linux-kernel - Re: [PATCH tip/core/rcu 3/7] rcu: Kick adaptive-ticks CPUs that are holding up RCU grace periods

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Message-ID: <CAFTL4hz9Lf6uPDNR865O3GUK3gEVJy-nSTu8okFJXV3U+KsbDA@mail.gmail.com>
Date:	Sat, 13 Apr 2013 16:06:58 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 3/7] rcu: Kick adaptive-ticks CPUs that are
 holding up RCU grace periods

2013/4/13 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>
> Adaptive-ticks CPUs inform RCU when they enter kernel mode, but they do
> not necessarily turn the scheduler-clock tick back on.  This state of
> affairs could result in RCU waiting on an adaptive-ticks CPU running
> for an extended period in kernel mode.  Such a CPU will never run the
> RCU state machine, and could therefore indefinitely extend the RCU state
> machine, sooner or later resulting in an OOM condition.
>
> This patch, inspired by an earlier patch by Frederic Weisbecker, therefore
> causes RCU's force-quiescent-state processing to check for this condition
> and to send an IPI to CPUs that remain in that state for too long.
> "Too long" currently means about three jiffies by default, which is
> quite some time for a CPU to remain in the kernel without blocking.
> The rcu_tree.jiffies_till_first_fqs and rcutree.jiffies_till_next_fqs
> sysfs variables may be used to tune "too long" if needed.
>
> Reported-by: Frederic Weisbecker <fweisbec@...il.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

It might be better if I take this patch to get it through
tip:timers/nohz so that I can keep it in sync with the rest. What do
you think?

Thanks.
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