Messages in this thread | | | Subject | Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs | From | Robin Murphy <> | Date | Wed, 27 Jan 2021 13:49:48 +0000 |
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On 2021-01-27 13:09, Marcelo Tosatti wrote: > 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.
True, if someone calls from a racy context then there's not much we can do to ensure that any CPU *remains* online after we initially observed it to be, but when it's called from somewhere safe like a cpuhp offline handler, then picking from cpu_online_mask *did* always do the right thing (by my interpretation), whereas picking from housekeeping_cpumask() might not.
This is why I decided to ask rather than just send a patch to fix what I think might be a bug - I have no objection if this *is* intended behaviour, other than suggesting we amend the "...selects an online CPU..." comment if that aspect was never meant to be relied upon.
Thanks, Robin.
> > 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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