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Date: Wed, 7 May 2014 15:41:34 +0200
From: Denys Vlasenko <dvlasenk@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: Denys Vlasenko <dvlasenk@...hat.com>,
Frederic Weisbecker <fweisbec@...il.com>,
Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Arjan van de Ven <arjan@...ux.intel.com>,
Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH 4/4 v2] nohz: Fix iowait overcounting if iowait task migrates
Before this change, if last IO-blocked task wakes up
on a different CPU, the original CPU may stay idle for much longer,
and the entire time it stays idle is accounted as iowait time.
This change adds struct tick_sched::iowait_exittime member.
On entry to idle, it is set to KTIME_MAX.
Last IO-blocked task, if migrated, sets it to current time.
Note that this can happen only once per each idle period:
new iowaiting tasks can't magically appear on idle CPU's rq.
If iowait_exittime is set, then (iowait_exittime - idle_entrytime)
gets accounted as iowait, and the remaining (now - iowait_exittime)
as "true" idle.
v2:
* Made iowait_exittime atomic64_t. This way, no locking
is necessary when setting/accessing it.
* Do more paranoid checking of iowait_exittime before using it:
now code checks that idle_entrytime <= iowait_exittime
and iowait_exittime <= now, and uses iowait_exittime only
if both are true.
Run-tested: /proc/stat counters no longer go backwards.
Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc: Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>
Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Oleg Nesterov <oleg@...hat.com>
---
include/linux/tick.h | 2 ++
kernel/sched/core.c | 19 ++++++++--
kernel/time/tick-sched.c | 91 +++++++++++++++++++++++++++++++++++++++++-------
3 files changed, 98 insertions(+), 14 deletions(-)
diff --git a/include/linux/tick.h b/include/linux/tick.h
index 4de1f9e..49f8b29 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -67,6 +67,7 @@ struct tick_sched {
ktime_t idle_exittime;
ktime_t idle_sleeptime;
ktime_t iowait_sleeptime;
+ atomic64_t iowait_exittime;
seqcount_t idle_sleeptime_seq;
ktime_t sleep_length;
unsigned long last_jiffies;
@@ -140,6 +141,7 @@ extern void tick_nohz_irq_exit(void);
extern ktime_t tick_nohz_get_sleep_length(void);
extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
+extern void tick_nohz_iowait_to_idle(int cpu);
# else /* !CONFIG_NO_HZ_COMMON */
static inline int tick_nohz_tick_stopped(void)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 268a45e..3137980 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4208,6 +4208,21 @@ EXPORT_SYMBOL_GPL(yield_to);
* This task is about to go to sleep on IO. Increment rq->nr_iowait so
* that process accounting knows that this is a task in IO wait state.
*/
+#ifdef CONFIG_NO_HZ_COMMON
+static __sched void io_wait_end(struct rq *rq)
+{
+ if (atomic_dec_and_test(&rq->nr_iowait)) {
+ if (raw_smp_processor_id() != cpu_of(rq))
+ tick_nohz_iowait_to_idle(cpu_of(rq));
+ }
+}
+#else
+static inline void io_wait_end(struct rq *rq)
+{
+ atomic_dec(&rq->nr_iowait);
+}
+#endif
+
void __sched io_schedule(void)
{
struct rq *rq = raw_rq();
@@ -4218,7 +4233,7 @@ void __sched io_schedule(void)
current->in_iowait = 1;
schedule();
current->in_iowait = 0;
- atomic_dec(&rq->nr_iowait);
+ io_wait_end(rq);
delayacct_blkio_end();
}
EXPORT_SYMBOL(io_schedule);
@@ -4234,7 +4249,7 @@ long __sched io_schedule_timeout(long timeout)
current->in_iowait = 1;
ret = schedule_timeout(timeout);
current->in_iowait = 0;
- atomic_dec(&rq->nr_iowait);
+ io_wait_end(rq);
delayacct_blkio_end();
return ret;
}
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7d0e14a..f3b214d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -411,17 +411,41 @@ enum {
IOWAIT_IDLE = 2,
};
+/* We always access iowait_exittime atomically. */
+static inline ktime_t fetch_iowait_exittime(struct tick_sched *ts)
+{
+ ktime_t v;
+
+ v.tv64 = atomic64_read(&ts->iowait_exittime);
+ return v;
+}
+
static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
{
- ktime_t delta;
+ ktime_t start, delta, end;
/* Updates the per cpu time idle statistics counters */
write_seqcount_begin(&ts->idle_sleeptime_seq);
- delta = ktime_sub(now, ts->idle_entrytime);
- if (ts->idle_active == IOWAIT_IDLE)
+ start = ts->idle_entrytime;
+ delta = ktime_sub(now, start);
+
+ if (ts->idle_active == IOWAIT_IDLE) {
+ /*
+ * If last iowaiting task on our rq wakes up on another
+ * CPU, it sets iowait_exittime.
+ * It's the only case it can satisfy "start <= end <= now".
+ */
+ end = fetch_iowait_exittime(ts);
+ if (ktime_compare(start, end) <= 0 && ktime_compare(end, now) <= 0) {
+ /* [end, now] goes to "true idle" counter */
+ delta = ktime_sub(now, end);
+ ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
+ delta = ktime_sub(end, start);
+ }
ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
- else
+ } else {
ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
+ }
ts->idle_active = 0;
write_seqcount_end(&ts->idle_sleeptime_seq);
@@ -435,6 +459,7 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
write_seqcount_begin(&ts->idle_sleeptime_seq);
ts->idle_entrytime = now;
ts->idle_active = nr_iowait_cpu(smp_processor_id()) ? IOWAIT_IDLE : TRUE_IDLE;
+ atomic64_set(&ts->iowait_exittime, KTIME_MAX);
write_seqcount_end(&ts->idle_sleeptime_seq);
sched_clock_idle_sleep_event();
@@ -442,6 +467,14 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
return now;
}
+void tick_nohz_iowait_to_idle(int cpu)
+{
+ struct tick_sched *ts = tick_get_tick_sched(cpu);
+ ktime_t now = ktime_get();
+
+ atomic64_set(&ts->iowait_exittime, now.tv64);
+}
+
/**
* get_cpu_idle_time_us - get the total idle time of a cpu
* @cpu: CPU number to query
@@ -458,7 +491,7 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
*/
u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
- struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+ struct tick_sched *ts;
ktime_t now, idle;
unsigned int seq;
@@ -469,14 +502,35 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
if (last_update_time)
*last_update_time = ktime_to_us(now);
+ ts = &per_cpu(tick_cpu_sched, cpu);
+
do {
- ktime_t delta;
+ ktime_t start, delta, iowait_exit;
seq = read_seqcount_begin(&ts->idle_sleeptime_seq);
idle = ts->idle_sleeptime;
- if (ts->idle_active == TRUE_IDLE) {
- delta = ktime_sub(now, ts->idle_entrytime);
+
+ if (ts->idle_active /* either IOWAIT_IDLE or TRUE_IDLE */) {
+ start = ts->idle_entrytime;
+
+ if (ts->idle_active == IOWAIT_IDLE) {
+ /* This idle period started as iowait */
+
+ iowait_exit = fetch_iowait_exittime(ts);
+ if (ktime_compare(start, iowait_exit) > 0 ||
+ ktime_compare(iowait_exit, now) > 0) {
+ /* And it still is (iowait_exit isn't set) */
+ goto skip;
+ }
+ /*
+ * This CPU used to be "iowait idle", but iowait task
+ * has migrated. The rest of idle time is "true idle":
+ */
+ start = iowait_exit;
+ }
+ delta = ktime_sub(now, start);
idle = ktime_add(idle, delta);
+ skip: ;
}
} while (read_seqcount_retry(&ts->idle_sleeptime_seq, seq));
@@ -500,7 +554,7 @@ EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
*/
u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
{
- struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+ struct tick_sched *ts;
ktime_t now, iowait;
unsigned int seq;
@@ -511,14 +565,27 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
if (last_update_time)
*last_update_time = ktime_to_us(now);
+ ts = &per_cpu(tick_cpu_sched, cpu);
+
do {
- ktime_t delta;
+ ktime_t start, delta, iowait_exit;
seq = read_seqcount_begin(&ts->idle_sleeptime_seq);
iowait = ts->iowait_sleeptime;
+
if (ts->idle_active == IOWAIT_IDLE) {
- delta = ktime_sub(now, ts->idle_entrytime);
- iowait = ktime_add(ts->iowait_sleeptime, delta);
+ start = ts->idle_entrytime;
+ iowait_exit = fetch_iowait_exittime(ts);
+ /*
+ * Did last iowaiting task on our rq wake up on other CPU
+ * sometime in the past, and updated ts->iowait_exittime?
+ */
+ if (ktime_compare(start, iowait_exit) > 0 ||
+ ktime_compare(iowait_exit, now) > 0) {
+ iowait_exit = now; /* no */
+ }
+ delta = ktime_sub(iowait_exit, start);
+ iowait = ktime_add(iowait, delta);
}
} while (read_seqcount_retry(&ts->idle_sleeptime_seq, seq));
--
1.8.1.4
--
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