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Date: Sun, 11 Jul 2004 22:17:53 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Con Kolivas <kernel@kolivas.org>
Cc: ck kernel mailing list <ck@vds.kolivas.org>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@osdl.org>, Arjan van de Ven <arjanv@redhat.com>
Subject: Re: [ck] Re: [announce] [patch] Voluntary Kernel Preemption Patch
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* Ingo Molnar <mingo@elte.hu> wrote:

> it was reporting more accurate latencies, except that there were
> strange spikes of latencies. It turned out that for whatever reason,
> userspace RDTSC is not always reliable on my box (!).
> 
> I've attached two fixes against latencytest - one makes rdtsc
> timestamps more reliable, the other one fixes an SMP bug in the kernel
> module (it would lock up under SMP otherwise.).

>  static inline unsigned long long int rdtsc(void)
>  {
> -	unsigned long long int x;
> -	__asm__ volatile ("rdtsc" : "=A" (x));
> -	return x;
> +	unsigned long long int x, y;
> +	for (;;) {
> +		__asm__ volatile ("rdtsc" : "=A" (x));
> +		__asm__ volatile ("rdtsc" : "=A" (y));
> +		if (y - x < 1000)
> +			return y;
> +	}
>  }

the same fix should be done to latencytest0.42 too.

	Ingo