On Fri, 5 Sep 2008, Mark Hounschell wrote: > 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. Note, I'm not just talking about that affinity being masked off, I'm talking about a task that is bounded to only that cpu that is about to go offline. If you have a mask (bit mask) 0xf to be on all 4 cpus, and cpu 3 goes offline, all those tasks will now be 0x7. But if you started with a task with a affinity of mask 0x8, then when it goes offline, it too will be 0x7. The difference here is that this task is now running on CPUS that it wasn't able to run on before going down. At least there's a warning about it in dmesg: "process 6560 (yes) no longer affine to cpu3" -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
