Message-Id: <20161012203417.GA8847@linux.vnet.ibm.com>
Date:   Wed, 12 Oct 2016 13:34:17 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Rich Felker <dalias@...c.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH] irqchip/jcore: fix lost per-cpu interrupts

On Wed, Oct 12, 2016 at 12:35:43PM -0400, Rich Felker wrote:
> On Wed, Oct 12, 2016 at 10:18:02AM +0200, Thomas Gleixner wrote:
> > On Tue, 11 Oct 2016, Rich Felker wrote:
> > > On Sun, Oct 09, 2016 at 09:23:58PM +0200, Thomas Gleixner wrote:
> > > > On Sun, 9 Oct 2016, Rich Felker wrote:
> > > > > On Sun, Oct 09, 2016 at 01:03:10PM +0200, Thomas Gleixner wrote:
> > > > > My preference would just be to keep the branch, but with your improved
> > > > > version that doesn't need a function call:
> > > > > 
> > > > > 	irqd_is_per_cpu(irq_desc_get_irq_data(desc))
> > > > >
> > > > > While there is some overhead testing this condition every time, I can
> > > > > probably come up with several better places to look for a ~10 cycle
> > > > > improvement in the irq code path without imposing new requirements on
> > > > > the DT bindings.
> > > > 
> > > > Fair enough. Your call.
> > > >  
> > > > > As noted in my followup to the clocksource stall thread, there's also
> > > > > a possibility that it might make sense to consider the current
> > > > > behavior of having non-percpu irqs bound to a particular cpu as part
> > > > > of what's required by the compatible tag, in which case
> > > > > handle_percpu_irq or something similar/equivalent might be suitable
> > > > > for both the percpu and non-percpu cases. I don't understand the irq
> > > > > subsystem well enough to insist on that but I think it's worth
> > > > > consideration since it looks like it would improve performance of
> > > > > non-percpu interrupts a bit.
> > > > 
> > > > Well, you can use handle_percpu_irq() for your device interrupts if you
> > > > guarantee at the hardware level that there is no reentrancy. Once you make
> > > > the hardware capable of delivering them on either core the picture changes.
> > > 
> > > One more concern here -- I see that handle_simple_irq is handling the
> > > soft-disable / IRQS_PENDING flag behavior, and irq_check_poll stuff
> > > that's perhaps important too. Since soft-disable is all we have
> > > (there's no hard-disable of interrupts), is this a problem? In other
> > > words, can drivers have an expectation of not receiving interrupts
> > > when the irq is disabled? I would think anything compatible with irq
> > > sharing can't have such an expectation, but perhaps the kernel needs
> > > disabling internally for synchronization at module-unload time or
> > > similar cases?
> > 
> > Sure. A driver would be surprised getting an interrupt when it is disabled,
> > but with your exceptionally well thought out interrupt controller a pending
> > (level) interrupt which is not handled will be reraised forever and just
> > hard lock the machine.
> 
> If you want to criticize the interrupt controller design (not my work
> or under my control) for limitations in the type of hardware that can
> be hooked up to it, that's okay -- this kind of input will actually be
> useful for designing the next iteration of it -- but I don't think
> this specific possibility is a concern.

Well, if this scenario does happen, the machine will likely either lock
up silently and hard, give you RCU CPU stall warning messages, or give
you soft-lockup messages.

							Thanx, Paul

>                                         The controller is designed for
> SoC-internal use with devices that behave well, and not for level
> interrupts that require device-specific action to clear (the clearing
> of pending status is non-device-specific and takes place at the time
> the interrupt is accepted by the cpu). It might end up being that it
> makes sense to keep the AIC2 as-is but attach a separate, more
> general-purpose global interrupt cuntroller that can route to any
> cpu's AIC and that's friendlier to diverse external hardware (like the
> type of level interrupts you described) but the hardware team would
> know better than me.
> 
> > > If you think any of these things are problems I'll switch back to the
> > > conditional version rather than using handle_percpu_irq for
> > > everything.
> > 
> > It might be the approach of least surprise, but it won't make a difference
> > for the situation described above.
> 
> I'm not seeing any easily measurable performance difference with the
> version using the conditional, so I'm going to submit that as a v3.
> Whether or not there's actually a safety concern, I'm not sure, but
> I'd rather use the functions the way they were intended to be used so
> we don't have to worry about unexpected bugs or regressions if the
> internals change.
> 
> Rich
> 

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