(2013年08月17日 01:46), Frederic Weisbecker wrote:
On Fri, Aug 16, 2013 at 06:26:54PM +0200, Oleg Nesterov wrote:
On 08/16, Frederic Weisbecker wrote:
On Fri, Aug 16, 2013 at 06:02:01PM +0200, Oleg Nesterov wrote:
+ do {
+ seq = read_seqcount_begin(&ts->sleeptime_seq);
+ if (ts->idle_active && nr_iowait_cpu(cpu) > 0) {
+ ktime_t delta = ktime_sub(now, ts->idle_entrytime);
+ iowait = ktime_add(ts->iowait_sleeptime, delta);
+ } else {
+ iowait = ts->iowait_sleeptime;
+ }
+ } while (read_seqcount_retry(&ts->sleeptime_seq, seq));
Unless I missread this patch, this is still racy a bit.
Suppose it is called on CPU_0 and cpu == 1. Suppose that
ts->idle_active == T and nr_iowait_cpu(cpu) == 1.
So we return iowait_sleeptime + delta.
Suppose that we call get_cpu_iowait_time_us() again. By this time
the task which incremented ->nr_iowait can be woken up on another
CPU, and it can do atomic_dec(rq->nr_iowait). So the next time
we return iowait_sleeptime, and this is not monotonic again.
Hmm, by the time it decrements nr_iowait, it returned from schedule() and
so idle had flushed the pending iowait sleeptime.
Suppose a task does io_schedule() on CPU_0, and increments the counter.
This CPU becomes idle and nr_iowait_cpu(0) == 1.
Then this task is woken up, but try_to_wake_up() selects another CPU != 0.
It returns from schedule() and decrements the same counter, it doesn't
do raw_rq/etc again. nr_iowait_cpu(0) becomes 0.
In fact the task can even migrate to another CPU right after raw_rq().
Ah I see now. So that indeed yet another race.
I am sorry for chiming in late.
That precisely the race I described here:
https://lkml.org/lkml/2013/7/2/3
I should have been more concise in my explanation. I apologize.
Should we flush that iowait to the src CPU? But then it means we must handle
concurrent updates to iowait_sleeptime, idle_sleeptime from the migration
code and from idle enter / exit.
So I fear we need a seqlock.
Or we can live with that and still account the whole idle time slept until
tick_nohz_stop_idle() to iowait if we called tick_nohz_start_idle() with nr_iowait
> 0.
All we need is just a new field in ts-> that records on which state we entered
idle.
Another approach could be to shadow ->iowait_sleeptime as
suggested here: https://lkml.org/lkml/2013/7/2/165
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