[Bug libc/29585] sched_getcpu returns invalid results

["mjbaars1977 at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29585

--- Comment #2 from Mischa Baars <mjbaars1977 at gmail dot com> ---
(In reply to Mischa Baars from comment #0)
> Hello,
> 
> I noticed the following while I was trying SCHED_FIFO, because FIFO
> processes are not supposed to migrate from processor to processor.
> 
> When looking at the manual page of sched_getcpu(), it states that:
> 
> The call
> 
>            cpu = sched_getcpu();
> 
>        is equivalent to the following getcpu(2) call:
> 
>            int c, s;
>            s = getcpu(&c, NULL, NULL);
>            cpu = (s == -1) ? s : c;
> 
> But when you compare the output of these two in a multi-threaded program
> (using clone()), the two produce very different outcomes. Only the second
> one will give you the right answer.
> 
> The results can be reproduced with sched_setaffinity() before calling
> clone() and a call to both methods above from within the child process. My
> version uses a timer_create() to make this call periodically.
> 
> 	sched_get_cpu_timer_init(0);
> //	core_from     =	sched_get_cpu() + 1;	
> 
> 	double x = 1.0;
> 	double y = (double) (random() + 1) * 1.00E-19f;
> 
> 	for	(long i = 0; i < ((long) 1 << 32); i++)	x *= 1.00f + y;
> 
> //	sleep	(1);
> //	usleep	(random() % 1000000L);
> 	
> //	core_upto     =	sched_get_cpu() + 1;
> 	sched_get_cpu_timer_free();
> 
> Compilation is done with -Ofast -mfpmath=387 -fno-toplevel-reorder, for sake
> of completeness.
> 
> Best regards,
> Mischa Baars.

(In reply to Florian Weimer from comment #1)
> Would you please post a complete, self-contained reproducer, and state your
> glibc and kernel versions? Thanks.

Yes... working on it.

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