affinity of main thread is bound to current core

Lange Norbert norbert.lange at andritz.com
Thu Aug 22 18:04:43 CEST 2019


> -----Original Message-----
> From: Philippe Gerum <rpm at xenomai.org>
> Sent: Donnerstag, 22. August 2019 17:29
> To: Lange Norbert <norbert.lange at andritz.com>; Jan Kiszka
> <jan.kiszka at siemens.com>; Xenomai (xenomai at xenomai.org)
> <xenomai at xenomai.org>
> Subject: Re: affinity of main thread is bound to current core
>
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>
> On 8/22/19 5:16 PM, Lange Norbert via Xenomai wrote:
> >> -----Original Message-----
> >> From: Jan Kiszka <jan.kiszka at siemens.com>
> >> Sent: Donnerstag, 22. August 2019 16:52
> >> To: Lange Norbert <norbert.lange at andritz.com>; Xenomai
> >> (xenomai at xenomai.org) <xenomai at xenomai.org>
> >> Subject: Re: affinity of main thread is bound to current core
> >>
> >> E-MAIL FROM A NON-ANDRITZ SOURCE: AS A SECURITY MEASURE,
> PLEASE
> >> EXERCISE CAUTION WITH E-MAIL CONTENT AND ANY LINKS OR
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> >>
> >>
> >> On 21.08.19 14:12, Lange Norbert via Xenomai wrote:
> >>> Hello,
> >>>
> >>> I use Xenomai master on ipipe-core-4.19.60-x86-5.
> >>> I start out with an affinity mask of 0xF, in the function
> >>> cobalt_init_2,
> >> pthread_setschedparam will get called, after the syscall
> >> sc_cobalt_thread_setschedparam_ex the affinity mask will contain a
> >> single CPU (supposedly the current one).
> >>>
> >>> All methods to control affinity are executed before this point
> >>> (cmdline args,
> >> /proc/xenomai/affinity), so there is no working way to control it.
> >>>
> >>
> >> Not sure I get the problem yet: "some-xenomai-app --cpu-affinity X"
> >> seems to work fine, so does setting /proc/xenomai/affinity. Can you
> >> describe more concretely what behaves unexpectedly, maybe with a test
> case?
> >
> > Well, it does not work for me.
> >
> > ------------------ file test.c
> > #define _GNU_SOURCE
> > #include <stdio.h>
> >
> > #include <sched.h>
> > #include <pthread.h>
> >
> > static void printaff()
> > {
> >     cpu_set_t cpuset;
> >     pthread_getaffinity_np(pthread_self(), sizeof(cpuset), &cpuset);
> >     printf("Affinity: ");
> >     for(unsigned i = 0; i < 256; ++i)
> >     {
> >         if(CPU_ISSET(i, &cpuset))
> >             printf("%u,", i);
> >     }
> >     printf("\n");
> > }
> >
> > int main(int argc, char const *argv[]) { printaff(); return 0; }
> > ------------------
> >
> > root at buildroot:~# cat /proc/xenomai/affinity 0000000f
> >
> > root at buildroot:~# /tmp/a.out
> > Affinity: 2,
> >
> > root at buildroot:~# /tmp/a.out app --cpu-affinity 0,1,2,3
> > Affinity: 3,
> >
> > I traced the point where affinity collapsed to the
> > sc_cobalt_thread_setschedparam_ex call.
> >
>
> Feature, note bug. CPU migration is at odds with (very) low-latency
> requirement, so cobalt pins the thread to one of the cores defined by --cpu-
> affinity, preferably the current one. There is no point in having more than a
> single core in the affinity mask for a real-time thread: we simply don't want
> that one to be involved in any load balancing strategy.

Why is all that trickery in setup reading /proc/xenomai/affinity,
parsing --cpu-affinity, and setting the resulting affinity when
soon thereafter *the same setup* will call pthread_setschedparam,
destroying its previous work?

Is that a feature to warm up the CPU, aside of sending me on a
goose chase...

This is the C main thread, even the most unnecessary thread I start with
__STD(pthread_create) will inherit that mask.
Sorry, but I have a hard time figuring out a single argument for that "feature".

I would understand if you bind the thread to a core, potentially affecting the affinity mask,
as soon the user explicitly kicked that thread into a realtime priority.
Is the affinity mask even affecting cobalt threads that don’t get demoted (would be a precondition for low-latency)?

Norbert
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