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 >