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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1704966538; x=1705571338; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=eaM81UE8HNBCwln5s5TJQ6eHQuRiuHL2tpn+YpV4pcA=; b=Qp8GesKfL2MnzXPcFzMWLgH8rjFfMrr7A5n+fwU1nDq5yOtD38DOiu+4sAtpk8enRV STPCVVVEM93gYXQLoc3jggxnCx+Yk9NXJAaVT28THywpztedhakLSD1F9zEqIMF70l6f Vz008RDNTc3ZDbAs2Bblea69H9kNFOfOKHmPMSd7pSvv2Pxoxg2wxLJKUdR4A+itoNNy dfbn9FqAjfVFQXMdQ5i785Rvd5m1yoy9jHeP4ZfpkudtYCBS7qJpDU1bOQhahSMLV6rM OdkXa0w8RQ56g4n3h6Jpp5MHxOksxYTSSXcmQEKG2Z7CkmlmdX4GozQivMNbFHQRE2vh wOAQ== X-Gm-Message-State: AOJu0YyQn5wHaOwpZlAZawMEesYSRI5w616WbAiMFRTWa8oizLy41Nye yYI4TfzUK717TJ14gQxooH0iVmkh5BV99vA+yUDGKChFeAQ9 X-Google-Smtp-Source: AGHT+IHOjyMfyL2jnUGJGBvS0vHHyjT8WL4gkvYRqPM9FPv9rtIY76L2L/PPfrYeqDs7i/HHossZLlXRco2LPjFq1Vo= X-Received: by 2002:a05:6122:a20:b0:4b6:eb5a:ee98 with SMTP id 32-20020a0561220a2000b004b6eb5aee98mr227842vkn.14.1704966537737; Thu, 11 Jan 2024 01:48:57 -0800 (PST) MIME-Version: 1.0 References: <9f81ffcc4bb422ebb6326a65a770bf1918634cbb.1700502145.git.andreyknvl@google.com> <87sf34lrn3.fsf@linux.intel.com> In-Reply-To: <87sf34lrn3.fsf@linux.intel.com> From: Marco Elver Date: Thu, 11 Jan 2024 10:48:19 +0100 Message-ID: Subject: Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock To: Andi Kleen Cc: Oscar Salvador , andrey.konovalov@linux.dev, Andrew Morton , Andrey Konovalov , Alexander Potapenko , Dmitry Vyukov , Vlastimil Babka , kasan-dev@googlegroups.com, Evgenii Stepanov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrey Konovalov Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: C85A440019 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: fsyfh3dosir6zkgkrh88bni9uiejaorr X-HE-Tag: 1704966538-886023 X-HE-Meta: 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 r18ewzJc allkC/9zWuLcC82P18NAcX1kLeJ9xyNVqQVk+L4jkDkDUkncolly9WO7ku6H86MKTg8qTc+OfRA40gMmUhWjYGv0Ozw3YsxsEtU52tsydAKV9WiQ+/l7cUvoKzEQGqU/LBrax2KHXKf6JZVG5AHSHtve6sYQwh8ZEVGi9E4m+/FOSJQsbHvmP2hJ4xUYLcInX/mDHkqu2bV1Viv4OCmGaiTFgay76/2Tc3X8qOZ1ajb6Tyq4rMPKFQ6o7OxVVfCv8Z135W54HlEDSdk2xQjpf+kDnyw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 11 Jan 2024 at 00:01, Andi Kleen wrote: > > Oscar Salvador writes: > >> > >> With this change, multiple users can still look up records in parallel. > > That's a severe misunderstanding -- rwlocks always bounce a cache line, > so the parallelism is significantly reduced. > > Normally rwlocks are only worth it if your critical region is quite long. > > >> > >> This is preparatory patch for implementing the eviction of stack records > >> from the stack depot. > >> > >> Reviewed-by: Alexander Potapenko > >> Signed-off-by: Andrey Konovalov > > > > Reviewed-by: Oscar Salvador > > > Has anyone benchmarked this on a high core count machine? It sounds > pretty bad if every lock aquisition starts bouncing a single cache line. > > Consider using RCU or similar. stackdepot is severely limited in what kernel facilities it may use due to being used by such low level facilities as the allocator itself. I've been suggesting percpu-rwsem here, but looking at it in more detail that doesn't work because percpu-rwsem wants to sleep, but stackdepot must work in non-sleepable contexts. :-/