On Mon, Nov 20, 2023 at 03:56:30PM -0800, Paul E. McKenney wrote: > 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. So again, what is task-trace RCU ? How does it differ from say preemptible rcu, which AFAICT could be used here too, no?
