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b=nXR2QW3sDHw+B2LZ8F8viG138u 0L8ihS3bQBKbQOxJ3ryPGk7sKYNEQNvWa+F9g44RaQa1OnPPQM2ahtmAfnjF/sGxZu6l9eYU5OVou k9bfETdHu0fmudYKb0DwT8sfK51WqjoQoE5tfDgDRjat/uDdIKyefd4mhrDw/aKUWn8Kp9ei0g+zQ Jkr+whQxrInGOUbzXn2tjr64rdbCnUNwI73IgA25z2Z1MY1rHDW5+o1NBMEFJ3tXQlQbdRvBuylsq tsfH0BRzvt/fKxN7cXR7TOV6MdveU9lFiMI52Ti4YcV6QPNBas6/ZajbC3jq9R7zCE8UJKZhEX/LV tv6fUWTQ==; Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNBZn-00000005hD1-2tpo; Sun, 23 Nov 2025 15:00:31 +0000 Date: Sun, 23 Nov 2025 15:00:31 +0000 From: Matthew Wilcox To: Mateusz Guzik Cc: oleg@redhat.com, brauner@kernel.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org Subject: Re: [PATCH 0/3] further damage-control lack of clone scalability Message-ID: References: <20251123063054.3502938-1-mjguzik@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251123063054.3502938-1-mjguzik@gmail.com> X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: E1B9220023 X-Stat-Signature: iw76onnh7bfwwnys8dq7qxz5usyx9r5f X-HE-Tag: 1763910035-829264 X-HE-Meta: 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 4gKjE909 P5vR7roLmvC937CDPefym5pNLoA43xUmW6RJekdCQwYHKIAy9K6lT93GeQaiWo8TsACcoB0RctM3Br6b+uKkH0GqtQPdu7ZJDFnx7VVHN9AIynD3BmMph7FH3B8S6EHcx/F3Vke46aiRMeXNhm0a6X11CFBGocxxYS2kaYH1hf7z/ChlJD6sVFQjayLuO2UbH/jOeJM1r2MFx2dbzycIY+zST1qQ0UAZJmvR0wpTpAnG6YP2w/yh1HPQhkg== 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 Sun, Nov 23, 2025 at 07:30:51AM +0100, Mateusz Guzik wrote: > When spawning and killing threads in separate processes in parallel the > primary bottleneck on the stock kernel is pidmap_lock, largely because > of a back-to-back acquire in the common case. > > Benchmark code at the end. > > With this patchset alloc_pid() only takes the lock once and consequently > alleviates the problem. While scalability improves, the lock remains the > primary bottleneck by a large margin. > > I believe idr is a poor choice for the task at hand to begin with, but > sorting out that out beyond the scope of this patchset. At the same time > any replacement would be best evaluated against a state where the > above relock problem is fixed. Good news! The IDR is deprecated. Bad news! I'm not 100% sure that the XArray is quite appropriate for this usecase. I am opposed to introducing more IDR APIs. Have you looked at converting to the XArray? Or do you have a better data structure in mind than the XArray?