linux-kernel - Re: [PATCH -v2] x86: Kill notsc

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Message-ID: <alpine.DEB.2.11.1511041120120.4032@nanos>
Date:	Wed, 4 Nov 2015 11:21:45 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Borislav Petkov <bp@...en8.de>
cc:	Peter Zijlstra <peterz@...radead.org>, x86-ml <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2] x86: Kill notsc

On Thu, 22 Oct 2015, Borislav Petkov wrote:
>  u64 native_sched_clock(void)
>  {
> -	if (static_branch_likely(&__use_tsc)) {
> -		u64 tsc_now = rdtsc();
> -
> -		/* return the value in ns */
> -		return cycles_2_ns(tsc_now);
> -	}
> -
> +#ifdef CONFIG_X86_TSC
> +	/* return the value in ns */
> +	return cycles_2_ns(rdtsc());
> +#else
>  	/*
> -	 * Fall back to jiffies if there's no TSC available:
> -	 * ( But note that we still use it if the TSC is marked
> -	 *   unstable. We do this because unlike Time Of Day,
> -	 *   the scheduler clock tolerates small errors and it's
> -	 *   very important for it to be as fast as the platform
> -	 *   can achieve it. )
> +	 * Fall back to jiffies if there's no TSC available: ( But note that we
> +	 * still use it if the TSC is marked unstable. We do this because unlike
> +	 * Time Of Day, the scheduler clock tolerates small errors and it's very
> +	 * important for it to be as fast as the platform can achieve it. )

This comment does not make any sense with this modification.

Thanks,

	tglx
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