Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs

[Marcelo Tosatti]

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@xxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxx>
> > Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
> > ---
> >   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.

> 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;
> >