can the scheduler decide to schedule an interrupted but runnable thread on another CPU core? What are the implications for code?

[Andriy Gapon]

on 15/02/2014 00:36 Andrey Chernov said the following:
> On 15.02.2014 2:08, Andrey Chernov wrote:
>> On 14.02.2014 23:10, John Baldwin wrote:
>>
>>>> Due to this bug, not fixed yet, the real picture is more complex:
>>>> http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/163585
>>>
>>> Eh, that bug report has no useful details, as in, it doesn't list the
>>> actual commands run.  If you do 'cpuset -l 6 -s 1' to force all
>>> processes to only use CPU6, then yes, of course the other CPUs are idle
>>> because that's what you _asked_ for.  AFAICT, that is all the original
>>> reporter did.  At work we regularly add and remove CPUs from the
>>> default set (set 1) on hundreds of machines every day with ULE without
>>> any issues.
>>
>> Probably original report lack certain commands, but I provide the link
>> to the port which reproduces this bug too. All threads there are
>> assigned to the _different_ CPUs and appears as result on single one
>> with SCHED_ULE (not with SCHED_4BSD). And it is what original reporter
>> mean too. It surely happens, maybe not the first time, but on 2nd-3rd.
>> It means that cpuset_setaffinity() is completely broken form SCHED_ULE
>> at least for 3 years.
>>
> 
> This is code example from cpuminer port, in case you are interested, it is very simple:
> 
> static inline void affine_to_cpu(int id, int cpu)
> {
>         cpuset_t set;
>         CPU_ZERO(&set);
>         CPU_SET(cpu, &set);
>         cpuset_setaffinity(CPU_LEVEL_WHICH, CPU_WHICH_CPUSET, -1, sizeof(cpuset_t), &set);

I think that CPU_WHICH_TID should have been used here.

> }
> ...
> 	num_processors = sysconf(_SC_NPROCESSORS_CONF);
> 	if (!opt_n_threads)
>                 opt_n_threads = num_processors;
> ...
> In the thread itself:
> 	struct thr_info *mythr = userdata;
>         int thr_id = mythr->id;
> 
>         if (num_processors > 1 && opt_n_threads % num_processors == 0) {
>                 if (!opt_quiet)
>                         applog(LOG_INFO, "Binding thread %d to cpu %d",
>                                thr_id, thr_id % num_processors);
>                 affine_to_cpu(thr_id, thr_id % num_processors);
>         }
> 


-- 
Andriy Gapon