linux-kernel - Re: [PATCH] Warn of incorrect cpu

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Message-ID: <20081106090118.GC4890@elte.hu>
Date:	Thu, 6 Nov 2008 10:01:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	mark.langsdorf@....com
Subject: Re: [PATCH] Warn of incorrect cpu_khz on AMD systems


* Prarit Bhargava <prarit@...hat.com> wrote:

> If none of the perfctrs are free when calculating cpu_khz we default 
> to using ctr 3 (ie, we just choose 3).  This may lead to an 
> incorrect tsc freq value which can cause the system to be unstable.
> 
> To aid in future debugging, WARN the user of a potential problem.

oh, nasty... when can this happen - are you using nmi_watchdog=2?

Cannot we avoid this situation somehow? The calibrate_cpu() function 
is quite ugly and does a dangerous thing by ignoring the reservation. 

This whole sequence is sloppy:

        for (i = 0; i < 4; i++)
                if (avail_to_resrv_perfctr_nmi_bit(i))
                        break;
        no_ctr_free = (i == 4);
        if (no_ctr_free) {
                i = 3;
                rdmsrl(MSR_K7_EVNTSEL3, evntsel3);
                wrmsrl(MSR_K7_EVNTSEL3, 0);
                rdmsrl(MSR_K7_PERFCTR3, pmc3);
        } else {
                reserve_perfctr_nmi(MSR_K7_PERFCTR0 + i);
                reserve_evntsel_nmi(MSR_K7_EVNTSEL0 + i);
        }

>  	no_ctr_free = (i == 4);
>  	if (no_ctr_free) {
> +		printk(KERN_WARN "Warning: AMD perfctrs busy ... "
> +		       "cpu_khz value may be incorrect.\n");

also, please use a WARN() instead so that kerneloops.org picks it up.

	Ingo
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