | 10 +++++--
 kernel/sched/core.c                | 42 ++++++++++++++++++++++++------
 kernel/sched/sched.h               |  1 +
 kernel/time/tick-sched.c           |  6 ++---
 8 files changed, 55 insertions(+), 16 deletions(-)

diff --git a/arch/s390/appldata/appldata_base.c b/arch/s390/appldata/appldata_base.c
index c2978cb03b36..6844b5294a8b 100644
--- a/arch/s390/appldata/appldata_base.c
+++ b/arch/s390/appldata/appldata_base.c
@@ -423,4 +423,4 @@ EXPORT_SYMBOL_GPL(si_swapinfo);
 #endif
 EXPORT_SYMBOL_GPL(nr_threads);
 EXPORT_SYMBOL_GPL(nr_running);
-EXPORT_SYMBOL_GPL(nr_iowait);
+EXPORT_SYMBOL_GPL(nr_iowait_acct);
diff --git a/arch/s390/appldata/appldata_os.c b/arch/s390/appldata/appldata_os.c
index a363d30ce739..fa4b278aca6c 100644
--- a/arch/s390/appldata/appldata_os.c
+++ b/arch/s390/appldata/appldata_os.c
@@ -100,7 +100,7 @@ static void appldata_get_os_data(void *data)
 
 	os_data->nr_threads = nr_threads;
 	os_data->nr_running = nr_running();
-	os_data->nr_iowait  = nr_iowait();
+	os_data->nr_iowait  = nr_iowait_acct();
 	os_data->avenrun[0] = avenrun[0] + (FIXED_1/200);
 	os_data->avenrun[1] = avenrun[1] + (FIXED_1/200);
 	os_data->avenrun[2] = avenrun[2] + (FIXED_1/200);
diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index da60956b2915..149be7a884fb 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -180,7 +180,7 @@ static int show_stat(struct seq_file *p, void *v)
 		(unsigned long long)boottime.tv_sec,
 		total_forks,
 		nr_running(),
-		nr_iowait());
+		nr_iowait_acct());
 
 	seq_put_decimal_ull(p, "softirq ", (unsigned long long)sum_softirq);
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ffe8f618ab86..1e198e268df1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -922,7 +922,13 @@ struct task_struct {
 
 	/* Bit to tell TOMOYO we're in execve(): */
 	unsigned			in_execve:1;
+	/* task is in iowait */
 	unsigned			in_iowait:1;
+	/*
+	 * task is in iowait and should be accounted as such. can only be set
+	 * if ->in_iowait is also set.
+	 */
+	unsigned			in_iowait_acct:1;
 #ifndef TIF_RESTORE_SIGMASK
 	unsigned			restore_sigmask:1;
 #endif
diff --git a/include/linux/sched/stat.h b/include/linux/sched/stat.h
index 0108a38bb64d..7c48a35f98ee 100644
--- a/include/linux/sched/stat.h
+++ b/include/linux/sched/stat.h
@@ -19,8 +19,14 @@ DECLARE_PER_CPU(unsigned long, process_counts);
 extern int nr_processes(void);
 extern unsigned int nr_running(void);
 extern bool single_task_running(void);
-extern unsigned int nr_iowait(void);
-extern unsigned int nr_iowait_cpu(int cpu);
+extern unsigned int nr_iowait_acct(void);
+extern unsigned int nr_iowait_acct_cpu(int cpu);
+unsigned int nr_iowait_cpu(int cpu);
+
+enum {
+	TASK_IOWAIT		= 1,
+	TASK_IOWAIT_ACCT	= 2,
+};
 
 static inline int sched_info_on(void)
 {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ecc6c26096e5..003208c758e4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3790,6 +3790,8 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 	if (p->in_iowait) {
 		delayacct_blkio_end(p);
 		task_rq(p)->nr_iowait--;
+		if (p->in_iowait_acct)
+			task_rq(p)->nr_iowait_acct--;
 	}
 
 	activate_task(rq, p, en_flags);
@@ -4359,6 +4361,8 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 
 				rq_lock(rq, &rf);
 				task_rq(p)->nr_iowait--;
+				if (p->in_iowait_acct)
+					task_rq(p)->nr_iowait_acct--;
 				rq_unlock(rq, &rf);
 				delayacct_blkio_end(p);
 			}
@@ -5466,9 +5470,9 @@ unsigned long long nr_context_switches(void)
  * it does become runnable.
  */
 
-unsigned int nr_iowait_cpu(int cpu)
+unsigned int nr_iowait_acct_cpu(int cpu)
 {
-	return cpu_rq(cpu)->nr_iowait;
+	return cpu_rq(cpu)->nr_iowait_acct;
 }
 
 /*
@@ -5501,16 +5505,21 @@ unsigned int nr_iowait_cpu(int cpu)
  * Task CPU affinities can make all that even more 'interesting'.
  */
 
-unsigned int nr_iowait(void)
+unsigned int nr_iowait_acct(void)
 {
 	unsigned int i, sum = 0;
 
 	for_each_possible_cpu(i)
-		sum += nr_iowait_cpu(i);
+		sum += nr_iowait_acct_cpu(i);
 
 	return sum;
 }
 
+unsigned int nr_iowait_cpu(int cpu)
+{
+	return cpu_rq(cpu)->nr_iowait;
+}
+
 #ifdef CONFIG_SMP
 
 /*
@@ -6687,6 +6696,8 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 
 			if (prev->in_iowait) {
 				rq->nr_iowait++;
+				if (prev->in_iowait_acct)
+					rq->nr_iowait_acct++;
 				delayacct_blkio_start();
 			}
 		}
@@ -8989,18 +9000,32 @@ int __sched yield_to(struct task_struct *p, bool preempt)
 }
 EXPORT_SYMBOL_GPL(yield_to);
 
+/*
+ * Returns a token which is comprised if the two bits of iowait wait state -
+ * one is whether we're making ourselves as in iowait for cpufreq reasons,
+ * and the other is if the task should be accounted as such.
+ */
 int io_schedule_prepare(void)
 {
-	int old_iowait = current->in_iowait;
+	int old_wait_flags = 0;
+
+	if (current->in_iowait)
+		old_wait_flags |= TASK_IOWAIT;
+	if (current->in_iowait_acct)
+		old_wait_flags |= TASK_IOWAIT_ACCT;
 
 	current->in_iowait = 1;
+	current->in_iowait_acct = 1;
 	blk_flush_plug(current->plug, true);
-	return old_iowait;
+	return old_wait_flags;
 }
 
-void io_schedule_finish(int token)
+void io_schedule_finish(int old_wait_flags)
 {
-	current->in_iowait = token;
+	if (!(old_wait_flags & TASK_IOWAIT))
+		current->in_iowait = 0;
+	if (!(old_wait_flags & TASK_IOWAIT_ACCT))
+		current->in_iowait_acct = 0;
 }
 
 /*
@@ -10034,6 +10059,7 @@ void __init sched_init(void)
 #endif
 #endif /* CONFIG_SMP */
 		hrtick_rq_init(rq);
+		rq->nr_iowait_acct = 0;
 		rq->nr_iowait = 0;
 
 #ifdef CONFIG_SCHED_CORE
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a1222a4bdc7b..ee390fc664ae 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1049,6 +1049,7 @@ struct rq {
 	u64			clock_idle_copy;
 #endif
 
+	unsigned int		nr_iowait_acct;
 	unsigned int		nr_iowait;
 
 #ifdef CONFIG_SCHED_DEBUG
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 01fb50c1b17e..f6709d543dac 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -669,7 +669,7 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
 	delta = ktime_sub(now, ts->idle_entrytime);
 
 	write_seqcount_begin(&ts->idle_sleeptime_seq);
-	if (nr_iowait_cpu(smp_processor_id()) > 0)
+	if (nr_iowait_acct_cpu(smp_processor_id()) > 0)
 		ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
 	else
 		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
@@ -742,7 +742,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 
 	return get_cpu_sleep_time_us(ts, &ts->idle_sleeptime,
-				     !nr_iowait_cpu(cpu), last_update_time);
+				     !nr_iowait_acct_cpu(cpu), last_update_time);
 }
 EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
 
@@ -768,7 +768,7 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 
 	return get_cpu_sleep_time_us(ts, &ts->iowait_sleeptime,
-				     nr_iowait_cpu(cpu), last_update_time);
+				     nr_iowait_acct_cpu(cpu), last_update_time);
 }
 EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
 
-- 
2.43.0

Re: [PATCHSET v2 0/2] Split iowait into two states

[Jens Axboe]

On 2/27/24 2:06 PM, Jens Axboe wrote:
> I haven't been able to properly benchmark patch 1, as the atomics are
> noise in any workloads that approximate normality. I can certainly
> concoct a synthetic test case if folks are interested. My gut says that
> we're trading 3 fast path atomics for none, and with the 4th case
> _probably_ being way less likely. There we grab the rq lock.

OK, so on Chris's suggestion, I tried his schbench to exercise the
scheduling side. It's very futex intensive, so I hacked up futex to set
iowait state when sleeping. I also added simple accounting to that path
so I knew how many times it ran. A run of:

./schbench -m 60 -t 10 -p 8

on a 2 socket Intel(R) Xeon(R) Platinum 8458P with 176 threads, there's
no regression in performance and try_to_wake_up() locking the rq of the
task being scheduled in from another CPU doesn't seem to register much.
On the previous run, I saw 2.21% there and now it's 2.36%. But it was
also a better performing run, which may have lead to the increase.

Each run takes 30 seconds, and during that time I see around 290-310M
hits of that path, or about ~10M/sec. Without modifying futex to use
iowait, we obviously rarely hit it. About 200 times for a run, which
makes sense as we're not really doing IO.

Anyway, just some data on this. If I leave the futex/pipe iowait in and
run the same test, I see no discernable difference in profiles. In fact,
the highest cost across the tests is bringing in the task->in_iowait
cacheline.

-- 
Jens Axboe