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Mon, 4 Nov 2024 23:58:44 +0000 Message-ID: <8a76c5df-3982-c53a-9643-4b6b1de290e2@intel.com> Date: Mon, 4 Nov 2024 15:59:03 -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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> > To a large degree Peter's is working around inefficiency in this housekeeping > call. > > Code may be running on a suitable CPU from the domain cpumask, but this > call will very likely pick the first CPU in that mask, rather than the current one. Agree. > > From that point it's all downhill unless you are lucky enough that the first > CPU is a tick_nohz_full_cpu() one and you take the > > smp_call_function_any(cpumask, mon_event_count, rr, 1); Whenever this function is called, the performance is degraded rather than improved because extra get_cpu()/put_cpu() are called in the fast path in the current patch. On platforms that have less housekeeping CPUs (e.g. a RT platform), there could be a higher chance that the first CPU is a nohz_full CPU and run smp_call_function_any(). > > path. It seems that on many systems you'll take the > > smp_call_on_cpu(cpu, smp_mon_event_count, rr, false); > > path and make a pointless IPI to get the data. Yes, that's right. But it's not conflicting with my suggested change. What I suggested is to move the fast path code to this case only. So the fast path is always checked/called in both cases if condition is met: 1. It's already checked/called inside smp_call_function_any() in nohz_full case. No need to call out the fast path separately. - No extra get_cpu() and put_cpu() are called. - The performance is better than the current patch. 2. It's called out in non nohz_full case. No performance difference from the current patch. Thanks. -Fenghua