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MINOR: cpu-topo: ignore excess of too small clusters

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On some Arm systems (typically A76/N1) where CPUs can be associated in
pairs, clusters are reported while they have no incidence on I/O etc.
Yet it's possible to have tens of clusters of 2 CPUs each, which is
counter productive since it does not even allow to start enough threads.

Let's detect this situation as soon as there are at least 4 clusters
having each 2 CPUs or less, which is already very suspcious. In this
case, all these clusters will be reset as meaningless. In the worst
case if needed they'll be re-assigned based on L2/L3.
This commit is contained in:
Willy Tarreau 2025-03-13 15:18:58 +01:00
parent aa4776210b
commit 7263366606
1 changed files with 68 additions and 0 deletions
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68
src/cpu_topo.c
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@ -531,6 +531,7 @@ void cpu_fixup_topology(void)
int cpu, cpu2;
int curr_id, prev_id;
int min_id, neg;
int cl_cpu, small_cl;
/* fill the package id, node id and thread_id. First we'll build a bitmap
* of all unassigned ones so that we can spot the lowest unassigned one
@ -601,6 +602,73 @@ void cpu_fixup_topology(void)
}
}
/* Some machines (typically ARM cortex A76 and Neoverse-N1) report 1
* cluster per pair of cores due to the internal architecture. While
* this can occasionally make sense (i.e. big.LITTLE etc), when there
* are many clusters of few cores, this is totally pointless. Here
* we'll check if there are at least 4 2-cpu clusters, and if so, all
* the 2-cpu clusters will be cancelled.
*/
cpu_reorder_by_cluster(ha_cpu_topo, cpu_topo_maxcpus);
curr_id = -1;
cl_cpu = small_cl = 0;
for (cpu = cpu2 = 0; cpu <= cpu_topo_lastcpu; cpu++) {
if (ha_cpu_topo[cpu].cl_gid < 0)
continue;
if (ha_cpu_topo[cpu].st & HA_CPU_F_EXCL_MASK)
continue;
if (ha_cpu_topo[cpu].cl_gid != curr_id) {
if (curr_id >= 0 && cl_cpu <= 2)
small_cl++;
cl_cpu = 0;
cpu2 = cpu;
curr_id = ha_cpu_topo[cpu].cl_gid;
}
cl_cpu++;
}
/* last one */
if (cl_cpu && cl_cpu <= 2)
small_cl++;
/* here we have the number of small clusters (<=2 cpu) in small_cl */
if (small_cl >= 4) {
for (cpu = cpu2 = 0; cpu <= cpu_topo_lastcpu; cpu++) {
if (ha_cpu_topo[cpu].cl_gid < 0)
continue;
if (ha_cpu_topo[cpu].st & HA_CPU_F_EXCL_MASK)
continue;
if (ha_cpu_topo[cpu].cl_gid != curr_id) {
if (curr_id >= 0 && cl_cpu <= 2) {
/* small cluster found for curr_id */
while (cpu2 < cpu) {
if (ha_cpu_topo[cpu2].cl_gid == curr_id)
ha_cpu_topo[cpu2].cl_gid = -1;
cpu2++;
}
}
cl_cpu = 0;
cpu2 = cpu;
curr_id = ha_cpu_topo[cpu].cl_gid;
}
cl_cpu++;
}
/* handle the last cluster */
while (curr_id >= 0 && cl_cpu <= 2 && cpu2 < cpu) {
if (ha_cpu_topo[cpu2].cl_gid == curr_id)
ha_cpu_topo[cpu2].cl_gid = -1;
cpu2++;
}
}
cpu_reorder_by_index(ha_cpu_topo, cpu_topo_maxcpus);
/* assign capacity if not filled, based on the number of threads on the
* core: in a same package, SMT-capable cores are generally those
* optimized for performers while non-SMT ones are generally those