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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. Link: https://lkml.kernel.org/r/20250520141236.2987309-1-joshua.hahnjy@gmail.com [joshua.hahnjy@gmail.com: wordsmithing changes, simplification, fixes] Link: https://lkml.kernel.org/r/20250511025840.2410154-1-joshua.hahnjy@gmail.com [joshua.hahnjy@gmail.com: remove auto_kobj_attr field from struct sysfs_wi_group] Link: https://lkml.kernel.org/r/20250512142511.3959833-1-joshua.hahnjy@gmail.com https://lore.kernel.org/linux-mm/20240202170238.90004-1-gregory.price@memverge.com/ [1] Link: https://lkml.kernel.org/r/20250505182328.4148265-1-joshua.hahnjy@gmail.com Co-developed-by: Gregory Price <gourry@gourry.net> Signed-off-by: Gregory Price <gourry@gourry.net> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com> Suggested-by: Yunjeong Mun <yunjeong.mun@sk.com> Suggested-by: Oscar Salvador <osalvador@suse.de> Suggested-by: Ying Huang <ying.huang@linux.alibaba.com> Suggested-by: Harry Yoo <harry.yoo@oracle.com> Reviewed-by: Harry Yoo <harry.yoo@oracle.com> Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com> Reviewed-by: Honggyu Kim <honggyu.kim@sk.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Jiang <dave.jiang@intel.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Joanthan Cameron <Jonathan.Cameron@huawei.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Len Brown <lenb@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
54 lines
2.1 KiB
Text
54 lines
2.1 KiB
Text
What: /sys/kernel/mm/mempolicy/weighted_interleave/
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Date: January 2024
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Contact: Linux memory management mailing list <linux-mm@kvack.org>
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Description: Configuration Interface for the Weighted Interleave policy
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What: /sys/kernel/mm/mempolicy/weighted_interleave/nodeN
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Date: January 2024
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Contact: Linux memory management mailing list <linux-mm@kvack.org>
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Description: Weight configuration interface for nodeN
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The interleave weight for a memory node (N). These weights are
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utilized by tasks which have set their mempolicy to
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MPOL_WEIGHTED_INTERLEAVE.
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These weights only affect new allocations, and changes at runtime
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will not cause migrations on already allocated pages.
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The minimum weight for a node is always 1.
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Minimum weight: 1
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Maximum weight: 255
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Writing invalid values (i.e. any values not in [1,255],
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empty string, ...) will return -EINVAL.
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Changing the weight to a valid value will automatically
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switch the system to manual mode as well.
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What: /sys/kernel/mm/mempolicy/weighted_interleave/auto
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Date: May 2025
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Contact: Linux memory management mailing list <linux-mm@kvack.org>
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Description: Auto-weighting configuration interface
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Configuration mode for weighted interleave. 'true' indicates
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that the system is in auto mode, and a 'false' indicates that
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the system is in manual mode.
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In auto mode, all node weights are re-calculated and overwritten
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(visible via the nodeN interfaces) whenever new bandwidth data
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is made available during either boot or hotplug events.
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In manual mode, node weights can only be updated by the user.
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Note that nodes that are onlined with previously set weights
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will reuse those weights. If they were not previously set or
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are onlined with missing bandwidth data, the weights will use
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a default weight of 1.
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Writing any true value string (e.g. Y or 1) will enable auto
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mode, while writing any false value string (e.g. N or 0) will
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enable manual mode. All other strings are ignored and will
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return -EINVAL.
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Writing a new weight to a node directly via the nodeN interface
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will also automatically switch the system to manual mode.
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