Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

[Paul E. McKenney]

On Mon, Nov 20, 2023 at 11:23:11PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 20, 2023 at 02:18:29PM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 20, 2023 at 10:47:42PM +0100, Peter Zijlstra wrote:
> > > On Mon, Nov 20, 2023 at 03:54:14PM -0500, Mathieu Desnoyers wrote:
> > > > When invoked from system call enter/exit instrumentation, accessing
> > > > user-space data is a common use-case for tracers. However, tracepoints
> > > > currently disable preemption around iteration on the registered
> > > > tracepoint probes and invocation of the probe callbacks, which prevents
> > > > tracers from handling page faults.
> > > > 
> > > > Extend the tracepoint and trace event APIs to allow defining a faultable
> > > > tracepoint which invokes its callback with preemption enabled.
> > > > 
> > > > Also extend the tracepoint API to allow tracers to request specific
> > > > probes to be connected to those faultable tracepoints. When the
> > > > TRACEPOINT_MAY_FAULT flag is provided on registration, the probe
> > > > callback will be called with preemption enabled, and is allowed to take
> > > > page faults. Faultable probes can only be registered on faultable
> > > > tracepoints and non-faultable probes on non-faultable tracepoints.
> > > > 
> > > > The tasks trace rcu mechanism is used to synchronize read-side
> > > > marshalling of the registered probes with respect to faultable probes
> > > > unregistration and teardown.
> > > 
> > > What is trace-trace rcu and why is it needed here? What's wrong with
> > > SRCU ?
> > 
> > Tasks Trace RCU avoids SRCU's full barriers and the array accesses in the
> > read-side primitives.  This can be important when tracing low-overhead
> > components of fast paths.
> 
> So why wasn't SRCU improved? That is, the above doesn't much explain.
> 
> What is the trade-off made to justify adding yet another RCU flavour?

We didn't think you would be all that happy about having each and
every context switch iterating through many tens or even hundreds of
srcu_struct structures.  For that matter, we didn't think that anyone
else would be all that happy either.  Us included.

							Thanx, Paul