Re: [PATCH v3 09/10] selftests/resctrl: Simplify perf usage in CAT test

[Ilpo Järvinen]

On Fri, 13 Mar 2026, Reinette Chatre wrote:

> The CAT test relies on the PERF_COUNT_HW_CACHE_MISSES event to determine if
> modifying a cache portion size is successful. This event is configured to
> report the data as part of an event group, but no other events are added to
> the group.
> 
> Remove the unnecessary PERF_FORMAT_GROUP format setting. This eliminates
> the need for struct perf_event_read and results in read() of the associated
> file descriptor to return just one value associated with the
> PERF_COUNT_HW_CACHE_MISSES event of interest.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Tested-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> ---
> Changes since v2:
> - Add Chen Yu's tag.
> ---
>  tools/testing/selftests/resctrl/cache.c    | 17 +++++------------
>  tools/testing/selftests/resctrl/cat_test.c |  4 +---
>  tools/testing/selftests/resctrl/resctrl.h  | 11 +----------
>  3 files changed, 7 insertions(+), 25 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cache.c b/tools/testing/selftests/resctrl/cache.c
> index bef71b6feacc..df9bea584a2d 100644
> --- a/tools/testing/selftests/resctrl/cache.c
> +++ b/tools/testing/selftests/resctrl/cache.c
> @@ -10,7 +10,6 @@ void perf_event_attr_initialize(struct perf_event_attr *pea, __u64 config)
>  	memset(pea, 0, sizeof(*pea));
>  	pea->type = PERF_TYPE_HARDWARE;
>  	pea->size = sizeof(*pea);
> -	pea->read_format = PERF_FORMAT_GROUP;
>  	pea->exclude_kernel = 1;
>  	pea->exclude_hv = 1;
>  	pea->exclude_idle = 1;
> @@ -37,19 +36,13 @@ int perf_event_reset_enable(int pe_fd)
>  	return 0;
>  }
>  
> -void perf_event_initialize_read_format(struct perf_event_read *pe_read)
> -{
> -	memset(pe_read, 0, sizeof(*pe_read));
> -	pe_read->nr = 1;
> -}
> -
>  int perf_open(struct perf_event_attr *pea, pid_t pid, int cpu_no)
>  {
>  	int pe_fd;
>  
>  	pe_fd = perf_event_open(pea, pid, cpu_no, -1, PERF_FLAG_FD_CLOEXEC);
>  	if (pe_fd == -1) {
> -		ksft_perror("Error opening leader");
> +		ksft_perror("Unable to set up performance monitoring");
>  		return -1;
>  	}
>  
> @@ -132,9 +125,9 @@ static int print_results_cache(const char *filename, pid_t bm_pid, __u64 llc_val
>   *
>   * Return: =0 on success. <0 on failure.
>   */
> -int perf_event_measure(int pe_fd, struct perf_event_read *pe_read,
> -		       const char *filename, pid_t bm_pid)
> +int perf_event_measure(int pe_fd, const char *filename, pid_t bm_pid)
>  {
> +	__u64 value;
>  	int ret;
>  
>  	/* Stop counters after one span to get miss rate */
> @@ -142,13 +135,13 @@ int perf_event_measure(int pe_fd, struct perf_event_read *pe_read,
>  	if (ret < 0)
>  		return ret;
>  
> -	ret = read(pe_fd, pe_read, sizeof(*pe_read));
> +	ret = read(pe_fd, &value, sizeof(value));
>  	if (ret == -1) {
>  		ksft_perror("Could not get perf value");
>  		return -1;
>  	}
>  
> -	return print_results_cache(filename, bm_pid, pe_read->values[0].value);
> +	return print_results_cache(filename, bm_pid, value);
>  }
>  
>  /*
> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> index 8bc47f06679a..6aac03147d41 100644
> --- a/tools/testing/selftests/resctrl/cat_test.c
> +++ b/tools/testing/selftests/resctrl/cat_test.c
> @@ -135,7 +135,6 @@ static int cat_test(const struct resctrl_test *test,
>  		    struct resctrl_val_param *param,
>  		    size_t span, unsigned long current_mask)
>  {
> -	struct perf_event_read pe_read;
>  	struct perf_event_attr pea;
>  	cpu_set_t old_affinity;
>  	unsigned char *buf;
> @@ -159,7 +158,6 @@ static int cat_test(const struct resctrl_test *test,
>  		goto reset_affinity;
>  
>  	perf_event_attr_initialize(&pea, PERF_COUNT_HW_CACHE_MISSES);
> -	perf_event_initialize_read_format(&pe_read);
>  	pe_fd = perf_open(&pea, bm_pid, uparams->cpu);
>  	if (pe_fd < 0) {
>  		ret = -1;
> @@ -192,7 +190,7 @@ static int cat_test(const struct resctrl_test *test,
>  
>  			fill_cache_read(buf, span, true);
>  
> -			ret = perf_event_measure(pe_fd, &pe_read, param->filename, bm_pid);
> +			ret = perf_event_measure(pe_fd, param->filename, bm_pid);
>  			if (ret)
>  				goto free_buf;
>  		}
> diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
> index 3bad2d80c09b..175101022bf3 100644
> --- a/tools/testing/selftests/resctrl/resctrl.h
> +++ b/tools/testing/selftests/resctrl/resctrl.h
> @@ -148,13 +148,6 @@ struct resctrl_val_param {
>  	struct fill_buf_param	*fill_buf;
>  };
>  
> -struct perf_event_read {
> -	__u64 nr;			/* The number of events */
> -	struct {
> -		__u64 value;		/* The value of the event */
> -	} values[2];
> -};
> -
>  /*
>   * Memory location that consumes values compiler must not optimize away.
>   * Volatile ensures writes to this location cannot be optimized away by
> @@ -210,11 +203,9 @@ unsigned int count_bits(unsigned long n);
>  int snc_kernel_support(void);
>  
>  void perf_event_attr_initialize(struct perf_event_attr *pea, __u64 config);
> -void perf_event_initialize_read_format(struct perf_event_read *pe_read);
>  int perf_open(struct perf_event_attr *pea, pid_t pid, int cpu_no);
>  int perf_event_reset_enable(int pe_fd);
> -int perf_event_measure(int pe_fd, struct perf_event_read *pe_read,
> -		       const char *filename, pid_t bm_pid);
> +int perf_event_measure(int pe_fd, const char *filename, pid_t bm_pid);
>  int measure_llc_resctrl(const char *filename, pid_t bm_pid);
>  int minimize_l2_occupancy(const struct resctrl_test *test,
>  			  const struct user_params *uparams,
> 

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>


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 i.