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Date:   Tue, 9 May 2023 08:13:59 +0200
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
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Subject: Re: [RFC][PATCH 6/9] s390/time: Provide sched_clock_noinstr()
Message-ID: <ZFnkp6dlOuJqm2II@osiris>
References: <20230508211951.901961964@infradead.org>
 <20230508213147.786238095@infradead.org>
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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.