RE: [PATCH] SN2 user-MMIO CPU migration

Chen, Kenneth W
2006-01-24T21:43:57Z

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.

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.

- Ken