linux-kernel - Re: [rfc patch v2] rt,nohz_full: fix nohz_full for PREEMPT_RT

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Message-ID: <20150316202409.GF28130@linutronix.de>
Date:	Mon, 16 Mar 2015 21:24:09 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, rostedt@...dmis.org,
	John Kacur <jkacur@...hat.com>
Subject: Re: [rfc patch v2] rt,nohz_full: fix nohz_full for PREEMPT_RT_FULL

* Mike Galbraith | 2015-03-13 05:53:25 [+0100]:

>First of all, a task being ticked and trying to shut the tick down will
>fail to do so due to having just awakened ksoftirqd, so let ksoftirqd
>try to do that after SOFTIRQ_TIMER processing.  Secondly, should the
>tick be shut down, we may livelock in hrtimer-cancel() because in -rt
>a callback may be running.  Break the loop, and let tick_nohz_restart()
>know that the timer is busy so it can bail.

I am a bit undecided on that one. I included it in the series but did
not enable it yet.
Just so we are on the same page here: you boot your machine with
something like
    "isolcpus=1-31 rcu_nocbs=1-31 nohz_full=1-31"
and pin all kernel threads to CPU0, right?

What you do is that you accept the fact that the timer-softirq is
scheduled for no reason and then you try to disable the tick from within
the timer-softirq. I assumed that it would work get the "expired timer"
somehow.

>Signed-off-by: Mike Galbraith <umgwanakikbuti@...il.com>

Sebastian
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