Message-ID: <20240819154259.215504-5-axboe@kernel.dk>
Date: Mon, 19 Aug 2024 09:39:49 -0600
From: Jens Axboe <axboe@...nel.dk>
To: linux-kernel@...r.kernel.org
Cc: peterz@...radead.org,
	tglx@...utronix.de,
	Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 4/4] sched/core: split iowait state into two states

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 would've otherwise have been 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 the task
needs boosting for short waits, and one that marks whether the task
needs to be accounted 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.

Utilize that there's enough space in rq->nr_iowait to store both values
in there, shifting the accounting side by half the size of the type.
Thank you to Thomas Gleixner for that [1] suggestion.

[1] https://lore.kernel.org/lkml/87sf1b6o9w.ffs@tglx/

Signed-off-by: Jens Axboe <axboe@...nel.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         |  5 ++--
 kernel/sched/core.c                | 45 +++++++++++++++++++++++-------
 kernel/sched/sched.h               |  1 +
 kernel/time/tick-sched.c           |  6 ++--
 8 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/arch/s390/appldata/appldata_base.c b/arch/s390/appldata/appldata_base.c
index 91a30e017d65..5b21225405fd 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 c1a65e19a3ac..1b40847b0a80 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -953,7 +953,13 @@ 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..31e8a44b3d71 100644
--- a/include/linux/sched/stat.h
+++ b/include/linux/sched/stat.h
@@ -19,8 +19,9 @@ 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);
+unsigned int nr_iowait_acct(void);
+unsigned int nr_iowait_acct_cpu(int cpu);
+unsigned int nr_iowait_cpu(int cpu);
 
 static inline int sched_info_on(void)
 {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d6c494e33206..a082919caaf8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3541,19 +3541,33 @@ static inline bool rq_has_pinned_tasks(struct rq *rq)
 
 #endif /* !CONFIG_SMP */
 
+/* Shift half the size of the type, atomic_long_t */
+#define IOWAIT_SHIFT	(4 * sizeof(atomic_long_t))
+
+/*
+ * Store iowait and iowait_acct state in the same variable. The lower bits
+ * hold the iowait state, and the upper bits hold the iowait_acct state.
+ */
 static void task_iowait_inc(struct task_struct *p)
 {
-	atomic_long_inc(&task_rq(p)->nr_iowait);
+	long val = 1 + ((long) p->in_iowait_acct << IOWAIT_SHIFT);
+	atomic_long_add(val, &task_rq(p)->nr_iowait);
 }
 
 static void task_iowait_dec(struct task_struct *p)
 {
-	atomic_long_dec(&task_rq(p)->nr_iowait);
+	long val = 1 + ((long) p->in_iowait_acct << IOWAIT_SHIFT);
+	atomic_long_sub(val, &task_rq(p)->nr_iowait);
 }
 
 int rq_iowait(struct rq *rq)
 {
-	return atomic_long_read(&rq->nr_iowait);
+	return atomic_long_read(&rq->nr_iowait) & ((1UL << IOWAIT_SHIFT) - 1);
+}
+
+int rq_iowait_acct(struct rq *rq)
+{
+	return atomic_long_read(&rq->nr_iowait) >> IOWAIT_SHIFT;
 }
 
 static void
@@ -5295,7 +5309,12 @@ 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 rq_iowait_acct(cpu_rq(cpu));
+}
+
+unsigned nr_iowait_cpu(int cpu)
 {
 	return rq_iowait(cpu_rq(cpu));
 }
@@ -5330,12 +5349,12 @@ 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;
 }
@@ -7415,18 +7434,24 @@ static inline void preempt_dynamic_init(void) { }
 
 #endif /* CONFIG_PREEMPT_DYNAMIC */
 
+/*
+ * Returns a token which is comprised of 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.
+ */
 long io_schedule_prepare(void)
 {
-	long old_iowait = current->in_iowait;
-
+	long token = current->in_iowait + ((long) current->in_iowait_acct << IOWAIT_SHIFT);
 	current->in_iowait = 1;
+	current->in_iowait_acct = 1;
 	blk_flush_plug(current->plug, true);
-	return old_iowait;
+	return token;
 }
 
 void io_schedule_finish(long token)
 {
-	current->in_iowait = token;
+	current->in_iowait = token & 0x01;
+	current->in_iowait_acct = token >> IOWAIT_SHIFT;
 }
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index da2e67621f39..be377ecd1301 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3580,6 +3580,7 @@ extern u64 avg_vruntime(struct cfs_rq *cfs_rq);
 extern int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se);
 
 int rq_iowait(struct rq *rq);
+int rq_iowait_acct(struct rq *rq);
 
 #ifdef CONFIG_RT_MUTEXES
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 753a184c7090..17a82771c3df 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -732,7 +732,7 @@ 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);
@@ -805,7 +805,7 @@ 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);
 
@@ -831,7 +831,7 @@ 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

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