From linux-arm-kernel Tue Apr 19 13:36:32 2022 From: Yichao Yu Date: Tue, 19 Apr 2022 13:36:32 +0000 To: linux-arm-kernel Subject: Re: Kernel perf counter support (for apple M1 and others) Message-Id: X-MARC-Message: https://marc.info/?l=linux-arm-kernel&m=165037755700982 On Tue, Apr 19, 2022 at 9:34 AM Yichao Yu wrote: > > On Tue, Apr 19, 2022 at 9:09 AM Marc Zyngier wrote: > > > > On Tue, 19 Apr 2022 13:06:37 +0100, > > Yichao Yu wrote: > > > > > > > - I don't think there is any value in stashing any of these HW events > > > > in the kernel. In most cases, the kernel definition only matches the > > > > x86 definition, and doesn't accurately describe the vast majority of > > > > the events implemented on an ARM CPU. The ARM architecture mentions > > > > a handful of architectural events that actually match the kernel > > > > definition, and for these CPUs the kernel carries the in-kernel > > > > description. > > > > > > > > - For the M1, none of the above applies, because there is *NO* > > > > architectural description for the events reported by the (non > > > > architectural) PMU, and there is no guarantee that they actually > > > > match the common understanding we have of these events. > > > > > > You mentioned documents from Apple on IRC and below. Why is that the > > > only acceptable source? > > > > Because that would be the only one giving an exact definition to what > > you are counting. Anything else is guess-work. Very good guess-work, > > I'm sure, but still very much a wet finger in the air. > > > > > The entire support for M1 is based on reverse engineering/testing of > > > the hardware so why would those not be acceptable sources here as > > > well? > > > > Because there is a difference between getting something to work (the > > PMU driver itself) and interpreting the results it gives. All we know > > is that it is counting something. You can sort of guess what, but you > > don't know for sure. > > > > > My understanding is that the current cycles and instructions counters > > > were figured out this way so I don't see why you want them to be > > > removed. > > > > Because you use them as an argument to pile more crap in the > > kernel. Gee, at this stage, it is the driver itself I am going to > > remove. > > I'm sorry but I'm not sure why you are so mad about this. That's > certainly not my intention. > I specifically said that I wasn't intended to submit anything to the > kernel at this point (which I assume is the "crap" you are talking > about) because I don't know what's acceptable and I want to understand > why. > For comparison to other M1 supporting code, I'm not talking about the > perf counter driver specifically, but all of the other related > drivers. I'm sure there is a lot of code that depends on what specific > thing a register is doing. For this particular question I'd like to > know why there's a difference between the two. The answer might be > bluntly obvious to you but that is certainly not the case for me. (And > FWIW, this very reason, that I think there might be some background > knowledge that I'm lacking is why I asked on IRC first) > > > Feel free to write a document or something else. The only thing I care > > about is in the kernel tree. > > That is fair, but my point is that this is literally the first time I > heard about hardware event type being essentially deprecated. I'm > certainly not qualified to write such a document myself (at least not > right now) and I won't be unless someone could explain to me what is > actually the expectation and why, or if there's existing document > explaining all these so that I can contribute to the document of other > projects. > > > > Currently, just by reading the document of the hardware event type, it > > > seems that it should work if the hardware supports such counters. > > > > Such document would be the JSON file I mentioned. But since you have > > stated that you don't intend to write anything that ends up in the > > kernel, I guess that's a moot point. > > By document I meant that `perf_event_open(2)` doesn't say anything > about, say the instruction hardware counter doesn't count all > instructions even when you get a non-zero value. > > > > > That boat has sailed a long time ago, when the BL PMU support was > > > > introduced, and all counters are treated equally: they are *NOT* > > > > counted globally. Changing this would be an ABI break, and I seriously > > > > doubt we want to go there. > > > > > > Sorry I'm not familiar with the names here. What's the "BL PMU" > > > support? And what are the counters that are not counted globally? > > > > BL stands for Big-Little. Asymmetric support, if you want. None of the > > counters are counted globally, only per PMU type. And this is an ABI > > we cannot break. > > Are you talking about the dynamic PMU type or the hardware or raw type? > > > > > It would also mean that the kernel would need to know which counters > > > > it can accumulate over the various CPU types (which is often more than > > > > 2, these days). All of that to save userspace adding things? I doubt > > > > this is worth it. > > > > > > > > > 4. There are other events that may not make as much sense to combine > > > > > (cycles for example). However, I feel like a combined cycle count > > > > > isn't going to be much tricker to use given that the cycle count on a > > > > > single core is still affected by frequency scaling and it can still be > > > > > used correctly by pinning the thread. > > > > > > > > I don't understand what frequency scaling has anything to do with this > > > > (a cycle is still a cycle at any frequency). > > > > > > Exactly, a cycle is still a cycle, so I don't see why it's that big a > > > problem to count it globally. > > > > Because you are going to walk the list of events generated during a > > time slice, work out which ones are to be merged and which ones > > aren't, and accumulate them into global, userspace visible counters? I > > dread to imagine the effect on scheduling latency. All that to avoid > > adding two values into userspace. Great. > > OK, if doing that will always incur a big overhead then I can take > that. What I imagined was that this only needs to be done if the > process is moved to a different CPU, and also I thought there should > already be some logic in scheduling related to perf counters (I was > imagining that's when the kernel decide to add/remove counters for > other cases) which is why I thought adding such logic shouldn't make a > big difference if no counters is used by the process. I can certainly > be wrong about that. > > Also, see below. > > > > What I meant exactly was that if a code runs for 100 cycles at 1 GHz, > > > it doesn't mean it'll also run (close to) 100 cycles at 3 GHz. > > > Similarly, if it runs for 100 cycles on the E core, it doesn't mean > > > it'll run for 100 cycles on the P core. > > > > And? What do you derive from this set of statements? > > And this is replying to the original argument you gave, saying that > counting cycles across different core types doesn't make sense. What > I'm saying here is that I don't believe counting across core types > makes any more or less sense than counting cycles across different > processor frequencies. > > > > We already allow the former case to count using the same counter > > > everywhere, I don't see why the latter can't be allowed. (ABI change > > > issue aside) > > > > *blink*. If you don't see a problem with changing the ABI, I'm at a > > loss. > > Yes I do see the issue with changing ABIs. However, there are multiple > arguments you brought up and I'd like to understand each of them > individually. It's certainly possible that some of what I was asking > about is impossible for some specific reason, but I'd like to > understand all of the arguments you brought up to fully understand the > issue. (also I intended to mean here that I get that there could be > ABI issue, although I don't fully get it yet which is why I was asking > above, however, I'd like to discuss this part without concerning the > ABI issue, I didn't intend to mean that we can just ignore all the ABI > issues and just change things. If that's not what I said actually > implies, I'm sorry about that) > > > > I don't have hardware to test this but it also seems that on the new > > > intel chips, the E core and the P core are counted together. (this is > > > purely based on the lack of multiple counter support in rr to support > > > the new chip...) > > > > Colour me uninterested on both count. x86 can do whatever they want. > > Again, this is just to show that counting globally on both E and P > cores isn't something that makes as little sense as you originally > said. > > > > > > > > > The main reasons I'm asking about 3 and 4 is that > > > > > 1. Right now, even to just count instructions without pinning the > > > > > thread, I need to create two counters. > > > > > > > > How bad is that? I mean, the counters are per-CPU anyway, so there > > > > *are* N counters (N being the number of CPUs). You only have to create > > > > a counter per PMU. > > > > > > > > > 2. Even if the number isn't exactly accurate, it can still be useful > > > > > as a general guideline. Right now, even if I just want to do a quick > > > > > check, I still need to manually specify a dozen of events in `perf > > > > > stat -e` rather than simply using `perf stat` (to make it worse, perf > > > > > doesn't even provide any useful warning about it). It is also much > > > > > harder to do things generically (which is at least partially because > > > > > of the lack of documentation....). > > > > > > > > I see this as a potential perf-tool improvement. Being able to say > > > > 'Count this event on all CPU PMUs' would certainly be valuable to all > > > > asymmetric systems. > > > > > > Short answer is not that bad if and only if there's a standard and > > > documented way to do this, userspace or kernel. > > > > Feel free to improve the kernel documentation[1], which is admittedly > > pretty sparse on the subject. > > > > The kernel gives you the tools to match PMUs and CPUs (just rummage in > > sysfs). If userspace knows which counter is what, you're in business. > > Do document your findings, by any mean. > > And as I said above, without understanding all the details I can't. > And it also seems that I don't know the right way to get such > information without putting up crap so I'll appreciate it if you could > let me know how I can find out more detail about it without annoy more > people. And just to clarify, I didn't originally ask about these to know how to fix/improve the document, I asked because I don't even know what/where to fix and apparently I knew even less than I thought I did in this regard. > > > > > M. > > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm64/perf.rst#n136 > > > > -- > > Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel