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

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Message-ID: <alpine.DEB.2.21.1806201214310.10546@nanos.tec.linutronix.de>
Date:   Wed, 20 Jun 2018 12:16:36 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>
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,
        peterz@...radead.org, prarit@...hat.com, pmladek@...e.com,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v10 7/7] x86/tsc: use tsc early

On Wed, 20 Jun 2018, Feng Tang wrote:
> On Wed, Jun 20, 2018 at 01:52:10AM +0200, Thomas Gleixner wrote:
> >  
> >  static DEFINE_STATIC_KEY_FALSE(__use_tsc);
> > +static DEFINE_STATIC_KEY_TRUE(tsc_early_enabled);
> 
> One potential problem may be the several static_keys used here,
> the "__use_tsc", the "__sched_clock_stable", it may not be used
> very early in boot phase. As the the static_branch_enable() will
> use pageing related code while the paging is not setup ready yet.

I know how static keys work and thats the reason for having the extra
conditional. The key is disabled at a point where paging is available.

> >  /*
> >   * Scheduler clock - returns current time in nanosec units.
> >   */
> >  u64 native_sched_clock(void)
> >  {
> > -	if (static_branch_likely(&__use_tsc)) {
> > -		u64 tsc_now = rdtsc();
> > +	if (static_branch_likely(&__use_tsc))
> > +		return cycles_2_ns(rdtsc());
> >  
> > -		/* return the value in ns */
> > -		return cycles_2_ns(tsc_now);
> > +	if (static_branch_unlikely(&tsc_early_enabled)) {
> > +		if (tsc_early_sched_clock)
> > +			return cycles_2_ns(rdtsc());
> >  	}

Thanks,

	tglx