Re: [PATCH v3 02/10] selftests/resctrl: Reduce interference from L2 occupancy during cache occupancy test

[Ilpo Järvinen]

On Fri, 13 Mar 2026, Reinette Chatre wrote:

> The CMT test creates a new control group that is also capable of monitoring
> and assigns the workload to it. The workload allocates a buffer that by
> default fills a portion of the L3 and keeps reading from the buffer,
> measuring the L3 occupancy at intervals. The test passes if the workload's
> L3 occupancy is within 15% of the buffer size.
> 
> The CMT test does not take into account that some of the workload's data
> may land in L2/L1. Matching L3 occupancy to the size of the buffer while
> a portion of the buffer can be allocated into L2 is not accurate.
> 
> Take the L2 cache into account to improve test accuracy:
>  - Reduce the workload's L2 cache allocation to the minimum on systems that
>    support L2 cache allocation. Do so with a new utility in preparation for
>    all L3 cache allocation tests needing the same capability.
>  - Increase the buffer size to accommodate data that may be allocated into
>    the L2 cache. Use a buffer size double the L3 portion to keep using the
>    L3 portion size as goal for L3 occupancy while taking into account that
>    some of the data may be in L2.
> 
> Running the CMT test on a sample system while introducing significant
> cache misses using "stress-ng --matrix-3d 0 --matrix-3d-zyx" shows
> significant improvement in L3 cache occupancy:
> 
> Before:
> 
>     # Starting CMT test ...
>     # Mounting resctrl to "/sys/fs/resctrl"
>     # Cache size :335544320
>     # Writing benchmark parameters to resctrl FS
>     # Write schema "L3:0=fffe0" to resctrl FS
>     # Write schema "L3:0=1f" to resctrl FS
>     # Benchmark PID: 7089
>     # Checking for pass/fail
>     # Pass: Check cache miss rate within 15%
>     # Percent diff=12
>     # Number of bits: 5
>     # Average LLC val: 73269248
>     # Cache span (bytes): 83886080
>     ok 1 CMT: test
> 
> After:
>     # Starting CMT test ...
>     # Mounting resctrl to "/sys/fs/resctrl"
>     # Cache size :335544320
>     # Writing benchmark parameters to resctrl FS
>     # Write schema "L3:0=fffe0" to resctrl FS
>     # Write schema "L3:0=1f" to resctrl FS
>     # Write schema "L2:1=0x1" to resctrl FS
>     # Benchmark PID: 7171
>     # Checking for pass/fail
>     # Pass: Check cache miss rate within 15%
>     # Percent diff=0
>     # Number of bits: 5
>     # Average LLC val: 83755008
>     # Cache span (bytes): 83886080
>     ok 1 CMT: test
> 
> Reported-by: Dave Martin <Dave.Martin@xxxxxxx>
> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Tested-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Link: https://lore.kernel.org/lkml/aO+7MeSMV29VdbQs@xxxxxxxxxxxxxxx/
> ---
> Changes since v2:
> - New patch split from v1's "selftests/resctrl: Improve accuracy of cache
>   occupancy test". (Ilpo)
> - Reword changelog. (Ilpo)
> - Update data used in changelog to match code after patch split.
> - Introduce utility to reduce L2 cache allocation. (Ilpo)
> - Add Chen Yu's tag.
> ---
>  tools/testing/selftests/resctrl/cache.c    | 13 +++++++++++++
>  tools/testing/selftests/resctrl/cmt_test.c | 14 ++++++++++----
>  tools/testing/selftests/resctrl/resctrl.h  |  3 +++
>  3 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cache.c b/tools/testing/selftests/resctrl/cache.c
> index 1ff1104e6575..bef71b6feacc 100644
> --- a/tools/testing/selftests/resctrl/cache.c
> +++ b/tools/testing/selftests/resctrl/cache.c
> @@ -173,6 +173,19 @@ int measure_llc_resctrl(const char *filename, pid_t bm_pid)
>  	return print_results_cache(filename, bm_pid, llc_occu_resc);
>  }
>  
> +/*
> + * Reduce L2 allocation to minimum when testing L3 cache allocation.
> + */
> +int minimize_l2_occupancy(const struct resctrl_test *test,
> +			  const struct user_params *uparams,
> +			  const struct resctrl_val_param *param)
> +{
> +	if (!strcmp(test->resource, "L3") && resctrl_resource_exists("L2"))
> +		return write_schemata(param->ctrlgrp, "0x1", uparams->cpu, "L2");
> +
> +	return 0;
> +}
> +
>  /*
>   * show_cache_info - Show generic cache test information
>   * @no_of_bits:		Number of bits
> diff --git a/tools/testing/selftests/resctrl/cmt_test.c b/tools/testing/selftests/resctrl/cmt_test.c
> index 7bc6cf49c1c5..ccb6fe881a94 100644
> --- a/tools/testing/selftests/resctrl/cmt_test.c
> +++ b/tools/testing/selftests/resctrl/cmt_test.c
> @@ -23,7 +23,9 @@
>   * Initialize capacity bitmasks (CBMs) of:
>   * - control group being tested per test parameters,
>   * - default resource group as inverse of control group being tested to prevent
> - *   other tasks from interfering with test.
> + *   other tasks from interfering with test,
> + * - L2 resource of control group being tested to minimize allocations into
> + *   L2 if possible to better predict L3 occupancy.
>   */
>  static int cmt_init(const struct resctrl_test *test,
>  		    const struct user_params *uparams,
> @@ -46,7 +48,11 @@ static int cmt_init(const struct resctrl_test *test,
>  		return ret;
>  
>  	snprintf(schemata, sizeof(schemata), "%lx", param->mask);
> -	return write_schemata(param->ctrlgrp, schemata, uparams->cpu, test->resource);
> +	ret = write_schemata(param->ctrlgrp, schemata, uparams->cpu, test->resource);
> +	if (ret)
> +		return ret;
> +
> +	return minimize_l2_occupancy(test, uparams, param);
>  }
>  
>  static int cmt_setup(const struct resctrl_test *test,
> @@ -175,11 +181,11 @@ static int cmt_run_test(const struct resctrl_test *test, const struct user_param
>  	span = cache_portion_size(cache_total_size, param.mask, long_mask);
>  
>  	if (uparams->fill_buf) {
> -		fill_buf.buf_size = span;
> +		fill_buf.buf_size = span * 2;
>  		fill_buf.memflush = uparams->fill_buf->memflush;
>  		param.fill_buf = &fill_buf;
>  	} else if (!uparams->benchmark_cmd[0]) {
> -		fill_buf.buf_size = span;
> +		fill_buf.buf_size = span * 2;
>  		fill_buf.memflush = true;
>  		param.fill_buf = &fill_buf;
>  	}
> diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
> index c72045c74ac4..7f2ab28be857 100644
> --- a/tools/testing/selftests/resctrl/resctrl.h
> +++ b/tools/testing/selftests/resctrl/resctrl.h
> @@ -216,6 +216,9 @@ 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 measure_llc_resctrl(const char *filename, pid_t bm_pid);
> +int minimize_l2_occupancy(const struct resctrl_test *test,
> +			  const struct user_params *uparams,
> +			  const struct resctrl_val_param *param);
>  void show_cache_info(int no_of_bits, __u64 avg_llc_val, size_t cache_span, bool lines);
>  
>  /*
> 

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

-- 
 i.