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Wed, 27 May 2026 01:57:25 +0000 Message-ID: Date: Wed, 27 May 2026 09:57:16 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] selftests/resctrl: Allow CAT testing when no exclusive cache exists To: Reinette Chatre , =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= CC: Tony Luck , , Dave Martin , James Morse , Babu Moger , LKML , , Hongyu Ning References: <20260523101715.3964456-1-yu.c.chen@intel.com> <09c77f99-0c8f-44c3-8971-aaec094053ca@intel.com> <3fbb001e-bc78-45d6-92fc-4a8dcf708c58@intel.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <3fbb001e-bc78-45d6-92fc-4a8dcf708c58@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SI3PR03CA0006.apcprd03.prod.outlook.com (2603:1096:4:297::8) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6020:EE_|DS7PR11MB9452:EE_ X-MS-Office365-Filtering-Correlation-Id: 1d5fe9ff-ddfe-4a3a-2eb4-08debb934c86 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|4143699003|11063799006|22082099003|18002099003|3023799007|6133799003|56012099006; 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On some platforms, all L3 >>>> cache ways are marked as potentially shared. >>>> >>>> The L3_CAT test should fall back to using the full CBM mask when no >>>> exclusive portion exists, since the test logic (writing different >>>> schemata and measuring cache misses) still works. The shareable mask > > How do you define "still works"? Allowing more interference will impact the > number of cache misses that is used to determine effectiveness of cache > allocation, no? > >>>> is a hint about potential interference, not a hard exclusion. >>>> >>>> Reported-by: Hongyu Ning >>>> Signed-off-by: Chen Yu >>>> --- >>>>   tools/testing/selftests/resctrl/cat_test.c | 2 +- >>>>   1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c >>>> index f00b622c1460..b6994ba4504b 100644 >>>> --- a/tools/testing/selftests/resctrl/cat_test.c >>>> +++ b/tools/testing/selftests/resctrl/cat_test.c >>>> @@ -248,7 +248,7 @@ static int cat_run_test(const struct resctrl_test *test, const struct user_param >>>>       /* Get the largest contiguous exclusive portion of the cache */ >>>>       ret = get_mask_no_shareable(test->resource, &long_mask); >>>>       if (ret) >>>> -        return ret; >>>> +        long_mask = full_cache_mask; >>> >>> Wouldn't it warrant to log something when the fallback occurs? It >>> certainly looks something that makes the results less reliable (so it >>> would be nice to know about it when looking at the test results). >>> >> >> Got it. Let me add the following message: >> ksft_print_msg("No exclusive cache portion found, fall back to full cache mask\n"); > > Allowing more interference will cause the test to fail while the system behaves correctly. > This log message may help a knowledgeable user running the test manually but I expect would > be confusing to folks not familiar with resctrl and definitely does not help test automation > that just looks for the pass/fail. > > The test can be fixed by skipping the test, instead of failing, if there are no bits > available for the test to run as intended. Of course, this will prevent CAT mechanics to > be tested on this platform so to test CAT on this platform there is a need a new functional > test since a performance test is not appropriate. > Got it. My thinking was that shareable_bits only describes portions that may be shared with non-CPU agents, and on an otherwise idle test system the CAT miss-rate trend can still be observed even if all ways are marked shareable. But I agree that this is not a good assumption for kselftest pass/fail. So I agree that falling back to the full CBM is not the right fix for this test. I will change the patch to skip L3_CAT when no exclusive cache portion is available, with a message explaining that the performance test cannot run reliably because all cache portions are shareable. For platforms where all cache ways are marked shareable, do you think providing a extra parameter to ask cat_test to "ignore" the share restriction is acceptable? thanks, Chenyu