Messages in this thread | | | Date | Tue, 22 Dec 2009 12:24:36 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [this_cpu_xx V8 11/16] Generic support for this_cpu_cmpxchg |
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* Christoph Lameter (cl@linux-foundation.org) 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
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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