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    SubjectRe: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs
    On Wed, Jan 27, 2021 at 12:36:30PM +0000, Robin Murphy wrote:
    > On 2021-01-27 12:19, Marcelo Tosatti wrote:
    > > On Wed, Jan 27, 2021 at 11:57:16AM +0000, Robin Murphy wrote:
    > > > Hi,
    > > >
    > > > On 2020-06-25 23:34, Nitesh Narayan Lal wrote:
    > > > > From: Alex Belits <abelits@marvell.com>
    > > > >
    > > > > 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 <abelits@marvell.com>
    > > > > Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
    > > > > ---
    > > > > 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?
    > >
    > > Robin,
    > >
    > > AFAICS online CPUs should be filtered.
    >
    > Apologies if I'm being thick, but can you explain how? In the case of
    > isolation being disabled or compiled out, housekeeping_cpumask() is
    > literally just "return cpu_possible_mask;". If we then iterate over that
    > with for_each_cpu() and just return the i'th possible CPU (e.g. in the
    > NUMA_NO_NODE case), what guarantees that CPU is actually online?
    >
    > Robin.

    Nothing, but that was the situation before 1abdfe706a579a702799fce465bceb9fb01d407c
    as well.

    cpumask_local_spread() should probably be disabling CPU hotplug.

    Thomas?

    >
    > > > 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.
    > > >
    > > > > /* 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;
    > > > >
    > >

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