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[2003:d8:2f25:9c00:e2c7:6eb5:8a51:1c60]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f73d3d6csm70659835e9.19.2025.05.21.06.04.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 21 May 2025 06:04:01 -0700 (PDT) Message-ID: <25283dbd-2867-4034-bb16-951e0fb81843@redhat.com> Date: Wed, 21 May 2025 15:04:00 +0200 Precedence: bulk X-Mailing-List: linux-acpi@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9] mm/mempolicy: Weighted Interleave Auto-tuning To: Joshua Hahn , gourry@gourry.net, akpm@linux-foundation.org Cc: harry.yoo@oracle.com, ying.huang@linux.alibaba.com, honggyu.kim@sk.com, yunjeong.mun@sk.com, gregkh@linuxfoundation.org, rakie.kim@sk.com, rafael@kernel.org, lenb@kernel.org, dan.j.williams@intel.com, Jonathan.Cameron@huawei.com, dave.jiang@intel.com, horen.chuang@linux.dev, hannes@cmpxchg.org, osalvador@suse.de, linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org, linux-mm@kvack.org, kernel-team@meta.com References: <20250520141236.2987309-1-joshua.hahnjy@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.05.25 16:12, Joshua Hahn wrote: > On machines with multiple memory nodes, interleaving page allocations > across nodes allows for better utilization of each node's bandwidth. > Previous work by Gregory Price [1] introduced weighted interleave, which > allowed for pages to be allocated across nodes according to user-set ratios. > > Ideally, these weights should be proportional to their bandwidth, so > that under bandwidth pressure, each node uses its maximal efficient > bandwidth and prevents latency from increasing exponentially. > > Previously, weighted interleave's default weights were just 1s -- which > would be equivalent to the (unweighted) interleave mempolicy, which goes > through the nodes in a round-robin fashion, ignoring bandwidth information. > > This patch has two main goals: > First, it makes weighted interleave easier to use for users who wish to > relieve bandwidth pressure when using nodes with varying bandwidth (CXL). > By providing a set of "real" default weights that just work out of the > box, users who might not have the capability (or wish to) perform > experimentation to find the most optimal weights for their system can > still take advantage of bandwidth-informed weighted interleave. > > Second, it allows for weighted interleave to dynamically adjust to > hotplugged memory with new bandwidth information. Instead of manually > updating node weights every time new bandwidth information is reported > or taken off, weighted interleave adjusts and provides a new set of > default weights for weighted interleave to use when there is a change > in bandwidth information. > > To meet these goals, this patch introduces an auto-configuration mode > for the interleave weights that provides a reasonable set of default > weights, calculated using bandwidth data reported by the system. In auto > mode, weights are dynamically adjusted based on whatever the current > bandwidth information reports (and responds to hotplug events). > > This patch still supports users manually writing weights into the nodeN > sysfs interface by entering into manual mode. When a user enters manual > mode, the system stops dynamically updating any of the node weights, > even during hotplug events that shift the optimal weight distribution. > > A new sysfs interface "auto" is introduced, which allows users to switch > between the auto (writing 1 or Y) and manual (writing 0 or N) modes. The > system also automatically enters manual mode when a nodeN interface is > manually written to. > > There is one functional change that this patch makes to the existing > weighted_interleave ABI: previously, writing 0 directly to a nodeN > interface was said to reset the weight to the system default. Before > this patch, the default for all weights were 1, which meant that writing > 0 and 1 were functionally equivalent. With this patch, writing 0 is invalid. > > [1] https://lore.kernel.org/linux-mm/20240202170238.90004-1-gregory.price@memverge.com/ > > Suggested-by: Yunjeong Mun > Suggested-by: Oscar Salvador > Suggested-by: Ying Huang > Suggested-by: Harry Yoo > Tested-by: Honggyu Kim > Reviewed-by: Honggyu Kim > Reviewed-by: Harry Yoo > Reviewed-by: Huang Ying > Co-developed-by: Gregory Price > Signed-off-by: Gregory Price > Signed-off-by: Joshua Hahn > --- [...] > -static void iw_table_free(void) > +static void wi_state_free(void) > { > - u8 *old; > + struct weighted_interleave_state *old_wi_state; > > - mutex_lock(&iw_table_lock); > - old = rcu_dereference_protected(iw_table, > - lockdep_is_held(&iw_table_lock)); > - rcu_assign_pointer(iw_table, NULL); > - mutex_unlock(&iw_table_lock); > + mutex_lock(&wi_state_lock); > + > + old_wi_state = rcu_dereference_protected(wi_state, > + lockdep_is_held(&wi_state_lock)); > + if (!old_wi_state) { > + mutex_unlock(&wi_state_lock); > + goto out; > + } > > + rcu_assign_pointer(wi_state, NULL); > + mutex_unlock(&wi_state_lock); Just one nit: if written as: ... rcu_assign_pointer(wi_state, NULL); mutex_unlock(&wi_state_lock); old_wi_state = ... if (old_wi_state) { synchronize_rcu(); kfree(old_wi_state); } kfree(&wi_group->wi_kobj); You can easily avoid the goto. > synchronize_rcu(); > - kfree(old); > + kfree(old_wi_state); > +out: > + kfree(&wi_group->wi_kobj); > } I'll note that this rather unrelated churn (renaming functions + variables) is a bit abd for review as it adds noise. Having that as part of a cleanup patch might have been better. Nothing else jumped at me (did not an in-depth review of the logic) Acked-by: David Hildenbrand -- Cheers, David / dhildenb