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Tue, 5 Nov 2024 03:28:41 +0000 Message-ID: <2e2636ef-eb0f-067d-ef8b-a95e762dbf9f@intel.com> Date: Mon, 4 Nov 2024 19:29:00 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0 Thunderbird/91.11.0 Subject: Re: [PATCH 2/2] x86/resctrl: Don't workqueue local event counter reads Content-Language: en-US To: "Luck, Tony" , Peter Newman , "Chatre, Reinette" CC: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "x86@kernel.org" , "H . 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Maybe costs less that the > cpumask_any_housekeeping() call that is avoided by Peter's patch. Quote from Peter: "AMD EPYC 7B12 64-Core Processor (250 mon groups) Local Domain: 3.25M -> 1.22M (-62.5%) Remote Domain: 7.91M -> 8.05M (+2.9%) Intel(R) Xeon(R) Gold 6268CL CPU @ 2.80GHz (190 mon groups) Local Domain: 2.98M -> 2.21M (-25.8%) Remote Domain: 4.49M -> 4.62M (+3.1%) Note that there is a small increase in overhead for remote domains, which results from the introduction of a put_cpu() call to reenable preemption after determining whether the fast path can be used." As his data shows, if the fast path is not taken, the extra put_cpu() itself costs +2.9% extra time on AMD machine and +3.1% extra time on Intel machine. And this ~3% overhead is on top of queued work, which is more expensive than cpumask_any_housekeeping() IIUC. > > Note that if Peter's patch doesn't take its fast path because the calling > CPU was on the wrong domain, then the subsequent code is going to > do an IPI whichever of the if/else path is taken. In this case, actually IPI is only taken in smp_call_function_any() and smp_call_on_cpu() invokes a queued work instead of IPI. My proposed change logically doesn't change Peter's fast path and performance for nohz_full/smp_call_on_cpu() case. It just utilizes the "built-in fast path already" inside smp_call_function_any() to save extra get_cpu() and put_cpu(). Hopefully the saved extra get_cpu() and put_cpu() can offset cost of cpumask_any_housekeeping(). From Peter's commit message, seems nohz_full case is not called/measured a lot if any. If only one or a very few housekeeping CPUs on a large system, the nohz_full case will be called frequently and fast path will fail most of time and the extra get_cpu()/put_cpu() around the fast path might impact more on both local and total domain. Thanks. -Fenghua