linux-kernel - Re: One potential issue with concurrent execution of RCU callbacks...

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Message-ID: <20201209000306.GZ2657@paulmck-ThinkPad-P72>
Date:   Tue, 8 Dec 2020 16:03:06 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     boqun.feng@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: One potential issue with concurrent execution of RCU callbacks...

On Tue, Dec 08, 2020 at 11:04:38PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 08, 2020 at 10:24:09AM -0800, Paul E. McKenney wrote:
> > > It reduces the code scope running with BH disabled.
> > > Also narrowing down helps to understand what it actually protects.
> > 
> > I thought that you would call out unnecessarily delaying other softirq
> > handlers.  ;-)
> > 
> > But if such delays are a problem (and they might well be), then to
> > avoid them on non-rcu_nocb CPUs would instead/also require changing the
> > early-exit checks to check for other pending softirqs to the existing
> > checks involving time, need_resched, and idle.  At which point, entering and
> > exiting BH-disabled again doesn't help, other than your point about the
> > difference in BH-disabled scopes on rcu_nocb and non-rcu_nocb CPUs.
> 
> Wise observation!
> 
> > Would it make sense to exit rcu_do_batch() if more than some amount
> > of time had elapsed and there was some non-RCU softirq pending?
> > 
> > My guess is that the current tlimit checks in rcu_do_batch() make this
> > unnecessary.
> 
> Right and nobody has complained about it so far.

If they did, my thought would be to add another early-exit check,
but under the tlimit check, so that pending non-RCU softirqs might
set a shorter time limit.  For example, instead of allowing up to the
current one second in rcu_do_batch(), allow only up to 100 milliseconds
or whatever.  But there are lots of choices, which is one reason to wait
until it becomes a problem.

> But I should add a comment explaining the reason for the BH-disabled
> section in my series.

That sounds like a most excellent idea, please do!

							Thanx, Paul