From: Frederic Weisbecker <frederic@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Uladzislau Rezki <urezki@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Boqun Feng <boqun.feng@gmail.com>,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Josh Triplett <josh@joshtriplett.org>,
Joel Fernandes <joel@joelfernandes.org>,
rcu@vger.kernel.org
Subject: Re: [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq
Date: Mon, 4 Oct 2021 15:42:27 +0200 [thread overview]
Message-ID: <20211004134227.GC273854@lothringen> (raw)
In-Reply-To: <877dewmy5v.mognet@arm.com>
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?
next prev parent reply other threads:[~2021-10-04 13:43 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-29 22:10 [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 01/11] rcu/nocb: Make local rcu_nocb_lock_irqsave() safe against concurrent deoffloading Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 02/11] rcu/nocb: Prepare state machine for a new step Frederic Weisbecker
2021-10-01 17:48 ` Valentin Schneider
2021-10-04 11:35 ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 03/11] rcu/nocb: Invoke rcu_core() at the start of deoffloading Frederic Weisbecker
2021-10-01 17:50 ` Valentin Schneider
2021-10-04 12:41 ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe Frederic Weisbecker
2021-10-01 17:50 ` Valentin Schneider
2021-09-29 22:10 ` [PATCH 05/11] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe Frederic Weisbecker
2021-10-01 17:50 ` Valentin Schneider
2021-10-04 13:14 ` Frederic Weisbecker
2021-09-29 22:10 ` [RFC PATCH 06/11] rcu/nocb: Check a stable offloaded state to manipulate qlen_last_fqs_check Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 07/11] rcu/nocb: Use appropriate rcu_nocb_lock_irqsave() Frederic Weisbecker
2021-10-01 17:50 ` Valentin Schneider
2021-10-04 13:33 ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq Frederic Weisbecker
2021-10-01 17:51 ` Valentin Schneider
2021-10-04 13:42 ` Frederic Weisbecker [this message]
2021-10-05 20:55 ` Paul E. McKenney
2021-09-29 22:10 ` [PATCH 09/11] rcu: Fix callbacks processing time limit retaining cond_resched() Frederic Weisbecker
2021-10-01 17:51 ` Valentin Schneider
2021-09-29 22:10 ` [PATCH 10/11] rcu: Apply callbacks processing time limit only on softirq Frederic Weisbecker
2021-10-01 17:51 ` Valentin Schneider
2021-10-04 13:47 ` Frederic Weisbecker
2021-10-06 15:12 ` Valentin Schneider
2021-10-07 0:18 ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 11/11] rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread Frederic Weisbecker
2021-09-30 15:37 ` Sebastian Andrzej Siewior
2021-10-11 12:11 ` Frederic Weisbecker
2021-10-01 17:47 ` [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes Valentin Schneider
2021-10-08 14:03 ` Valentin Schneider
2021-10-08 15:49 ` Paul E. McKenney
2021-10-06 15:13 ` Paul E. McKenney
2021-10-07 8:49 ` Sebastian Andrzej Siewior
2021-10-07 19:12 ` Paul E. McKenney
-- strict thread matches above, loose matches on Subject: below --
2021-10-11 14:51 [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes v2 Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq Frederic Weisbecker
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