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[195.38.113.95]) by smtp.gmail.com with ESMTPSA id h17-20020a5d4311000000b003373f723dcbsm5136543wrq.42.2024.01.02.01.49.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Jan 2024 01:49:34 -0800 (PST) Sender: Ingo Molnar Date: Tue, 2 Jan 2024 10:49:32 +0100 From: Ingo Molnar To: David Laight Cc: "'linux-kernel@vger.kernel.org'" , "'peterz@infradead.org'" , "'longman@redhat.com'" , "'mingo@redhat.com'" , "'will@kernel.org'" , "'boqun.feng@gmail.com'" , 'Linus Torvalds' , "'virtualization@lists.linux-foundation.org'" , 'Zeng Heng' Subject: Re: [PATCH next v2 2/5] locking/osq_lock: Optimise the vcpu_is_preempted() check. Message-ID: References: <2b4e8a5816a742d2bd23fdbaa8498e80@AcuMS.aculab.com> <3a9d1782cd50436c99ced8c10175bae6@AcuMS.aculab.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3a9d1782cd50436c99ced8c10175bae6@AcuMS.aculab.com> * David Laight wrote: > The vcpu_is_preempted() test stops osq_lock() spinning if a virtual > cpu is no longer running. > > Although patched out for bare-metal the code still needs the cpu number. Comma missing. > Reading this from 'prev->cpu' is a pretty much guaranteed have a cache miss > when osq_unlock() is waking up the next cpu. > > Instead save 'prev->cpu' in 'node->prev_cpu' and use that value instead. > Update in the osq_lock() 'unqueue' path when 'node->prev' is changed. > > This is simpler than checking for 'node->prev' changing and caching > 'prev->cpu'. Throughout the series, in changelogs and comments, please do: s/cpu /CPU Please be more careful about changelog quality. > struct optimistic_spin_node { > struct optimistic_spin_node *next, *prev; > - int locked; /* 1 if lock acquired */ > - int cpu; /* encoded CPU # + 1 value */ > + int locked; /* 1 if lock acquired */ > + int cpu; /* encoded CPU # + 1 value */ > + int prev_cpu; /* encoded CPU # + 1 value */ > }; s/ encoded CPU # + 1 value / encoded value: CPU+1 Thanks, Ingo