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Wed, 4 Sep 2024 21:15:50 +0000 Message-ID: <9b2da518-89ce-4f9b-92f2-d317ed892886@intel.com> Date: Wed, 4 Sep 2024 14:15:47 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/6] selftests/resctrl: Do not compare performance counters and resctrl at low bandwidth To: =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= CC: , , , , , =?UTF-8?Q?Maciej_Wiecz=C3=B3r-Retman?= , , LKML References: <9bbefa3b9a62319698907d10e8b78f1b999c311b.1724970211.git.reinette.chatre@intel.com> <5d063290-9da4-c9ca-e5c5-cb0083d7483f@linux.intel.com> <87e4788c-6407-41a8-b201-e3f05064e5a6@intel.com> <238af9fe-0d7b-9bc1-9923-35ef74aad360@linux.intel.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <238af9fe-0d7b-9bc1-9923-35ef74aad360@linux.intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR03CA0216.namprd03.prod.outlook.com (2603:10b6:303:b9::11) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|IA1PR11MB6266:EE_ X-MS-Office365-Filtering-Correlation-Id: 60eadd0e-4a04-4ba6-fc1a-08dccd26c05e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; 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For example RAS features or memory performance >>>> features that generate memory traffic may drive accesses that are >>>> counted differently by performance counters and MBM respectively, >>>> for instance generating "overhead" traffic which is not counted >>>> against any specific RMID. As a ratio, this different view of memory >>>> bandwidth becomes more apparent at low memory bandwidths. >>> >>> Interesting. >>> >>> I did some time back prototype with a change to MBM test such that instead >>> of using once=false I changed fill_buf to be able to run N passes through >>> the buffer which allowed me to know how many reads were performed by the >>> benchmark. This yielded numerical difference between all those 3 values >>> (# of reads, MBM, perf) which also varied from arch to another so it >>> didn't end up making an usable test. >>> >>> I guess I now have an explanation for at least a part of the differences. >>> >>>> It is not practical to enable/disable the various features that >>>> may generate memory bandwidth to give performance counters and >>>> resctrl an identical view. Instead, do not compare performance >>>> counters and resctrl view of memory bandwidth when the memory >>>> bandwidth is low. >>>> >>>> Bandwidth throttling behaves differently across platforms >>>> so it is not appropriate to drop measurement data simply based >>>> on the throttling level. Instead, use a threshold of 750MiB >>>> that has been observed to support adequate comparison between >>>> performance counters and resctrl. >>>> >>>> Signed-off-by: Reinette Chatre >>>> --- >>>> tools/testing/selftests/resctrl/mba_test.c | 7 +++++++ >>>> tools/testing/selftests/resctrl/resctrl.h | 6 ++++++ >>>> 2 files changed, 13 insertions(+) >>>> >>>> diff --git a/tools/testing/selftests/resctrl/mba_test.c >>>> b/tools/testing/selftests/resctrl/mba_test.c >>>> index cad473b81a64..204b9ac4b108 100644 >>>> --- a/tools/testing/selftests/resctrl/mba_test.c >>>> +++ b/tools/testing/selftests/resctrl/mba_test.c >>>> @@ -96,6 +96,13 @@ static bool show_mba_info(unsigned long *bw_imc, >>>> unsigned long *bw_resc) >>>> avg_bw_imc = sum_bw_imc / (NUM_OF_RUNS - 1); >>>> avg_bw_resc = sum_bw_resc / (NUM_OF_RUNS - 1); >>>> + if (avg_bw_imc < THROTTLE_THRESHOLD || avg_bw_resc < >>>> THROTTLE_THRESHOLD) { >>>> + ksft_print_msg("Bandwidth below threshold (%d MiB). >>>> Dropping results from MBA schemata %u.\n", >>>> + THROTTLE_THRESHOLD, >>>> + ALLOCATION_MAX - ALLOCATION_STEP * >>>> allocation); >>> >>> The second one too should be %d. >>> >> >> hmmm ... I intended to have it be consistent with the ksft_print_msg() that >> follows. Perhaps allocation can be made unsigned instead? > > If you go that way, then it would be good to make the related defines and > allocation in mba_setup() unsigned too, although the latter is a bit scary Sure, will look into that. > because it does allocation -= ALLOCATION_STEP which could underflow if the > defines are ever changed. > Is this not already covered in the following check: if (allocation < ALLOCATION_MIN || allocation > ALLOCATION_MAX) return END_OF_TESTS; We are talking about hardcoded constants though. Reinette