Subject: Re: [PATCH] oprofile, powerpc: Handle events that raise an exception without overflowing

From: Maynard Johnson <[email-masked]>
Date: Mon, 23 May 2011 15:04:20 -0500

Eric B Munson wrote:
> On Mon, 23 May 2011, Eric B Munson wrote:
> 
>> Commit 0837e3242c73566fc1c0196b4ec61779c25ffc93 fixes a situation on POWER7
>> where events can roll back if a specualtive event doesn't actually complete.
>> This can raise a performance monitor exception.  We need to catch this to ensure
>> that we reset the PMC.  In all cases the PMC will be less than 256 cycles from
>> overflow.
>>
>> This patch lifts Anton's fix for the problem in perf and applies it to oprofile
>> as well.
>>
>> Signed-off-by: Eric B Munson <[email-masked]>
>> Cc: <[email-masked]> # as far back as it applies cleanly
> 
> I'd like to get this patch into mainline this merge window if at all possible.
Ack.  I've been able to create a system hang profiling with speculative events on POWER7.  This patch fixes that problem.

-Maynard

> 
>> ---
>>  arch/powerpc/oprofile/op_model_power4.c |   24 +++++++++++++++++++++++-
>>  1 files changed, 23 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/powerpc/oprofile/op_model_power4.c b/arch/powerpc/oprofile/op_model_power4.c
>> index 8ee51a2..e6bec74 100644
>> --- a/arch/powerpc/oprofile/op_model_power4.c
>> +++ b/arch/powerpc/oprofile/op_model_power4.c
>> @@ -261,6 +261,28 @@ static int get_kernel(unsigned long pc, unsigned long mmcra)
>>  	return is_kernel;
>>  }
>>  
>> +static bool pmc_overflow(unsigned long val)
>> +{
>> +	if ((int)val < 0)
>> +		return true;
>> +
>> +	/*
>> +	 * Events on POWER7 can roll back if a speculative event doesn't
>> +	 * eventually complete. Unfortunately in some rare cases they will
>> +	 * raise a performance monitor exception. We need to catch this to
>> +	 * ensure we reset the PMC. In all cases the PMC will be 256 or less
>> +	 * cycles from overflow.
>> +	 *
>> +	 * We only do this if the first pass fails to find any overflowing
>> +	 * PMCs because a user might set a period of less than 256 and we
>> +	 * don't want to mistakenly reset them.
>> +	 */
>> +	if (__is_processor(PV_POWER7) && ((0x80000000 - val) <= 256))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
>>  static void power4_handle_interrupt(struct pt_regs *regs,
>>  				    struct op_counter_config *ctr)
>>  {
>> @@ -281,7 +303,7 @@ static void power4_handle_interrupt(struct pt_regs *regs,
>>  
>>  	for (i = 0; i < cur_cpu_spec->num_pmcs; ++i) {
>>  		val = classic_ctr_read(i);
>> -		if (val < 0) {
>> +		if (pmc_overflow(val)) {
>>  			if (oprofile_running && ctr[i].enabled) {
>>  				oprofile_add_ext_sample(pc, regs, i, is_kernel);
>>  				classic_ctr_write(i, reset_value[i]);
>> -- 
>> 1.7.4.1
>>
>>
>>
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