Re: [PATCH v2 2/2] iopoll: Do not use timekeeping in read_poll_timeout_atomic()

[Geert Uytterhoeven]

Hi Ulf,

On Thu, May 11, 2023 at 12:27 PM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
> On Wed, 10 May 2023 at 15:23, Geert Uytterhoeven
> <geert+renesas@xxxxxxxxx> wrote:
> > read_poll_timeout_atomic() uses ktime_get() to implement the timeout
> > feature, just like its non-atomic counterpart.  However, there are
> > several issues with this, due to its use in atomic contexts:
> >
> >   1. When called in the s2ram path (as typically done by clock or PM
> >      domain drivers), timekeeping may be suspended, triggering the
> >      WARN_ON(timekeeping_suspended) in ktime_get():
> >
> >         WARNING: CPU: 0 PID: 654 at kernel/time/timekeeping.c:843 ktime_get+0x28/0x78
> >
> >      Calling ktime_get_mono_fast_ns() instead of ktime_get() would get
> >      rid of that warning.  However, that would break timeout handling,
> >      as (at least on systems with an ARM architectured timer), the time
> >      returned by ktime_get_mono_fast_ns() does not advance while
> >      timekeeping is suspended.
> >      Interestingly, (on the same ARM systems) the time returned by
> >      ktime_get() does advance while timekeeping is suspended, despite
> >      the warning.
>
> Interesting, looks like we should spend some time to further
> investigate this behaviour.

Probably, I was a bit surprised by this behavior, too.

> >   2. Depending on the actual clock source, and especially before a
> >      high-resolution clocksource (e.g. the ARM architectured timer)
> >      becomes available, time may not advance in atomic contexts, thus
> >      breaking timeout handling.
> >
> > Fix this by abandoning the idea that one can rely on timekeeping to
> > implement timeout handling in all atomic contexts, and switch from a
> > global time-based to a locally-estimated timeout handling.  In most
> > (all?) cases the timeout condition is exceptional and an error
> > condition, hence any additional delays due to underestimating wall clock
> > time are irrelevant.
>
> I wonder if this isn't an oversimplification of the situation. Don't
> we have timeout-error-conditions that we expected to happen quite
> frequently?

We may have some.  But they definitely do not happen when time
does not advance, or they would have been mitigated long ago
(the loop would never terminate).

> If so, in these cases, we really don't want to continue looping longer
> than actually needed, as then we will remain in the atomic context
> longer than necessary.
>
> I guess some information about how big these additional delays could
> be, would help to understand better. Of course, it's not entirely easy
> to get that data, but did you run some tests to see how this changes?

I did some timings (when timekeeping is available), and the differences
are rather minor.  The delay and timeout parameters are in µs, and
1 µs is already a few orders of magnitude larger than the cycle time
of a contemporary CPU.

Under-estimates are due to the time spent in op() (depends on the
user, typical use is a hardware device register read), udelay()
(architecture/platform-dependent accuracy), and general loop overhead.

> > Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> > ---
> > Alternatively, one could use a mixed approach (use both
> > ktime_get_mono_fast_ns() and a local (under)estimate, and timeout on the
> > earliest occasion), but I think that would complicate things without
> > much gain.
>
> Another option could be to provide two different polling APIs for the
> atomic use-case.
>
> One that keeps using ktime, which is more accurate and generally
> favourable - and another, along the lines of what you propose, that
> should be used by those that can't rely on timekeeping.

At the risk of people picking the wrong one, leading to hard to
find bugs?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds