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

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Date:   Mon, 16 Mar 2020 17:32:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>, rcu <rcu@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-team@...com," <kernel-team@...com>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude
 variant

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