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. Thanx, Paul
