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Date: Tue, 9 Jun 2015 11:29:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Huang Rui <ray.huang@amd.com>
Cc: Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, Len Brown <lenb@kernel.org>,
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        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Fengguang Wu <fengguang.wu@intel.com>, Aaron Lu <aaron.lu@intel.com>,
        Suravee Suthikulanit <suravee.suthikulpanit@amd.com>,
        Tony Li <tony.li@amd.com>, Ken Xue <ken.xue@amd.com>
Subject: Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a
 configurable timer
Message-ID: <20150609092952.GR3644@twins.programming.kicks-ass.net>
References: <1433819621-15093-1-git-send-email-ray.huang@amd.com>
 <1433819621-15093-4-git-send-email-ray.huang@amd.com>
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On Tue, Jun 09, 2015 at 11:13:40AM +0800, Huang Rui wrote:
> +static void delay_mwaitx(unsigned long __loops)
> +{
> +	u32 end, now, delay, addr;
> +
> +	delay = __loops;
> +	rdtsc_barrier();
> +	rdtscl(end);
> +	end += delay;
> +
> +	while (1) {
> +		__monitorx(&addr, 0, 0);
> +		mwaitx(delay, true);
> +
> +		rdtsc_barrier();
> +		rdtscl(now);
> +		if (end <= now)
> +			break;
> +		delay = end - now;
> +	}

How about you think instead and do something like:

	rdtsc(start);
	rdtsc_barrier();

	for (;;) {
		delay = min(MWAIT_MAX_LOOPS, loops);

		__monitorx(&addr, 0, 0);
		mwaitx(delay, true);

		rdtsc_barrier();
		rdtsc(end);
		rdtsc_barrier();

		loops -= end - start;
		if (loops <= 0)
			break;

		start = end;
	}

> +}
> +
> +/*
>   * Since we calibrate only once at boot, this
>   * function should be set once at boot and not changed
>   */
> @@ -118,7 +145,12 @@ int read_current_timer(unsigned long *timer_val)
>  
>  void __delay(unsigned long loops)
>  {
> -	delay_fn(loops);
> +	if (loops > MWAITX_MAX_LOOPS ||
> +			!static_cpu_has_safe(X86_FEATURE_MWAITT) ||
> +			boot_option_delay != DELAY_MWAITX)
> +		delay_fn(loops);
> +	else
> +		delay_mwaitx(loops);
>  }

Then you can do away with that fallback entirely.