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

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Date:	Thu, 06 Nov 2008 08:42:50 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	mark.langsdorf@....com
Subject: Re: [PATCH] Warn of incorrect cpu_khz on AMD systems


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

Yes, I noticed that too -- it's really the crux of the problem.  If 
no_ctr_free is a last resort boot option.  But I wonder if it should 
exist at all.

I was originally thinking the system should just stop booting if 
no_ctr_free and panic() ... and I'm willing to make that patch.

The likelihood of hitting this is low, below .0125% of the time.  But 
when it does hit, it is nasty and difficult to diagnose.
The last thing that I thought could be wrong was that cpu_khz was busted.

> 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.
>
>   

Will do -- but do you think a panic() is more appropriate?

P.

> 	Ingo
>   
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