* Linus Torvalds <torva...@linux-foundation.org> wrote:
> On Sat, 13 Jun 2009, Linus Torvalds wrote:
> >
> > On Sat, 13 Jun 2009, Paul Mackerras wrote:
> > >
> > > Linus, Andrew: OK if this goes in via the powerpc tree?
> >
> > Ok by me.
>
> Btw, do 32-bit architectures really necessarily want 64-bit
> performance counters?
>
> I realize that 32-bit counters will overflow pretty easily, but I
> do wonder about the performance impact of doing things like hashed
> spinlocks for 64-bit counters. Maybe the downsides of 64-bit perf
> counters on such architectures might outweight the upsides?
We account all sorts of non-hw bits via atomic64_t as well - for
example time related counters in nanoseconds - which wrap 32 bits at
4 seconds.
There's also security/stability relevant bits:
counter->id = atomic64_inc_return(&perf_counter_id);
We dont really want that ID to wrap ever - it could create a leaking
of one PMU context into another. (We could rewrite it by putting a
global lock around it, but still - this is a convenient primitive.)
In select places we might be able to reduce the use of atomic64_t
(that might make performance sense anyway) - but to get rid of all
of them would be quite painful. We initially started with a 32-bit
implementation and it was quite painful with fast-paced units.
So since Paul has already coded the wrappers up ... i'd really
prefer that, unless there's really compelling reasons not to do it.
Ingo
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