Re: [PATCH] rt-numa: optionally ignore runtime cpumask

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Jan 24, 2022 at 05:26:26PM +0100, Sebastian Andrzej Siewior wrote:
> On 2022-01-24 09:58:31 [-0300], Marcelo Tosatti wrote:
> > You mean "using all CPUs which are part of the current affinity mask by
> > default" ? (where current affinity mask would mean user specified CPU
> > mask).
> > 
> > > And then either specify the
> > > requested CPU mask or use explicitly all CPUs.
> > 
> > Do you mean to drop 
> > 
> >         /*
> >          * Clear bits that are not set in both the cpuset from the
> >          * environment, and in the user specified affinity.
> > 
> > And just attempt to use the user specified mask? (which will then return
> > failure to the user in which case he can correct it).
> > 
> After reading it again, I don't get it.
>   cyclictest -a
> 
> Uses all CPUs in the system. 
> 
>   cyclictest -a $CPU
> 
> Uses the $CPU (mask) specified. If $CPU is not part of the current CPU
> mask, why shouldn't it work?

       -a, --affinity[=PROC-SET]
              Run threads on the set of processors given by PROC-SET.  If PROC-SET is not specified, all processors will be used.  Threads will be assigned to processors
              in the set in numeric order, in a round-robin fashion.
              The set of processors can be specified as A,B,C, or A-C, or A-B,D-F, and so on*.  The ! character can be used to negate a set.  For example, !B-D means to
              use all available CPUs except B through D.  The cpu numbers are the same as shown in the processor field in /proc/cpuinfo.  See numa(3) for more
              information on specifying CPU sets.  * Support for CPU sets requires libnuma version >= 2.  For libnuma v1, PROC-SET, if specified, must be a single CPU
              number.


/*
 * After this function is called, affinity_mask is the intersection of
 * the user supplied affinity mask and the affinity mask from the run
 * time environment
 */
static void use_current_cpuset(int max_cpus, struct bitmask *cpumask)
{
        struct bitmask *curmask;
        int i;

        curmask = numa_allocate_cpumask();
        numa_sched_getaffinity(getpid(), curmask);

        /*
         * Clear bits that are not set in both the cpuset from the
         * environment, and in the user specified affinity.
         */
        for (i = 0; i < max_cpus; i++) {
                if ((!numa_bitmask_isbitset(cpumask, i)) ||
                    (!numa_bitmask_isbitset(curmask, i)))
                        numa_bitmask_clearbit(cpumask, i);
        }

        numa_bitmask_free(curmask);
}

Consider 8 CPU system booted with isolcpus=3-7, and execution of 
"cyclictest -a 3-7".

sched_getaffinity() returns mask with bits set for CPUs 0 and 1.
The user supplied mask has bits 3-7 set.

The intersection between the user supplied mask and the affinity mask
from the run time environment has no bits set.