Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant

Steven Rostedt
2020-03-16T21:32:24Z

On Mon, 16 Mar 2020 13:32:41 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> > Just curious, why is the "rude" version better than SRCU? Seems the
> > schedule_on_each_cpu() would be much slower than SRCU especially if
> > there are 1000s of CPUs involved. Is there any reason that is a better
> > alternative?
>
> The rude version has much faster readers, and the story I hear is that
> there are not expected to be all that many concurrent updaters.
>
> But to get more detail, why not ask Steven why he chose not to use SRCU?
> (I know the story for the BPF guys, and it is because of SRCU's read-side
> overhead.)

Same for the function side (if not even more so). This would require adding
a srcu_read_lock() to all functions that can be traced! That would be a huge
kill in performance. Probably to the point no one would bother even using
function tracer.

-- Steve