Messages in this thread | | | Date | Thu, 16 Dec 2010 14:48:53 -0500 | From | Jason Baron <> | Subject | Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1) |
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On Thu, Dec 16, 2010 at 08:41:41PM +0100, Peter Zijlstra wrote: > On Thu, 2010-12-16 at 14:36 -0500, Jason Baron wrote: > > On Thu, Dec 16, 2010 at 08:33:51PM +0100, Peter Zijlstra wrote: > > > On Thu, 2010-12-16 at 14:23 -0500, Jason Baron wrote: > > > > > > > > For the jump label disabled case, perf is using atomic_inc/dec and atomic_read > > > > to check if enabled. While other consumers (tracepoints) are just using an > > > > 'int'. I didn't want hurt the jump label disabled case for tracepoints. > > > > If we can agree to use atomic ops for tracepoints, or drop atomics from > > > > perf, that would simplify things. > > > > > > I had a quick look at the tracepoint stuff but got lost, but surely it > > > has a reference count somewhere as well, it needs to know when the last > > > probe goes away.. or does it check if the list is empty? > > > > > > Anyway, tracepoint enable/disable isn't a real fast-path, surely it > > > could suffer an atomic op? > > > > It is the atomic_read() at the tracepoint site that I am concerned > > about. > > Look at the implementation :-), its just wrapper foo, its a regular read
i did.
> for everything except some really weird archs (you really shouldn't care > about).
right, I wasn't sure how much those mattered.
> > static inline int atomic_read(const atomic_t *v) > { > return (*(volatile int *)&(v)->counter); > } > > The volatile simply forces a load to be emitted.
Mathieu, what do you think? Are you ok with an atomic_read() for checking if a tracepoint is enabled, when jump labels are disabled?
thanks,
-Jason
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