On 1/16/2025 3:58 AM, 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 NUMA 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.
At the same time, we want these weights to be as small as possible.
Having ratios that involve large co-prime numbers like 7639:1345:7 leads
to awkward and inefficient allocations, since the node with weight 7
will remain mostly unused (and despite being proportional to bandwidth,
will not aid in relieving the bandwidth pressure in the other two nodes).
This patch introduces an auto-configuration mode for the interleave
weights that aims to balance the two goals of setting node weights to be
proportional to their bandwidths and keeping the weight values low.
In order to perform the weight re-scaling, we use an internal
"weightiness" value (fixed to 32) that defines interleave aggression.
In this auto configuration mode, node weights are dynamically updated
every time there is a hotplug event that introduces new bandwidth.
Users can also enter manual mode by writing "manual" to the new "mode"
sysfs interface. When a user enters manual mode, the system stops
dynamically updating any of the node weights, even during hotplug events
that can shift the optimal weight distribution. The system also enters
manual mode any time a user sets a node's weight by hand, using the
nodeN interface introduced in [1]. On the other hand, auto mode is
only entered by explicitly writing "auto" to the mode interface.
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.
This patch introduces "real" defaults, but we have decided to move away
from letting users use 0 as a "set to default" interface. Rather, users
who want to use system defaults should use "auto" mode. This patch seems
to be the appropriate place to make this change, since we would like to
remove this usage before users begin to rely on the feature in
userspace. Moreover, users will not be losing any functionality; they
can still write 1 into a node if they want a weight of 1. Thus, we
deprecate the "write zero to reset" feature in favor of returning an
error, the same way we would return an error when the user writes any
other invalid weight to the interface.
[1] https://lore.kernel.org/linux-mm/20240202170238.90004-1-gregory.price@xxxxxxxxxxxx/
Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Co-developed-by: Gregory Price <gourry@xxxxxxxxxx>
Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
---
Changelog
v3:
- Weightiness (max_node_weight) is now fixed to 32.
- Instead, the sysfs interface now exposes a "mode" parameter, which
can either be "auto" or "manual".
- Thank you Hyeonggon and Honggyu for the feedback.
- Documentation updated to reflect new sysfs interface, explicitly
specifies that 0 is invalid.
- Thank you Gregory and Ying for the discussion on how best to
handle the 0 case.
- Re-worked nodeN sysfs store to handle auto --> manual shifts
- mempolicy_set_node_perf internally handles the auto / manual
caes differently now. bw is always updated, iw updates depend on
what mode the user is in.
- Wordsmithing comments for clarity.
- Removed RFC tag.
v2:
- Name of the interface is changed: "max_node_weight" --> "weightiness"
- Default interleave weight table no longer exists. Rather, the
interleave weight table is initialized with the defaults, if bandwidth
information is available.
- In addition, all sections that handle iw_table have been changed
to reference iw_table if it exists, otherwise defaulting to 1.
- All instances of unsigned long are converted to uint64_t to guarantee
support for both 32-bit and 64-bit machines
- sysfs initialization cleanup
- Documentation has been rewritten to explicitly outline expected
behavior and expand on the interpretation of "weightiness".
- kzalloc replaced with kcalloc for readability
- Thank you Gregory and Hyeonggon for your review & feedback!
...fs-kernel-mm-mempolicy-weighted-interleave | 30 ++-
drivers/acpi/numa/hmat.c | 1 +
drivers/base/node.c | 7 +
include/linux/mempolicy.h | 4 +
mm/mempolicy.c | 212 ++++++++++++++++--
5 files changed, 227 insertions(+), 27 deletions(-)
Hi Joshua, thanks for the update!
It actually is what I was intended in the manual / auto mode description.
I don't have a strong opinion on the weight of the hot-plugged NUMA node
in manual mode, as it's not ideal whatever weight we choose and the user
need to update the weight after hot-plug events anyway.
Some comments inlined below:
diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-mempolicy-weighted-interleave b/Documentation/ABI/testing/sysfs-kernel-mm-mempolicy-weighted-interleave
index 0b7972de04e9..d30dc29c53ff 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-mempolicy-weighted-interleave
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-mempolicy-weighted-interleave
@@ -20,6 +20,30 @@ Description: Weight configuration interface for nodeN
Minimum weight: 1
Maximum weight: 255
- Writing an empty string or `0` will reset the weight to the
- system default. The system default may be set by the kernel
- or drivers at boot or during hotplug events.
+ Writing invalid values (i.e. any values not in [1,255],
+ empty string, ...) will return -EINVAL.
+
+What: /sys/kernel/mm/mempolicy/weighted_interleave/mode
+Date: January 2025
+Contact: Linux memory management mailing list <linux-mm@xxxxxxxxx>
+Description: Auto-weighting configuration interface
+
+ Configuration modes for weighted interleave. Can take one of
+ two options: "manual" and "auto". Default is "auto".
+
+ In auto mode, all node weights are re-calculated and overwritten
+ (visible via the nodeN interfaces) whenever new bandwidth data
+ is made available either during boot or hotplug events.
+
+ In manual mode, node weights can only be updated by the user.
+ If a node is hotplugged while the user is in manual mode,
+ the node will have a default weight of 1.
+
+ Modes can be changed by writing either "auto" or "manual" to the
+ interface. All other strings will be ignored, and -EINVAL will
+ be returned. If "auto" is written to the interface but the
+ recalculation / updates fail at any point (-ENOMEM or -ENODEV)
+ then the mode will remain in manual mode.
+
+ Writing a new weight to a node directly via the nodeN interface
+ will also automatically update the system to manual mode.
I think the last paragraph should also be included in the nodeX parameter.
@@ -2450,16 +2548,8 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
if (!weights)
return total_allocated;
- rcu_read_lock();
- table = rcu_dereference(iw_table);
- if (table)
- memcpy(weights, table, nr_node_ids);
- rcu_read_unlock();
-
- /* calculate total, detect system default usage */
for_each_node_mask(node, nodes) {
- if (!weights[node])
- weights[node] = 1;
+ weights[node] = get_il_weight(node);
weight_total += weights[node];
}
Uh-hum...
Looks like it now allows copying weights from different versions of iw_tables?
Otherwise this patch looks good to me.
Best,
Hyeonggon
