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Date:   Wed, 20 Jun 2018 12:16:36 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Feng Tang <feng.tang@intel.com>
cc:     Pavel Tatashin <pasha.tatashin@oracle.com>,
        steven.sistare@oracle.com, daniel.m.jordan@oracle.com,
        linux@armlinux.org.uk, schwidefsky@de.ibm.com,
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        sboyd@codeaurora.org, x86@kernel.org, linux-kernel@vger.kernel.org,
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        peterz@infradead.org, prarit@redhat.com, pmladek@suse.com,
        gnomes@lxorguk.ukuu.org.uk
Subject: Re: [PATCH v10 7/7] x86/tsc: use tsc early
In-Reply-To: <20180620101241.5vhmyvdjbenldp4r@shbuild888>
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References: <20180615174204.30581-1-pasha.tatashin@oracle.com> <20180615174204.30581-8-pasha.tatashin@oracle.com> <alpine.DEB.2.21.1806192335390.10546@nanos.tec.linutronix.de> <20180620101241.5vhmyvdjbenldp4r@shbuild888>
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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