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

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Message-ID: <20151021190109.GC2508@worktop.programming.kicks-ass.net>
Date:	Wed, 21 Oct 2015 21:01:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	x86-ml <x86@...nel.org>, Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86: Kill notsc

On Wed, Oct 21, 2015 at 07:58:03PM +0200, Borislav Petkov wrote:
> > +#ifdef CONFIG_X86_TSC
> > +	/* return the value in ns */
> > +	return cycles_2_ns(rdtsc());
> > +#else
> 
> Ok, we have a problem here:
> 
> See the splat below. On the init path
> 
> start_kernel
> |-> sched_init
>     |-> init_idle
> 
> we're calling sched_clock() which does the cycles_2_ns() thing.
> 
> However, that cycles_2_ns() thing gets called only in tsc_init() which
> comes later in start_kernel().
> 
> Which means, data in this line
> 
>       ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, data->cyc2ns_shift);
> 
> is NULL.
> 
> Now, the question is, can I push only the cyc2ns_init() call up, before
> sched_init()?

I _think_ so, but its late. It would result in sched_clock() being 0
until you hit that other bit, but that should be fine (maybe).
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