linux-kernel - Re: Interface for enabling context tracking

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Message-ID: <a9d1144e-0f75-4594-b85f-d66b1de623a0@paulmck-laptop>
Date: Thu, 10 Apr 2025 12:10:19 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Junxuan Liao <ljx@...wisc.edu>
Cc: Frederic Weisbecker <frederic@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: Interface for enabling context tracking

On Thu, Apr 10, 2025 at 01:51:39PM -0500, Junxuan Liao wrote:
> Hi all,
> 
> From what I can tell, tracepoints context_tracking:user_enter and
> user_exit might be useful for performance analysis. e.g. Figuring out how
> much time is spent handling page faults or in system calls. However
> context tracking is by default inactive and the only way to enable it is
> to turn on nohz_full for some CPUs.
> 
> Is it a good idea to support turning on and off context tracking through
> some interface accessible from the userspace?

There is some in-kernel support for turning the rcu_nocbs portion
of nohz_full on and off on a per-CPU basis, but a given CPU must be
offline in order to do this transition.  Last I heard, there are still
issues preventing this support from being generalized to cover all of
the nohz_full functionality, and I doubt that it would be exposed to
user level until all of nohz_full is supported.

The rcu_nocbs in-kernel functionality is tested regularly.

Are you interested in working on joining the noble quest of getting the
rest of the nohz_full support in place?  (Full disclosure: This stuff
is non-trivial.)

							Thanx, Paul