Steven Rostedt wrote:
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
Right, thats what I was referring to also. A task bound to a single cpu
all by its self.
Mark
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