RE: [PATCH] SN2 user-MMIO CPU migration

Brent Casavant
2006-01-24T22:12:09Z

On Tue, 24 Jan 2006, Chen, Kenneth W wrote:

> 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.

Done.

> 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.

Actually, IA64_HAS_EXTRA_STATE() reads the TIF_* flags, and is called
in __switch_to(), which is in very close proximity to checking last_cpu.
I don't think there'll be a problem here, particularly as I've now moved
last_cpu to neighbor cpu.

Updated patch following in a few minutes.

Brent

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Brent Casavant All music is folk music. I ain't
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