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@xxxxxxxxxxxx/ > > Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx> > Co-developed-by: Gregory Price <gourry@xxxxxxxxxx> > Signed-off-by: Gregory Price <gourry@xxxxxxxxxx> > Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> > --- 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
