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[2003:cb:c73f:8500:f83c:3602:5300:88af]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36bbd01619csm12403076f8f.36.2024.08.06.02.12.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Aug 2024 02:12:52 -0700 (PDT) Message-ID: <04bff7a7-3901-4adc-9f6a-466627cc1a21@redhat.com> Date: Tue, 6 Aug 2024 11:12:49 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 2/4] arm64: use IPIs to pause/resume remote CPUs To: Yu Zhao , Catalin Marinas , Will Deacon Cc: Andrew Morton , David Rientjes , Douglas Anderson , Frank van der Linden , Mark Rutland , Muchun Song , Nanyong Sun , Yang Shi , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240806022114.3320543-1-yuzhao@google.com> <20240806022114.3320543-3-yuzhao@google.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit [...] > + > +void resume_remote_cpus(void) > +{ > + cpumask_t cpus_to_resume; > + > + lockdep_assert_cpus_held(); > + lockdep_assert_preemption_disabled(); > + > + cpumask_copy(&cpus_to_resume, cpu_online_mask); > + cpumask_clear_cpu(smp_processor_id(), &cpus_to_resume); > + > + spin_lock(&cpu_pause_lock); > + > + cpumask_setall(&resumed_cpus); > + /* A typical example for sleep and wake-up functions. */ > + smp_mb(); > + while (cpumask_intersects(&cpus_to_resume, &paused_cpus)) { > + sev(); > + cpu_relax(); > + barrier(); > + } > I'm curious, is there a fundamental reason why we wait for paused CPUs to actually start running, or is it simply easier to get the implementation race-free, in particular when we have two pause_remote_cpus() calls shortly after each other and another remote CPU might still be on its way out of pause_local_cpu() from the first pause. -- Cheers, David / dhildenb