linux-kernel - Re: [PATCH v2] ARM: Don't use complete() during __cpu

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Date:	Thu, 5 Feb 2015 11:30:47 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	Arnd Bergmann <arnd@...db.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH v2] ARM: Don't use complete() during __cpu_die

On Thu, Feb 05, 2015 at 11:28:05AM +0000, Mark Rutland wrote:
> Hi Russell,
> 
> On Thu, Feb 05, 2015 at 10:50:35AM +0000, Russell King - ARM Linux wrote:
> > On Thu, Feb 05, 2015 at 11:14:30AM +0100, Krzysztof Kozlowski wrote:
> > > The complete() should not be used on offlined CPU. Rewrite the
> > > wait-complete mechanism with wait_on_bit_timeout().
> > 
> > Yuck.
> > 
> > I think that the IPI idea would be far better, and a much smaller patch.
> > We can continue using the completions, but instead of running the
> > completion on the dying CPU, the dying CPU triggers an IPI which does
> > the completion on the requesting CPU.
> 
> This does look _much_ nicer than the bitmask approach.
> 
> [...]
> 
> > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> > index 194df2f1aa87..c623e27a9c85 100644
> > --- a/arch/arm/kernel/smp.c
> > +++ b/arch/arm/kernel/smp.c
> > @@ -73,6 +73,9 @@ enum ipi_msg_type {
> >  	IPI_IRQ_WORK,
> >  	IPI_COMPLETION,
> >  	IPI_CPU_BACKTRACE,
> > +#ifdef CONFIG_HOTPLUG_CPU
> > +	IPI_CPU_DEAD,
> > +#endif
> >  };
> 
> [...]
> 
> >  static const char *ipi_types[NR_IPI] __tracepoint_string = {
> >  #define S(x,s)	[x] = s
> >  	S(IPI_WAKEUP, "CPU wakeup interrupts"),
> 
> We'll probably want to add an entry here ("CPU teardown interrupts"?),
> and bump NR_IPI in asm/hardirq.h.

I'd need to move IPI_CPU_BACKTRACE out of the way then - that'll mostly
always be zero (even if the NMI IPI happens.)  I'll sort that when I
backport the patch to mainline kernels. :)

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