Re: [PATCH] cpuidle: avoid using smp_processor_id() in preemptible code (nr_iowait_cpu) v4

Peter Zijlstra
2010-06-25T14:39:33Z

On Thu, 2010-06-17 at 09:29 +0300, Sergey Senozhatsky wrote:
> Fix
>
> BUG: using smp_processor_id() in preemptible [00000000] code: s2disk/3392

> The initial fix was to use get_cpu/put_cpu in nr_iowait_cpu. However,
> Arjan stated that "the bug is that it needs to be nr_iowait_cpu(int cpu)".
>
> This patch introduces nr_iowait_cpu(int cpu) and changes to its callers.
>
> Arjan also pointed out that we can't use get_cpu/put_cpu in update_ts_time_stats
> since we "pick the current cpu, rather than the one denoted by ts" in that case.
> To match given *ts and cpu denoted by *ts we use new field in the struct tick_sched: int cpu.

> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index b232ccc..db14691 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -51,6 +51,7 @@ struct tick_sched {
> unsigned long check_clocks;
> enum tick_nohz_mode nohz_mode;
> ktime_t idle_tick;
> + int cpu;
> int inidle;
> int tick_stopped;
> unsigned long idle_jiffies;

> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 1d7b9bc..1907037 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -38,6 +38,9 @@ static ktime_t last_jiffies_update;
>
> struct tick_sched *tick_get_tick_sched(int cpu)
> {
> + /*FIXME: Arjan van de Ven:
> + can we do this bit once, when the ts structure gets initialized?*/
> + per_cpu(tick_cpu_sched, cpu).cpu = cpu;
> return &per_cpu(tick_cpu_sched, cpu);
> }

> @@ -161,7 +164,7 @@ update_ts_time_stats(struct tick_sched *ts, ktime_t now, u64 *last_update_time)
> if (ts->idle_active) {
> delta = ktime_sub(now, ts->idle_entrytime);
> ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> - if (nr_iowait_cpu() > 0)
> + if (nr_iowait_cpu(ts->cpu) > 0)
> ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> ts->idle_entrytime = now;
> }

This all seems extremely silly, why not something like:

---
kernel/time/tick-sched.c | 16 ++++++++--------
1 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 5f171f0..1363d3a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -154,14 +154,14 @@ static void tick_nohz_update_jiffies(ktime_t now)
* Updates the per cpu time idle statistics counters
*/
static void
-update_ts_time_stats(struct tick_sched *ts, ktime_t now, u64 *last_update_time)
+update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_update_time)
{
ktime_t delta;

if (ts->idle_active) {
delta = ktime_sub(now, ts->idle_entrytime);
ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
- if (nr_iowait_cpu() > 0)
+ if (nr_iowait_cpu(cpu) > 0)
ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
ts->idle_entrytime = now;
}
@@ -175,19 +175,19 @@ static void tick_nohz_stop_idle(int cpu, ktime_t now)
{
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);

- update_ts_time_stats(ts, now, NULL);
+ update_ts_time_stats(cpu, ts, now, NULL);
ts->idle_active = 0;

sched_clock_idle_wakeup_event(0);
}

-static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
+static ktime_t tick_nohz_start_idle(int cpu, struct tick_sched *ts)
{
ktime_t now;

now = ktime_get();

- update_ts_time_stats(ts, now, NULL);
+ update_ts_time_stats(cpu, ts, now, NULL);

ts->idle_entrytime = now;
ts->idle_active = 1;
@@ -216,7 +216,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
if (!tick_nohz_enabled)
return -1;

- update_ts_time_stats(ts, ktime_get(), last_update_time);
+ update_ts_time_stats(cpu, ts, ktime_get(), last_update_time);

return ktime_to_us(ts->idle_sleeptime);
}
@@ -242,7 +242,7 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
if (!tick_nohz_enabled)
return -1;

- update_ts_time_stats(ts, ktime_get(), last_update_time);
+ update_ts_time_stats(cpu, ts, ktime_get(), last_update_time);

return ktime_to_us(ts->iowait_sleeptime);
}
@@ -284,7 +284,7 @@ void tick_nohz_stop_sched_tick(int inidle)
*/
ts->inidle = 1;

- now = tick_nohz_start_idle(ts);
+ now = tick_nohz_start_idle(cpu, ts);

/*
* If this cpu is offline and it is the one which updates