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

Heiko Carstens
2023-05-09T06:16:20Z

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.