linux-kernel - Re: Regression with suspicious RCU usage splats with cpu

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Message-Id: <20170717123938.GP2393@linux.vnet.ibm.com>
Date:   Mon, 17 Jul 2017 05:39:38 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Alex Shi <alex.shi@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <Will.Deacon@....com>
Subject: Re: Regression with suspicious RCU usage splats with cpu_pm change

On Sun, Jul 16, 2017 at 11:08:07PM -0700, Tony Lindgren wrote:
> * Alex Shi <alex.shi@...aro.org> [170716 16:25]:
> > I reused the rcu_irq_enter_irqson() from RCU_NONIDLE to avoid this issue. 
> > It works fine.
> > 
> > Tony, Could you like to give a tested-by if this patch works for you.
> 
> Yeah that keeps things booting for me with no splats so:
> 
> Tested-by: Tony Lindgren <tony@...mide.com>
> 
> In general, it seems we're missing the knowledge in Linux kernel
> of when the entire system is idle. Right now it seems that only
> cpuidle_coupled knows that?
> 
> We could probably simplify things by adding some PM state for
> entire system idle. Then cpuidle code and timer code could use
> that to test when it's safe to do whatever the SoC needs to do
> to enter deeper power states.
> 
> If we already have something like that, please do let me know :)

Well, we used to have CONFIG_NO_HZ_FULL_SYSIDLE, which detected
full-system idle lazily so as to avoid scalability bottlenecks.
https://lwn.net/Articles/558284/

No one was using it, so I removed it last merge window.  The
patch that removed it is at sysidle.2017.05.11a, which can
probably still be reverted cleanly.  Or just use v4.11 or earlier.

						Thanx, Paul