Re: [PATCH v5] mm/mempolicy: Weighted Interleave Auto-tuning

[Oscar Salvador <osalvador@suse.de>]

On Fri, Feb 07, 2025 at 12:13:35PM -0800, 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.
> 
> 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 "N" or "0" to the new "auto"
> 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 directly by using the
> nodeN interface introduced in [1]. On the other hand, auto mode is
> only entered by explicitly writing "Y" or "1" to the auto 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 moves 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@memverge.com/
> 
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> Co-developed-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> ---

Hi Joshua

> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 0ea653fa3433..16e7a5a8ebe7 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -7,6 +7,7 @@
>  #include <linux/init.h>
>  #include <linux/mm.h>
>  #include <linux/memory.h>
> +#include <linux/mempolicy.h>
>  #include <linux/vmstat.h>
>  #include <linux/notifier.h>
>  #include <linux/node.h>
> @@ -214,6 +215,12 @@ void node_set_perf_attrs(unsigned int nid, struct access_coordinate *coord,
>  			break;
>  		}
>  	}
> +
> +	/* When setting CPU access coordinates, update mempolicy */
> +	if (access == ACCESS_COORDINATE_CPU) {
> +		if (mempolicy_set_node_perf(nid, coord))
> +			pr_info("failed to set node%d mempolicy attrs\n", nid);

Not a big deal but I think you want to make that consistent with the error
pr_info? that is: "failed to set mempolicy attrs for node %d".

Also, I guess we cannot reach here with a memoryless node, right?

> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 04f35659717a..51edd3663667 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -109,6 +109,7 @@
>  #include <linux/mmu_notifier.h>
>  #include <linux/printk.h>
>  #include <linux/swapops.h>
> +#include <linux/gcd.h>
>  
>  #include <asm/tlbflush.h>
>  #include <asm/tlb.h>
> @@ -138,16 +139,18 @@ static struct mempolicy default_policy = {
>  
>  static struct mempolicy preferred_node_policy[MAX_NUMNODES];
>  
> +static uint64_t *node_bw_table;
> +
>  /*
> - * iw_table is the sysfs-set interleave weight table, a value of 0 denotes
> - * system-default value should be used. A NULL iw_table also denotes that
> - * system-default values should be used. Until the system-default table
> - * is implemented, the system-default is always 1.
> - *
> + * iw_table is the interleave weight table.
> + * If bandwidth data is available and the user is in auto mode, the table
> + * is populated with default values in [1,255].
>   * iw_table is RCU protected
>   */
>  static u8 __rcu *iw_table;
>  static DEFINE_MUTEX(iw_table_lock);
> +static const int weightiness = 32;

You explain why you chose this value in the changelog, but I would either
drop a comment, probably in reduce_interleave_weights() elaborating a
little bit, otherwise someone who stumbles upon that when reading that
code will have to go through the changelog.


-- 
Oscar Salvador
SUSE Labs