RE: [PATCH] SN2 user-MMIO CPU migration

Chen, Kenneth W
2006-01-24T22:31:00Z

Brent Casavant wrote on Tuesday, January 24, 2006 1:13 PM
> The new patch below implements pretty much exactly that. I'm not
> thrilled about carrying around the thread_info.last_cpu field even
> on non-SN2, but making the presence/updating of the field conditional
> led to huge headaches in the "should we call platform_switch_from"
> conditionals/logic.

Chen, Kenneth wrote on Tuesday, January 24, 2006 1:44 PM
> Yeah, I'm not thrilled either. Currently, on context switch kernel
> doesn't read thread_info.cpu, nor any other neighboring field. With
> the patch, it needs to do a cacheline read and possibly an update.
> If unlucky, it might be two cache lines with last_cpu 92 bytes away
> from thread_info.cpu. Perhaps, last_cpu should be next to cpu field.
> On the other hand, it may not be too bad because on kernel exit, flags
> field in thread_info is accessed for TIF_* and if kernel exit path is
> taken back-to-back with context switch, the net effect maybe small.

Thinking about it, I wonder if Ingo would entertain the idea of moving
p->thread_info.cpu into p->thread.cpu. Ingo?

- Ken