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

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Date:   Wed, 20 Jun 2018 18:29:46 +0800
From:   Feng Tang <feng.tang@...el.com>
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,
        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, Jun 20, 2018 at 12:16:36PM +0200, Thomas Gleixner wrote:
> 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.

Maybe I'm wrong, for printk timestamp, it uses the local_clock() thus
    sched_clock_cpu()
	sched_clock_stable
	    check the static_key "__sched_clock_stable" which is
	    defined FALSE, and need a static_branch_enable() to
	    take effect

Thanks,
Feng

> 
> > >  /*
> > >   * 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