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

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Message-ID: <CAEXW_YREzQ8hMP_vGiQFiNAtwxPn_C0TG6mH68QaS8cES-Jr3Q@mail.gmail.com>
Date:   Mon, 16 Mar 2020 16:17:51 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     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>,
        Steven Rostedt <rostedt@...dmis.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, Mar 16, 2020 at 3:47 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Thu, Mar 12, 2020 at 11:16:55AM -0700, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> >
> > This commit adds a "rude" variant of RCU-tasks that has as quiescent
> > states schedule(), cond_resched_tasks_rcu_qs(), userspace execution,
> > and (in theory, anyway) cond_resched().  Updates make use of IPIs and
> > force an IPI and a context switch on each online CPU.  This variant
> > is useful in some situations in tracing.
>
> Would it be possible to better clarify that the "rude version" works only
> from preempt-disabled regions? Is that also true for the "non-rude" version?
>
> Also it would be good to clarify better in cover letter, how these new
> flavors relate to the existing Tasks-RCU implementation.
>
> In the existing one, a quiescent state is a task updating its context switch
> counters such that it went to sleep at least once, implying there is no
> chance it is on an about to be destroyed trampoline.
>
> However, here we are trying to determine if a task state is no longer on an
> RQ (which I gleaned from the first patch). Sounds very similar, would the
> context switch counters not help in that determination as well? If it is Ok,
> it would be good to describe in cover letter about what is exactly is a
> quiescent state and what exactly is a reader section in the cover letter, for
> both non-rude and rude version. Thanks!

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?

thanks,

 - Joel