On Wed, Jan 29, 2025 at 9:16 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote: > > On Wed, Jan 29, 2025 at 05:16:58PM -0800, Ian Rogers wrote: > > On Wed, Jan 29, 2025 at 1:55 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote: > > > On Wed, Jan 15, 2025 at 01:20:32PM -0800, Ian Rogers wrote: > > > > On Wed, Jan 15, 2025 at 9:59 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote: > > > > > I think the behavior should be: > > > > > > > > > > cycles -> PERF_COUNT_HW_CPU_CYCLES > > > > > cpu-cycles -> PERF_COUNT_HW_CPU_CYCLES > > > > > cpu_cycles -> no legacy -> sysfs or json > > > > > cpu/cycles/ -> sysfs or json > > > > > cpu/cpu-cycles/ -> sysfs or json > > > > > > > > So I disagree as if you add a PMU to an event name the encoding > > > > shouldn't change: > > > > 1) This historically was perf's behavior. > > > > > > Well.. I'm not sure about the history. I believe the logic I said above > > > is the historic and (I think) right behavior. > > > > You're wrong as you are describing the behavior post: > > https://lore.kernel.org/r/20231123042922.834425-1-irogers@xxxxxxxxxx > > commit a24d9d9dc096fc0d0bd85302c9a4fe4fe3b1107b from Nov 2022, but > > somehow without legacy event fall backs which Intel added with a PMU > > for hybrid. > > > > The behavior in this patch series is best for RISC-V, presumably ARM > > (particularly for Apple M? CPUs), carries ARM and Intel's tags, > > implements the behavior Arnaldo asked for, and solves the > > inconsistency that I think is fundamentally wrong in the tool that PMU > > names shouldn't matter on an event name (an inconsistency my past > > fixes introduced). It is also part of solving other problems: > > https://lore.kernel.org/linux-perf-users/20250127-counter_delegation-v3-0-64894d7e16d5@xxxxxxxxxxxx/ > > So you think the below behavior is preferred, right? > > cycles -> cpu/cycles/ (or whatever PMU name) -> sysfs or json > > And there's no way to use legacy event encodings anymore? This is absolutely the right thing to do! If sysfs/json knows better than to allow a legacy event named cycles, advertises it, then perf should select it. Not doing this was the cause of the ARM Apple M? breakage - because their PMUs looked uncore before hybrid fixes and so weren't known previously to accept legacy events and always used the sysfs/json encodings in preference. Why would or not having the PMU in the event name imply a different and sometimes known broken encoding? And then in the perf stat uniquification we can rename the event to be the version with a different encoding. It is madness to me. If a user wants to force a legacy event, even though most typically the driver is saying it knows better, they can use a raw event encoding or in the case of cycles its alias cpu-cycles. If there really is a use-case for using legacy encodings, we could introduce new legacy-cpu and legacy-cache PMUs that advertise the events, but then the wildcard behavior would be weird. To be clear, I do not know of a single use-case where the legacy encodings are actually wanted when sysfs/json have an encoding. The opposite is very much true, that legacy encodings are not wanted - hence wanting the lowering of their priority everywhere originally by ARM to fix Apple M? and then by RISC-V. > > > > You've not pointed at anything wrong in the scheme that these patches > > introduce, and are supported by vendors, except that it is a behavior > > change. I can, and have, pointed at many issues with your proposal > > above and the current behavior. The behavior change came about to work > > around PMU bugs over 2 years ago but only partially did so. It makes > > sense to remedy this and for the clean, consistent behavior this > > series achieves. It is unfortunate that it is a behavior change, but > > the first step for that was made 2 years ago. I think it also makes > > sense that something self described as legacy is a lower priority and > > of the past (wrt event naming moving forward). > > I want to clarify the event parsing behavior and to find the right way > to deal with various cases. I haven't followed the activities in this > area closely so I missed some changes in the past. Maybe the problem > is that the behavior is complex and not clarified. Hopefully we can > write it down in a doc. I think what is typical in the kernel is the source is the best documentation. By simplifying event parsing, for example, parse-events.y has been reduced from 952 lines (in v5.10) to 762 lines - so we're about 25% simpler whilst being more correct (I've fixed all the memory leaks, etc.) and avoiding expensive start-up costs, lazy initialization, etc. Having a single priority for which events are preferred, legacy vs sysfs/json with or without PMU, will further make the code base simpler and easy to understand. Thanks, Ian
