linux-kernel - Re: Tasks RCU vs Preempt RCU

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Message-Id: <20180519022918.GV3803@linux.vnet.ibm.com>
Date:   Fri, 18 May 2018 19:29:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     rostedt@...dmis.org, byungchul.park@....com,
        mathieu.desnoyers@...icios.com,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: Tasks RCU vs Preempt RCU

On Fri, May 18, 2018 at 11:36:23AM -0700, Joel Fernandes wrote:
> Hi,
> 
> I was thinking about tasks-RCU and why its needed. Since preempt-RCU allows
> tasks to be preempted in read-sections, can we not just reuse that mechanism
> for the trampolines since we track all preempted tasks so we would wait on
> all tasks preempted within a trampoline?
> 
> I am trying to understand what will _not_ work if we did that.. I'm guessing
> the answer is that that would mean the trampoline has to be wrapped with
> rcu_read_{lock,unlock} which may add some overhead, but please let me know
> if I'm missing something else..
> 
> The advantage I guess is possible elimination of an RCU variant, and also
> possibly eliminating the tasks RCU thread that monitors.. Anyway I was
> thinking more in terms of the effort of reduction of the RCU flavors etc and
> reducing complexity ideas.

The problem is that if they are preempted while executing in a trampoline,
RCU-preempt doesn't queue them nor does it wait on them.

And the problem with wrapping them with rcu_read_{lock,unlock} is that
there would be a point before the trampoline executed rcu_read_lock()
but while it was on the trampoline.  Nothing good comes from this.  ;-)

							Thanx, Paul