Re: [this_cpu_xx V8 11/16] Generic support for this_cpu_cmpxchg

[Mathieu Desnoyers]

* Christoph Lameter (cl@xxxxxxxxxxxxxxxxxxxx) wrote:
> On Sat, 19 Dec 2009, Mathieu Desnoyers wrote:
> 
> > I am a bit concerned about the "generic" version of this_cpu_cmpxchg.
> > Given that what LTTng needs is basically an atomic, nmi-safe version of
> > the primitive (on all architectures that have something close to a NMI),
> > this means that it could not switch over to your primitives until we add
> > the equivalent support we currently have with local_t to all
> > architectures. The transition would be faster if we create an
> > atomic_cpu_*() variant which would map to local_t operations in the
> > initial version.
> >
> > Or maybe have I missed something in your patchset that address this ?
> 
> NMI safeness is not covered by this_cpu operations.
> 
> We could add nmi_safe_.... ops?
> 
> The atomic_cpu reference make me think that you want full (LOCK)
> semantics? Then use the regular atomic ops?

nmi_safe would probably make sense here.

But given that we have to disable preemption to add precision in terms
of trace clock timestamp, I wonder if we would really gain something
considerable performance-wise.

I also thought about the design change this requires for the per-cpu
buffer commit count pointer which would have to become a per-cpu pointer
independent of the buffer structure, and I foresee a problem with
Steven's irq off tracing which need to perform buffer exchanges while
tracing is active. Basically, having only one top-level pointer for the
buffer makes it possible to exchange it atomically, but if we have to
have two separate pointers (one for per-cpu buffer, one for per-cpu
commit count array), then we are stucked.

So given that per-cpu ops limits us in terms of data structure layout, I
am less and less sure it's the best fit for ring buffers, especially if
we don't gain much performance-wise.

Thanks,

Mathieu

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Mathieu Desnoyers
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