Re: [PATCH v3 1/2] timers: Fix usleep_range() in the context of wake_up_process()

Thomas Gleixner
2016-10-21T07:08:23Z

On Thu, 20 Oct 2016, Doug Anderson wrote:
> > And this is broken because schedule_hrtimeout_range() returns with task
> > state TASK_RUNNING so the next schedule_hrtimeout_range() will return
> > -EINTR again because nothing sets the task state back to UNINTERRUPTIBLE.
> > So instead of sleeping you busy loop.
>
> That explains the mystery of why my delays were always so precise in
> the test. I was a bit baffled by the fact that I was ending up with a
> delay of almost exactly 50001 or 50002 in my test case.

Well, if you see something as a mystery or you are baffled, then you
certainly should figure out why and not just declare: Works for me, but I
don't know why. That's stuff which comes back to hunt you sooner than
later.

> > What you really want to do is something like this:
> >
> > void __sched usleep_range(unsigned long min, unsigned long max)
> > {
> > ktime_t expires = ktime_add_us(ktime_get(), min * NSEC_PER_USEC);
> > ktime_t delta = ktime_set(0, (u64)(max - min) * NSEC_PER_USEC);
> >
> > for (;;) {
> > __set_current_state(TASK_UNINTERRUPTIBLE);
> > /* Do not return before the requested sleep time has elapsed */
> > if (!schedule_hrtimeout_range(&expires, delta, HRTIMER_MODE_ABS))
> > break;
> > }
> > }
>
> The above mostly works other than some small fixups. Thanks!

Yeah, delta is u64. Was too lazy to look it up.

> ...other than small fixups, the one substantive change I'll make is to
> actually check the timeout in the loop above. If I use your code with
> my test, I find that, even though I'm waking up every millisecond I
> still end up not exiting the loop until the upper bound of the delay.
>
> Presumably this happens because:
>
> a_time_in_the_past = ktime_sub_us(ktime_get(), 10);
> schedule_hrtimeout_range(&a_time_in_the_past, delta, HRTIMER_MODE_ABS)
>
> ...delays "delta" nano seconds even though "a_time_in_the_past" is in
> the past. I presume that behavior is OK (let me know if it's not).

It does not delay delta nano seconds. It delays to

(now - 10us) + deltans

The core is free to use the full range for batching or extending idle sleep
times and with a delta > 10us it's expected and correct behaviour.

> In the case of usleep_range() if we're waking up anyway, it seems
> sensible to spend a few cycles to see if the minimum bound has already
> past.

We really can do without that. You are over engineering for something which
is pointless in 99.9% of the use cases. That stuff is what causes bloat. A
few lines of code here and there, which sums up in the end.

Thanks,

tglx