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

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Message-ID: <1532007405.2632.29.camel@infradead.org>
Date:   Thu, 19 Jul 2018 15:36:45 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>,
        mhillenb@...zon.de, linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested



On Thu, 2018-07-19 at 15:14 +0200, Frederic Weisbecker wrote:
> > I'm not sure about the context tracking condition in the code snippet
> > cited above, though. I think that's what caused my problem in the first
> > place — I have CONTEXT_TRACKING_FORCE && !NO_HZ_FULL. So in 4.15, that
> > means rcu_user_enter() did nothing and rcu_virt_note_context_switch()
> > wasn't called. Hence the observed stalls.
> > 
> > Should rcu_user_enter() itself be conditional on CONTEXT_TRACKING not
> > NO_HZ_FULL? 
> 
> Ah, CONTEXT_TRACKING_FORCE is only for testing purpose, you should not select
> it, it's going to introduce overhead. Actually I should remove that. Although
> since we have removed CONFIG_NOHZ_FULL_ALL it's the last way we have to test
> NOHZ_FULL from config alone.

I didn't select it. It defaults to y if !NO_HZ_FULL.
I'm turning it off now...


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