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Date:   Tue, 9 Apr 2019 13:28:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
        Ashok Raj <ashok.raj@intel.com>,
        Andi Kleen <andi.kleen@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Ricardo Neri <ricardo.neri@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Tony Luck <tony.luck@intel.com>,
        Clemens Ladisch <clemens@ladisch.de>,
        Arnd Bergmann <arnd@arndb.de>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Jan Kiszka <jan.kiszka@siemens.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Nayna Jain <nayna@linux.ibm.com>
Subject: Re: [RFC PATCH v2 12/14] x86/watchdog/hardlockup/hpet: Determine if
 HPET timer caused NMI
Message-ID: <20190409112817.GT4038@hirez.programming.kicks-ass.net>
References: <1551283518-18922-1-git-send-email-ricardo.neri-calderon@linux.intel.com>
 <1551283518-18922-13-git-send-email-ricardo.neri-calderon@linux.intel.com>
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On Wed, Feb 27, 2019 at 08:05:16AM -0800, Ricardo Neri wrote:
> @@ -62,7 +67,18 @@ static inline void set_comparator(struct hpet_hld_data *hdata,
>  static void kick_timer(struct hpet_hld_data *hdata, bool force)
>  {
>  	bool kick_needed = force || !(hdata->flags & HPET_DEV_PERI_CAP);
> -	unsigned long new_compare, count;
> +	unsigned long tsc_curr, tsc_delta, new_compare, count;
> +
> +	/* Start obtaining the current TSC and HPET counts. */
> +	tsc_curr = rdtsc();
> +
> +	if (kick_needed)
> +		count = get_count();
> +
> +	tsc_delta = (unsigned long)watchdog_thresh * (unsigned long)tsc_khz
> +		    * 1000L;
> +	hdata->tsc_next = tsc_curr + tsc_delta;
> +	hdata->tsc_next_error = tsc_delta >> 6;

What do we need a per hld_data tsc_next_error for? It is basically a
global 'constant'.

>  	/*
>  	 * Update the comparator in increments of watch_thresh seconds relative
> @@ -74,8 +90,6 @@ static void kick_timer(struct hpet_hld_data *hdata, bool force)
>  	 */
>  
>  	if (kick_needed) {
> -		count = get_count();
> -
>  		new_compare = count + watchdog_thresh * hdata->ticks_per_second;
>  
>  		set_comparator(hdata, new_compare);
> @@ -147,6 +161,14 @@ static void set_periodic(struct hpet_hld_data *hdata)
>   */
>  static bool is_hpet_wdt_interrupt(struct hpet_hld_data *hdata)
>  {
> +	if (smp_processor_id() == hdata->handling_cpu) {
> +		unsigned long tsc_curr;

TSC is u64

> +
> +		tsc_curr = rdtsc();
> +		if (abs(tsc_curr - hdata->tsc_next) < hdata->tsc_next_error)
> +			return true;

You can write that as:

		(tsc_curr - hdata->tsc_next) + tsc_error < 2*tsc_error

which doesn't contain any branches what so ever.

> +	}
> +
>  	return false;
>  }