linux-kernel - Re: [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq

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Message-ID: <20211004134227.GC273854@lothringen>
Date:   Mon, 4 Oct 2021 15:42:27 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only
 on softirq

On Fri, Oct 01, 2021 at 06:51:08PM +0100, Valentin Schneider wrote:
> On 30/09/21 00:10, Frederic Weisbecker wrote:
> > The current condition to limit the number of callbacks executed in a
> > row checks the offloaded state of the rdp. Not only is it volatile
> > but it is also misleading: the rcu_core() may well be executing
> > callbacks concurrently with NOCB kthreads, and the offloaded state
> > would then be verified on both cases. As a result the limit would
> > spuriously not apply anymore on softirq while in the middle of
> > (de-)offloading process.
> >
> > Another issue with the condition is that rcu_is_callbacks_kthread()
> > doesn't check if we are actually running callbacks from rcuc itself or
> > from a softirq interrupting rcuc.
> >
> 
> Doesn't rcutree.use_softirq imply rcuc is never woken, in which case
> RCU_SOFTIRQ can't interrupt rcuc (e.g. while run atop an IRQ exit)?
> I suppose during the (de)offload sequence we could have RCU_SOFTIRQ running
> atop the NOCB CB kthread, but that's not something
> rcu_is_callbacks_kthread() detects.

Yes good point, I don't know if rcuc can be ever be interrupted by
irq_exit() -> do_softirq() -> rcu_core() itself after all.

Paul can probably confirm your point?

> 
> Also, why is rcu_is_callbacks_kthread() hardcoded to false for
> !CONFIG_RCU_BOOST? Isn't it relevant for do_rcu_batch() ratelimiting
> regardless (at least before your patches)?

I believe rcuc is only used on CONFIG_RCU_BOOST?