linux-kernel - Re: [PATCH rcu 1/2] docs: Add documentation for rude and trace RCU flavors

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Date:   Wed, 20 Apr 2022 11:32:31 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH rcu 1/2] docs: Add documentation for rude and trace RCU
 flavors

On Mon, 18 Apr 2022 15:50:32 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> This commit belatedly adds documentation of Tasks Rude RCU and Tasks
> Trace RCU to RCU's requirements document.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  .../RCU/Design/Requirements/Requirements.rst  | 32 +++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
> index 45278e2974c0..ff2be1ac54c4 100644
> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> @@ -2654,6 +2654,38 @@ synchronize_rcu(), and rcu_barrier(), respectively. In
>  three APIs are therefore implemented by separate functions that check
>  for voluntary context switches.
>  
> +Tasks Rude RCU
> +~~~~~~~~~~~~~~
> +
> +Some forms of tracing need to wait for all preemption-disabled regions
> +of code running on any online CPU, including those executed when RCU is
> +not watching.  This means that synchronize_rcu() is insufficient, and
> +Tasks Rude RCU must be used instead.  This flavor of RCU does its work by
> +forcing a workqueue to be scheduled on each online CPU, hence the "Rude"
> +moniker.  And this operation is considered to be quite rude by real-time
> +workloads that don't want their ``nohz_full`` CPUs receiving IPIs and
> +by battery-powered systems that don't want their idle CPUs to be awakened.
> +
> +The tasks-rude-RCU API is also reader-marking-free and thus quite compact,
> +consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(),
> +and rcu_barrier_tasks_rude().

Are we going to be able to get rid of the "rude" version once we have all
tracing in a RCU visible section?

> +
> +Tasks Trace RCU
> +~~~~~~~~~~~~~~~
> +
> +Some forms of tracing need to sleep in readers, but cannot tolerate
> +SRCU's read-side overhead, which includes a full memory barrier in both
> +srcu_read_lock() and srcu_read_unlock().  This need is handled by a
> +Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with
> +readers.  Real-time systems that cannot tolerate IPIs may build their
> +kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at
> +the expense of adding full memory barriers to the read-side primitives.

If NOHZ_FULL is enabled, is there a way to also be able to have this full
mb on RT removed as well?

> +
> +The tasks-trace-RCU API is also reasonably compact,
> +consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(),
> +rcu_read_lock_trace_held(), call_rcu_tasks_trace(),
> +synchronize_rcu_tasks_trace(), and rcu_barrier_tasks_trace().
> +
>  Possible Future Changes
>  -----------------------
>  

For the documentation:

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve