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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=iBw406SW; 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 5765B606A322; Mon, 26 Oct 2026 12:12:19 +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=iBw406SW Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3F041F031E1; Mon, 26 Oct 2026 12:12:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793016739; bh=EKPDpnDsxP95BvhQy6gMrx0mg3B9bgjTY7R3pORfF0o=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=iBw406SWNAyN0iypY6z9eMu7GvnlQWgx71O//+605r5yzwZ0nPS5QYtL2h4pXli4b jQ/8nviFCI0Ni4rjvPmue0JD1nP7zd9iiL+Vbr+PT5RwYMUo5U4P8srFZooN8q67/O OS5ASw439ShiiovDUOTFVjawzHGh8ZT0ZO79ney7YB8LWOCivypf9hPnO/TLbgVZ9Z zwJMbwJIGNsuKg5xVyu63tT+4rAywuC2G/YzpOPeoC4QIpXEd80c0uIVBCaSr+jdfu 3MZtaCRlaL6j16jSTIDbjNvxRDP896Db8tLalPtJcZ7P+R43IhNMGOKbYVg7PygMDo Lfd9/xbqy5Jqw== Message-ID: Date: Mon, 26 Oct 2026 12:19:21 +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] 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, honggyu.kim@sk.com, stable@vger.kernel.org References: <20261024170415.4121901-1-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 10/24/26 19:04, 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. > > When a node loses its memory, the other nodes keep weights computed > with its bandwidth until another node reports. If one does while the > node is away, the reduction skips the node and resets its weight to 1, > which it keeps once its memory returns. > > 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 > > Toggling weighted_interleave/auto to 0 and back to 1 gives the correct > 6, 3 and 1. > > Recompute the weights from the recorded bandwidth whenever a node gains > its first memory or loses its last. > > 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 the helper reads node_bw_table and the mode under the lock, > so a racing writer only makes it work from newer state. > > With this patch, weighted_interleave/nodeN reads 6, 3 and 1 as soon as > node 4 is back. > > Fixes: e341f9c3c841 ("mm/mempolicy: Weighted Interleave Auto-tuning") > Cc: stable@vger.kernel.org > Signed-off-by: Gregory Price (Meta) > --- > mm/mempolicy.c | 66 +++++++++++++++++++++++++++++--------------------- > 1 file changed, 38 insertions(+), 28 deletions(-) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 0884c629670d..ffb293a4cc56 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -238,27 +238,45 @@ static void reduce_interleave_weights(unsigned int *bw, u8 *new_iw) > new_iw[nid] /= iw_gcd; > } > > +/* Recompute the weights from node_bw_table, unless they were set manually */ > +static int wi_state_refresh(void) > +{ > + struct weighted_interleave_state *new_wi_state, *old_wi_state; > + int i; > + > + new_wi_state = kmalloc_flex(*new_wi_state, iw_table, nr_node_ids); > + if (!new_wi_state) > + return -ENOMEM; > + new_wi_state->mode_auto = true; > + for (i = 0; i < nr_node_ids; i++) > + new_wi_state->iw_table[i] = 1; > + > + mutex_lock(&wi_state_lock); > + old_wi_state = rcu_dereference_protected(wi_state, > + lockdep_is_held(&wi_state_lock)); > + /* NULL wi_state assumes auto=true */ > + if (!node_bw_table || (old_wi_state && !old_wi_state->mode_auto)) { I was confused about the node_bw_table check at first. Maybe /* * We can get hotplug events before any BW data was recorded. * ... */ > > /** > @@ -3900,8 +3903,15 @@ static int wi_node_notifier(struct notifier_block *nb, > case NODE_REMOVED_LAST_MEMORY: > sysfs_wi_node_delete(nid); > break; > + default: > + return NOTIFY_OK; > } > > + /* > + * HMAT and CXL report a node's bandwidth only when it first gains > + * memory, so recompute the weights whenever N_MEMORY changes. > + */ > + wi_state_refresh(); If the function fails, should we at least pr_warn_once() ? Acked-by: David Hildenbrand (Arm) -- Cheers, David