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Fri, 13 Dec 2024 01:27:07 -0800 (PST) Date: Fri, 13 Dec 2024 17:27:02 +0800 From: Richard Clark To: Mark Kettenis Cc: Marc Zyngier , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, will@kernel.org, linux@armlinux.org.uk, mark.rutland@arm.com, torvalds@linux-foundation.org Subject: Re: Question about interrupt prioriyt of ARM GICv3/4 Message-ID: References: <86cyi5tanz.wl-maz@kernel.org> <86ldwlryzz.wl-maz@kernel.org> <871pydxde2.fsf@bloch.sibelius.xs4all.nl> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <871pydxde2.fsf@bloch.sibelius.xs4all.nl> Hi Mark, On Thu, Dec 12, 2024 at 02:02:45PM +0100, Mark Kettenis wrote: > > Date: Thu, 12 Dec 2024 10:12:32 +0000 > > From: Marc Zyngier > > Hi Marc, Richard, > > > On Thu, 12 Dec 2024 09:18:56 +0000, > > richard clark wrote: > > > > > > Hi M, > > > > Hi r, > > > > > > > > On Fri, Dec 6, 2024 at 5:37 PM Marc Zyngier wrote: > > > > > > > > On Fri, 06 Dec 2024 08:33:11 +0000, > > > > richard clark wrote: > > > > > > > > > > Hi, > > > > > Currently seems the GICv3/4 irqchip configures all the interrupts as > > > > > the same priority, I am thinking about to minimize the latency of the > > > > > interrupt for a particular device, e.g, the arm arch_timer in the RTL > > > > > system. The question is, > > > > > 1. Why don't we provide a /proc or /sys interface for the enduser to > > > > > set the priority of a specific interrupt(SPI/PPI)? > > > > > > > > I'm afraid this really has nothing to do with any particular interrupt > > > > architecture. > > > > > > > > Before thinking of exposing the interrupt priority to userspace, you > > > > should look at what this translates into for the kernel once you allow > > > > interrupts to be preempted by another one with a higher priority. > > > > > > > Interrupt priority doesn't necessarily mean the preemption, seems > > > you're talking about the interrupt preemption harm according to your > > > statement followed.Frankly I am just thinking higher priority will win > > > the lower ones in case massive external interrupts received in the GIC > > > level (you see I am still talking about GIC, not kernel) > > > > As I stated at the end of my email, the GIC only gives guarantee that > > you will ack the highest priority interrupt in finite time. Not right > > when it is made pending. Yes, it has the concept of HPPI. But that > > from the PoV of the CPU interface, not that of the distributor. Factor > > in the Stream interface, and you realise that expecting to always ack > > the highest priority pending interrupt is akin to expecting no > > reordering of packets in a network. > > > > > > > > > > This means that at every point where you would normally see a > > > > local_irq_save(), spinlock_irqsave() or equivalent, you would need to > > > > explicitly specify the priority that you allow for preemption. You > > > > should then make sure that any code that can be run during an > > > > interrupt is reentrant. You need to define which data structures can > > > > be manipulated at which priority level... The list goes on. > > > > > > > irqsave just masks the interrupt from the point of cpu, I don't think > > > it will mess things up if preemption really happens (no? then what the > > > side-effect is for the nested interrupt handling in the softirq part. > > > damage the semantic of the lock primitives?) > > > > > > > > If you want a small taste of the complexity, just look at what > > > > handling NMIs (or even pseudo-NMIs in the case of GICv3) means, and > > > > generalise it to not just two, but an arbitrary range of priorities. > > > > > > > > If you want the full blown experience, look at the BSDs and their use > > > > of spl*(). I don't think anyone has any plan to get there, and the RT > > > > patches have shown that there is little need for it. > > > > > > > As supplement，the fiq is suggested to be used as an alternative to the > > > higher priority in the RT area... > > > > > > FIQ's dead, baby. FIQ's dead. > > > > Hah, tell that to Apple! ;). > Suppose you're kiding, seems neither Apple employee nor working on its HW :) > > > > > > 2. Is there any way to verify the higher priority interrupt will have > > > > > more dominant to be selected to the CPU (IOW, the priority is really > > > > > working) in case of multiple different interrupts asserted to the GIC > > > > > at the same time(some debug registers of GIC like GICD_REEMPT_CNT :-) > > > > > to record higher priority wins)? > > > > > > > > The GIC architecture makes no promise that the interrupt you > > > > acknowledge is the highest priority pending interrupt, because this is > > > > by definition a very racy process. > > > > > > > > Also, even on busy systems, you will very rarely see two interrupts > > > > targeting the same CPU being made pending at the same time, so that > > > > the interrupt delivery system would have to arbitrate between the two. > > > > That's because interrupts are vanishingly rare in the grand scheme of > > > > things. > > > > > > > 1. I am trying to stress the external interrupts to the core#0 via the > > > stress-ng tool with one of interrupt being configured as higher > > > priority to see the benchmark data, it's time consuming as you can > > > image, still is in progress(BTW, I can't see any lockup similar hang > > > in the system with a higher priority configured) > > > > If you don't have preemption, I don't think anything wrong will > > happen. But I don't expect any benefit either. > > Based on my experience with OpenBSD, I'm not even sure there is much > benefit even if you have preemtion. > is OpenBSD has this priority feature supported? or do you have some related perf data on BSP... > > And regarding anything wrong happening: there is an interesting bug in > the RK3399 GIC integration where it gets the priorities wrong: > > https://github.com/openbsd/src/blob/feb3ea439d8f49b3c0e33f54c34631a611b98e21/sys/arch/arm64/dev/agintc.c#L395 > > (that comment is my interpretation of what's happening; I might be > misinterpreting what's really going on) > > As far as I can tell the Linux code doesn't handle that quirk. > Probably it doesn't matter because Linux only uses the priority > mechanisms to implement pseudo-NMI functionality and/or doesn't do > preemption of interrupts. > seems the BSP has the priority support but encounter the bug/quirk, correct me if I am wrong. Frankly I have no time to read the code of your link r. > > > > 2. This raises a very interesting question and I am also very curious > > > about is, what is the purpose for the GIC to introduce the interrupt > > > priority features, a placeholder feature reserved for the future? Ah, > > > hardware prefer to provide more functionalities than its being > > > actually used by software, any other justification except that? > > > > You realise that the HW is not exclusively designed for Linux, right? > > > > M. > > > > -- > > Without deviation from the norm, progress is not possible. > > > >