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

From: Eric B Munson <[email-masked]>
Date: Mon, 23 May 2011 15:37:36 -0400

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.

> ---
>  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
>