* Andi Kleen ([email protected]) wrote:
> On Fri, Jul 06, 2007 at 10:41:44AM -0400, Mathieu Desnoyers wrote:
> > I haven't thought about making it the default for kernel space
> > preemption, but yes, it would make sense.
>
> Now it's too late -- getcpu() has infected the kernel everywhere.
> It would have made sense a few years ago.
>
>
> > ... getcpu()...
>
> Hmm ok, although i suspect it's rare to assume that. But understood
> you don't want to audit all getcpu users because of this.
>
> > using a short instead of an int on modern x86 will cause pipeline stalls
> > due to partial register use.
>
> Sorry, that's totally bogus. Primarily because the access would be directly
> on memory and there is no partial register tracking there.
>
> Besides pipeline stall is not the correct description on what would
> happen if you used a register, the worst you get is a single false dependency
> but no pipeline flush.
>
> Besides the latest x86 cpus (C2, K8) don't have much trouble with these
> false dependencies in general.
>
Yes, false dependency is what I meant. And hrm, yeah I guess that mostly
movzbl would be used to get the byte from memory and zero-extend, making
sure there is no false dependency.
> > usage, since it is followed by an unsigned long; gcc structure alignment
>
>
> > will put padding instead of the integer, which does not buy us anything
>
> on i386 unsigned long is 4 bytes.
>
Since we have, on i386:
int preempt_count; /* 0 => preemptable, <0 => BUG */
int migrate_count;/* 0: can migrate, <0: BUG */
mm_segment_t addr_limit; /* thread address space:
and:
typedef struct {
unsigned long seg;
} mm_segment_t;
Turning migrate_count into a char would only put padding between
migrate_count and addr_limit. So I still do not see a clear improvement
in memory usage there... but I don't specially care about it being an
integer or a byte though :)
Mathieu
> -Andi
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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