linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Date:   Mon, 09 Jul 2018 21:35:38 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested



On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote:
> 
> So here are the possible code paths when .rcu_urgent_qs is set to true:
> 
> 1.      A context switch will record the quiescent state and clear
>         .rcu_urgent_qs.  (The failure to do the clearing in current -rcu
>         for PREEMPT builds is a performance bug that I need to fix.)

What if there's nothing else runnable and there is no actual context
switch?

> 2.      A cond_resched() will cause rcu_all_qs() to be invoked, which
>         will record a quiescent state and clear .rcu_urgent_qs.
> 
> 3.      With the patch below, a scheduling-clock interrupt of a
>         non-idle non-userspace task will force a reschedule, which
>         will result in #1 above happening.
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