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

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Message-ID: <20201208220438.GC3916@lothringen>
Date:   Tue, 8 Dec 2020 23:04:38 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...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 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.

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

Thanks.

> 
> Thoughts?
> 
> 							Thanx, Paul