Re: [RFC][PATCH 6/9] s390/time: Provide sched_clock_noinstr()

Peter Zijlstra
2023-05-09T06:43:21Z

On Tue, May 09, 2023 at 08:13:59AM +0200, Heiko Carstens wrote:
>
> 1;115;0cOn Mon, May 08, 2023 at 11:19:57PM +0200, Peter Zijlstra wrote:
> > With the intent to provide local_clock_noinstr(), a variant of
> > local_clock() that's safe to be called from noinstr code (with the
> > assumption that any such code will already be non-preemptible),
> > prepare for things by providing a noinstr sched_clock_noinstr()
> > function.
> >
> > Specifically, preempt_enable_*() calls out to schedule(), which upsets
> > noinstr validation efforts.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> > arch/s390/include/asm/timex.h | 13 +++++++++----
> > arch/s390/kernel/time.c | 11 ++++++++++-
> > 2 files changed, 19 insertions(+), 5 deletions(-)
> ...
> > +static __always_inline unsigned long __get_tod_clock_monotonic(void)
> > +{
> > + return get_tod_clock() - tod_clock_base.tod;
> > +}
> > +
> > /**
> > * get_clock_monotonic - returns current time in clock rate units
> > *
> > @@ -216,7 +221,7 @@ static inline unsigned long get_tod_cloc
> > unsigned long tod;
> >
> > preempt_disable_notrace();
> > - tod = get_tod_clock() - tod_clock_base.tod;
> > + tod = __get_tod_clock_monotonic();
> > preempt_enable_notrace();
> > return tod;
> > }
> ...
> > +unsigned long long noinstr sched_clock_noinstr(void)
> > +{
> > + return tod_to_ns(__get_tod_clock_monotonic());
> > +}
> > +
> > /*
> > * Scheduler clock - returns current time in nanosec units.
> > */
> > unsigned long long notrace sched_clock(void)
> > {
> > - return tod_to_ns(get_tod_clock_monotonic());
> > + unsigned long long ns;
> > + preempt_disable_notrace();
> > + ns = tod_to_ns(get_tod_clock_monotonic());
> > + preempt_enable_notrace();
> > + return ns;
> > }
> > NOKPROBE_SYMBOL(sched_clock);
>
> This disables preemption twice within sched_clock(). So this should either
> call __get_tod_clock_monotonic() instead, or the function could stay as it
> is, which I would prefer.

Duh. Will fix.