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Date: Sat, 19 Oct 2013 17:17:21 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: Oleg Nesterov <oleg@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Frederic Weisbecker <fweisbec@...il.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>,
Andrew Morton <akpm@...ux-foundation.org>,
Arjan van de Ven <arjan@...ux.intel.com>
Subject: [PATCH 5/5] nohz: Synchronize sleep time stats with seqlock
When some call site uses get_cpu_*_time_us() to read a sleeptime
stat, it deduces the total sleeptime by adding the pending time
to the last sleeptime snapshot if the CPU target is idle.
Namely this sums up to:
sleeptime = ts($CPU)->idle_sleeptime;
if (ts($CPU)->idle_active)
sleeptime += NOW() - ts($CPU)->idle_entrytime
But this only works if idle_sleeptime, idle_entrytime and idle_active are
read and updated under some disciplined order.
Lets consider the following scenario:
CPU 0 CPU 1
(seq 1) ts(CPU 0)->idle_active = 1
ts(CPU 0)->idle_entrytime = NOW()
(seq 2) sleeptime = NOW() - ts(CPU 0)->idle_entrytime
ts(CPU 0)->idle_sleeptime += sleeptime sleeptime = ts(CPU 0)->idle_sleeptime;
if (ts(CPU 0)->idle_active)
ts(CPU 0)->idle_entrytime = NOW() sleeptime += NOW() - ts(CPU 0)->idle_entrytime
The resulting value of sleeptime in CPU 1 can vary depending of some
ordering scenario:
* If it sees the value of idle_entrytime after seq 1 and the value of idle_sleeptime
after seq 2, the value of sleeptime will be buggy because it accounts the delta twice,
so it will be too high.
* If it sees the value of idle_entrytime after seq 2 and the value of idle_sleeptime
after seq 1, the value of sleeptime will be buggy because it misses the delta, so it
will be too low.
* If it sees the value of idle_entrytime and idle_sleeptime, both as seen after seq 1 or 2,
the value will be correct.
Some more tricky scenario can also happen if idle_active value is read from a former sequence.
Hence we must honour the following constraints:
- idle_sleeptime, idle_active and idle_entrytime must be updated and read
under some correctly enforced SMP ordering
- The three variable values as read by CPU 1 must belong to the same update
sequences from CPU 0. The update sequences must be delimited such that the
resulting three values after a sequence completion produce a coherent result
together when read from the CPU 1.
- We need to prevent from fetching middle-state sequence values.
The ideal solution to implement this synchronization is to use a seqcount. Lets
use one here around these three values to enforce sequence synchronization between
updates and read.
This fixes a reported bug where non-monotonic sleeptime stats are returned by /proc/stat
when it is frequently read. And potential cpufreq governor bugs.
Reported-by: Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>
Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.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 | 1 +
kernel/time/tick-sched.c | 22 ++++++++++++++++------
2 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/include/linux/tick.h b/include/linux/tick.h
index 7a66bf6..4d6131e 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -65,6 +65,7 @@ struct tick_sched {
ktime_t idle_waketime;
ktime_t idle_exittime;
ktime_t idle_sleeptime;
+ seqcount_t idle_sleeptime_seq;
ktime_t sleep_length;
unsigned long last_jiffies;
unsigned long next_jiffies;
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a038d4e..da53cbf 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -407,9 +407,11 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
ktime_t delta;
/* Updates the per cpu time idle statistics counters */
+ write_seqcount_begin(&ts->idle_sleeptime_seq);
delta = ktime_sub(now, ts->idle_entrytime);
ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
ts->idle_active = 0;
+ write_seqcount_end(&ts->idle_sleeptime_seq);
sched_clock_idle_wakeup_event(0);
}
@@ -418,9 +420,13 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
{
ktime_t now = ktime_get();
+ write_seqcount_begin(&ts->idle_sleeptime_seq);
ts->idle_entrytime = now;
ts->idle_active = 1;
+ write_seqcount_end(&ts->idle_sleeptime_seq);
+
sched_clock_idle_sleep_event();
+
return now;
}
@@ -442,6 +448,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
ktime_t now, idle;
+ unsigned int seq;
u64 iowait;
if (!tick_nohz_enabled)
@@ -451,12 +458,16 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
if (last_update_time)
*last_update_time = ktime_to_us(now);
- if (ts->idle_active) {
- ktime_t delta = ktime_sub(now, ts->idle_entrytime);
- idle = ktime_add(ts->idle_sleeptime, delta);
- } else {
+ do {
+ ktime_t delta;
+
+ seq = read_seqcount_begin(&ts->idle_sleeptime_seq);
idle = ts->idle_sleeptime;
- }
+ if (ts->idle_active) {
+ delta = ktime_sub(now, ts->idle_entrytime);
+ idle = ktime_add(idle, delta);
+ }
+ } while (read_seqcount_retry(&ts->idle_sleeptime_seq, seq));
iowait = get_cpu_iowait_time_us(cpu, NULL);
@@ -468,7 +479,6 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
idle = ktime_sub_us(idle, iowait);
return ktime_to_us(idle);
-
}
EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);
--
1.8.3.1
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