Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs
From: Robin Murphy <robin.murphy@arm.com>
Date: 2021-01-27 12:01:20
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linux-pci, lkml
Hi, On 2020-06-25 23:34, Nitesh Narayan Lal wrote:
quoted hunk ↗ jump to hunk
From: Alex Belits <redacted> The current implementation of cpumask_local_spread() does not respect the isolated CPUs, i.e., even if a CPU has been isolated for Real-Time task, it will return it to the caller for pinning of its IRQ threads. Having these unwanted IRQ threads on an isolated CPU adds up to a latency overhead. Restrict the CPUs that are returned for spreading IRQs only to the available housekeeping CPUs. Signed-off-by: Alex Belits <redacted> Signed-off-by: Nitesh Narayan Lal <redacted> --- lib/cpumask.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-)diff --git a/lib/cpumask.c b/lib/cpumask.c index fb22fb266f93..85da6ab4fbb5 100644 --- a/lib/cpumask.c +++ b/lib/cpumask.c@@ -6,6 +6,7 @@ #include <linux/export.h> #include <linux/memblock.h> #include <linux/numa.h> +#include <linux/sched/isolation.h> /** * cpumask_next - get the next cpu in a cpumask@@ -205,22 +206,27 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask) */ unsigned int cpumask_local_spread(unsigned int i, int node) { - int cpu; + int cpu, hk_flags; + const struct cpumask *mask; + hk_flags = HK_FLAG_DOMAIN | HK_FLAG_MANAGED_IRQ; + mask = housekeeping_cpumask(hk_flags);
AFAICS, this generally resolves to something based on cpu_possible_mask rather than cpu_online_mask as before, so could now potentially return an offline CPU. Was that an intentional change? I was just looking at the current code since I had the rare presence of mind to check if something suitable already existed before I start open-coding "any online CPU, but local node preferred" logic for handling IRQ affinity in a driver - cpumask_local_spread() appears to be almost what I want (if a bit more heavyweight), if only it would actually guarantee an online CPU as the kerneldoc claims :( Robin.
quoted hunk ↗ jump to hunk
/* Wrap: we always want a cpu. */ - i %= num_online_cpus(); + i %= cpumask_weight(mask); if (node == NUMA_NO_NODE) { - for_each_cpu(cpu, cpu_online_mask) + for_each_cpu(cpu, mask) { if (i-- == 0) return cpu; + } } else { /* NUMA first. */ - for_each_cpu_and(cpu, cpumask_of_node(node), cpu_online_mask) + for_each_cpu_and(cpu, cpumask_of_node(node), mask) { if (i-- == 0) return cpu; + } - for_each_cpu(cpu, cpu_online_mask) { + for_each_cpu(cpu, mask) { /* Skip NUMA nodes, done above. */ if (cpumask_test_cpu(cpu, cpumask_of_node(node))) continue;