linux-kernel - Re: [PATCH v10 7/7] x86/tsc: use tsc early

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Message-ID: <20180620091532.GK2476@hirez.programming.kicks-ass.net>
Date:   Wed, 20 Jun 2018 11:15:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        prarit@...hat.com, feng.tang@...el.com, pmladek@...e.com,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v10 7/7] x86/tsc: use tsc early

On Wed, Jun 20, 2018 at 01:52:10AM +0200, Thomas Gleixner wrote:
>  u64 native_sched_clock(void)
>  {
> +	if (static_branch_likely(&__use_tsc))
> +		return cycles_2_ns(rdtsc());
>  
> +	if (static_branch_unlikely(&tsc_early_enabled)) {
> +		if (tsc_early_sched_clock)
> +			return cycles_2_ns(rdtsc());
>  	}

I'm still puzzled by the entire need for tsc_early_enabled and all that.
Esp. since both branches do the exact same thing:

	return cycles_2_ns(rdtsc());