Re: [PATCH] perf/x86/intel/uncore: fix broken read_counter() for SNB IMC PMU

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2022-08-03 12:00 p.m., Stephane Eranian wrote:
> Existing code was generating bogus counts for the SNB IMC bandwidth counters:
> 
> $ perf stat -a -I 1000 -e uncore_imc/data_reads/,uncore_imc/data_writes/
>      1.000327813           1,024.03 MiB  uncore_imc/data_reads/
>      1.000327813              20.73 MiB  uncore_imc/data_writes/
>      2.000580153         261,120.00 MiB  uncore_imc/data_reads/
>      2.000580153              23.28 MiB  uncore_imc/data_writes/
> 
> The problem was introduced by commit:
>   07ce734dd8ad ("perf/x86/intel/uncore: Clean up client IMC")
> 
> Where the read_counter callback was replace to point to the generic
> uncore_mmio_read_counter() function.
> 
> The SNB IMC counters are freerunnig 32-bit counters laid out contiguously in
> MMIO. But uncore_mmio_read_counter() is using a readq() call to read from
> MMIO therefore reading 64-bit from MMIO. Although this is okay for the
> uncore_perf_event_update() function because it is shifting the value based
> on the actual counter width to compute a delta, it is not okay for the
> uncore_pmu_event_start() which is simply reading the counter  and therefore
> priming the event->prev_count with a bogus value which is responsible for
> causing bogus deltas in the perf stat command above.
> 
> The fix is to reintroduce the custom callback for read_counter for the SNB
> IMC PMU and use readl() instead of readq(). With the change the output of
> perf stat is back to normal:
> $ perf stat -a -I 1000 -e uncore_imc/data_reads/,uncore_imc/data_writes/
>      1.000120987             296.94 MiB  uncore_imc/data_reads/
>      1.000120987             138.42 MiB  uncore_imc/data_writes/
>      2.000403144             175.91 MiB  uncore_imc/data_reads/
>      2.000403144              68.50 MiB  uncore_imc/data_writes/
> 
> Signed-off-by: Stephane Eranian <eranian@google.com>

Reviewed-by: Kan Liang <kan.liang@linux.intel.com>

Thanks,
Kan

> ---
>  arch/x86/events/intel/uncore_snb.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/uncore_snb.c b/arch/x86/events/intel/uncore_snb.c
> index ce440011cc4e..1ef4f7861e2e 100644
> --- a/arch/x86/events/intel/uncore_snb.c
> +++ b/arch/x86/events/intel/uncore_snb.c
> @@ -841,6 +841,22 @@ int snb_pci2phy_map_init(int devid)
>  	return 0;
>  }
>  
> +static u64 snb_uncore_imc_read_counter(struct intel_uncore_box *box, struct perf_event *event)
> +{
> +	struct hw_perf_event *hwc = &event->hw;
> +
> +	/*
> +	 * SNB IMC counters are 32-bit and are laid out back to back
> +	 * in MMIO space. Therefore we must use a 32-bit accessor function
> +	 * using readq() from uncore_mmio_read_counter() causes problems
> +	 * because it is reading 64-bit at a time. This is okay for the
> +	 * uncore_perf_event_update() function because it drops the upper
> +	 * 32-bits but not okay for plain uncore_read_counter() as invoked
> +	 * in uncore_pmu_event_start().
> +	 */
> +	return (u64)readl(box->io_addr + hwc->event_base);
> +}
> +
>  static struct pmu snb_uncore_imc_pmu = {
>  	.task_ctx_nr	= perf_invalid_context,
>  	.event_init	= snb_uncore_imc_event_init,
> @@ -860,7 +876,7 @@ static struct intel_uncore_ops snb_uncore_imc_ops = {
>  	.disable_event	= snb_uncore_imc_disable_event,
>  	.enable_event	= snb_uncore_imc_enable_event,
>  	.hw_config	= snb_uncore_imc_hw_config,
> -	.read_counter	= uncore_mmio_read_counter,
> +	.read_counter	= snb_uncore_imc_read_counter,
>  };
>  
>  static struct intel_uncore_type snb_uncore_imc = {