Re: Question regarding sched_setaffinity

[Mark Hounschell <markh@compro.net>]

Steven Rostedt wrote:
> On Sun, 31 Aug 2008, Karthik Singaram Lakshmanan wrote:
> 
>> Hi,
>>  I am a noob to SMP real-time scheduling in linux, I have a question of
>> sched_setaffinity(). If I restrict an application to a single CPU
>> using the appropriate cpumask for sched_setaffinity(), Is it
>> guaranteed that the kernel will never schedule it on any other CPU? I
>> am asking this because looking at kernel/sched.c seems to use
>> cpu_clear during load balancing. I am interested in knowing whether the
>> cache will be invalidated when my task is assigned to a single CPU.
>> Are there exceptions where the cpu-mask will be overridden?
>> Thanks for taking the time to answer my query.
> 
> A task should never be scheduled on a CPU that is not in its affinity. 
> (although I hear a rumor that if a task is bound to a single CPU, and that 
> CPU is taken offline, it will be migrated. But I doubt this is true, since 
> there are tasks that would crash the system if this were true).
> 
> -- Steve
> 

It seems to be true. It's affinity mask gets changed to the CPUs left online.
At least according to sched_getaffinity.

Mark