linux-kernel - Re: [PATCH] oprofile: re-arm APIC_DM_NMI in ppro_check

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Date:	Tue, 11 Nov 2008 09:32:12 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Robert Richter <robert.richter@....com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] oprofile: re-arm APIC_DM_NMI in ppro_check_ctrs()

Andi Kleen a écrit :
>>  			oprofile_add_sample(regs, i);
>> +			/*
>> +			 * We need to unmask the apic vector *before*
>> +			 * writing reset_value to msr counter
>> +			 */
>> +			apic_write(APIC_LVTPC, APIC_DM_NMI);
>>  			wrmsrl(msrs->counters[i].addr, -reset_value[i]);
>>  		}
>>  	}
>>  
>> -	/* Only P6 based Pentium M need to re-unmask the apic vector but it
>> -	 * doesn't hurt other P6 variant */
>> -	apic_write(APIC_LVTPC, apic_read(APIC_LVTPC) & ~APIC_LVT_MASKED);
> 
> Did you also test if it really needs to be inside the if () or 
> just before the wrmsrl? 
> 

Andi, the following patch also works well for me. The key is we must
unmask APIC before the wrmsrl. after/before the rdmsrl has no impact.

I wonder then if the final comment in ppro_check_ctrs() is still applicable.

PATCH] oprofile: un-mask APIC before resetting counter in ppro_check_ctrs()

While using oprofile on my HP BL460c G1, (two quad core intel E5450 CPU),
I noticed that one CPU after the other could not get anymore NMI.

After a while, all cores where blocked (ie not generating events for oprofile)
I tried all major linux versions and all where affected by this freeze.

I found that we have to un-mask APIC *before* writing to MSR counter
when we get event notification, because we use APIC_LVTPC in edge triggered mode.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
---
 arch/x86/oprofile/op_model_ppro.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

View attachment "oprofile_msr.patch" of type "text/plain" (1010 bytes)