Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Jason Baron (jbaron@redhat.com) wrote:
> 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?

[Steven:]

Note, I'm fine with this method too. An atomic_read() is extremely fast.
The worse that it does is to prevent gcc from optimizing a little, which
we already cause it to do due to the asm goto that we use.

[my reply to Steven]

How does the asm goto we use prohibit the compiler from moving code in
any way more than an standard branch ? It's not an "asm volatile goto",
just an asm goto.

Thanks,

Mathieu


> 
> thanks,
> 
> -Jason

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com