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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CtNTYGWH; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A23F9606A330; Tue, 3 Nov 2026 16:50:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=CtNTYGWH Received: by smtp.kernel.org (Postfix) with ESMTPSA id 345BB1F0093A; Tue, 3 Nov 2026 16:50:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793724654; bh=EtsuxpFaru5bCJVGsaTDOqLmkcEo7AgKuEWc5zgUH/c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=CtNTYGWHPzRk14MitXVi1MzDjUGP5Rm6vsULElGy6l1XszG9eoY0hvO0ia8qJ+ERk hSoD0pQy+xuQeJvsDmlwJYaaot8dr1g69hDk/GtieK9SrkIuy6KbRU66IWza+63otD lURNrr8tHaShaZCvhubnoBBJG65sTdXacUiePyf+KTLieywfDCUNw3Rkkrx0HgYWz1 /DQmm+heyD/iiwWLhBiilBhitBm3zwfIfW0lXVd7fG+qFMw98S/CTHPmLEgNPibZTA Y5xal+/1KVYPL3unucuzubdyh6mvUQYJnXW+CnYS+CbIFUsH76brKuClXLSPxtynvK t849VHMquTSsg== Message-ID: Date: Tue, 3 Nov 2026 17:50:48 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/mempolicy: recompute automatic interleave weights on memory hotplug To: "Gregory Price (Meta)" , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, ziy@nvidia.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, harry@kernel.org, stable@vger.kernel.org References: <20261030161234.1505367-1-gourry@gourry.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/30/26 17:12, Gregory Price (Meta) wrote: > In auto mode, weighted interleave derives each node's weight from its > bandwidth, reduced over the nodes in N_MEMORY. HMAT and CXL report a > node's bandwidth once, when the node first gains memory, and nothing > else on the hotplug path recomputes the weights. > > This can result in a major skew in distribution and overload tiers > with more data than their respective bandwidth can support, resulting > in a performance regression. > > With HMAT bandwidths of 100G for nodes 0-3, and 50G and 25G for kmem > nodes 4 and 5, weighted_interleave/nodeN reads: > > event node 0-3 node 4 node 5 > node 4 online 7 3 - > node 4 removed 7 - - > node 5 online 7 - 1 > node 4 back 7 1 1 > Expected: 6 3 1 > > Toggling auto off and on generates the correct result, but we should > detect this and fix it during hotplug. Recompute the weights from the > recorded bandwidth whenever a node gains/loses its first/last memory. > > The notifier cannot fail the hotplug, which has already happened, so > warn once if the new weights cannot be allocated. The next recompute > starts from the full bandwidth table and corrects them. > > mempolicy_set_node_perf() now only records the bandwidth and calls the > same helper, so the bandwidth is kept even if allocating the new weights > fails. wi_state_lock is dropped between the two steps, which should be > safe because racing writer simply causes it to run with newer state. > > Fixes: e341f9c3c841 ("mm/mempolicy: Weighted Interleave Auto-tuning") > Cc: stable@vger.kernel.org > Signed-off-by: Gregory Price (Meta) > --- It's a good idea to spell out what changed (and why) to make review easier ;) @Andrew I can pick this for the next merge window, or do you want to merge this through your tree? -- Cheers, David