Re: [RFC][PATCH 03/14 v2] function_graph: Allow multiple users to attach to function graph

[Peter Zijlstra]

On Fri, May 24, 2019 at 08:57:44AM -0400, Steven Rostedt wrote:
> On Fri, 24 May 2019 14:27:24 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > Believe me, I rather not have that array, but I couldn't come up with a
> > > better solution to handle freeing of fgraph_ops.  
> > 
> > The trivial answer would be to refcount the thing, but can't we make
> > rcu_tasks do this?
> 
> But wouldn't refcounts require atomic operations, something that would
> be excruciatingly slow for something that runs on all functions.

Obviously, which is why I suggested something else :-)

> rcu_tasks doesn't cross voluntary sleeps, which this does.

Sure, but we can 'fix' that, surely. Alternatively we use SRCU, or
something else, a blend between SRCU and percpu-rwsem for example, SRCU
has that annoying smp_mb() on the read side, where percpu-rwsem doesn't
have that.

> > And delay the unreg until all active users are gone -- who gives a crap
> > that can take a while.
> 
> It could literally be forever (well, until the machine reboots). And
> something that could appear to be a memory leak, although a very slow
> one. But probably be hard to have more than the number of tasks on the
> system.

Again, who cares.. ? How often do you have return trace functions that
dissapear, afaict that only happens with modules, and neither
function_graph_trace nor kprobes are modules.

It'll just mean the module unload will be stuck, possibly forever.
That's not something I care about. Also, if we _really_ care, we can
mandate that module users use some sort of ugly trampoline that covers
their asses at the cost of some performance.

Getting rid of that array makes this code far saner (and I suspect
faster too).