Date: Fri, 06 Aug 2021 09:26:31 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=257641 Stefan Eßer <se@FreeBSD.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |se@FreeBSD.org --- Comment #1 from Stefan Eßer <se@FreeBSD.org> --- Maybe the best solution is to just have a per-core check for validity of the requested PMC and verify that it is available on *all* CPUs in the applicable cpuset. You can deal with both system-scope and process-scope PMCs in exactly the same way, then. This will obviously require a list of supported PMC register number ranges per architecture attached to the per-core data. By formalizing a format (e.g. a range of start,end values) the check could become MI. For process-scope PMCs the user has to explicitly specify a cpuset that excludes the cores that do not support the PMC, giving him full control over the measured setup. If the process is not bound only to cores that support the PMC, the request must be rejected. An implicit cpuset() to limit the process-scope PMC to use just those cores that support some particular PMC might give surprising results, since the user might compare different runs with different PMCs without being aware that some of them were measured on a limited set of cores and the others on all cores. In the case of system-scope PMCs you may be able to request one PMC on cores that support it and another less optimal PMC on cores that don't. To support such a use case, the selection of cores to use for the measurement should again be explicit and based on core numbers (i.e., not just implicitly based on whether a core supports the requested PMC). In either case I'd reject the request if not all selected cores (cpuset of the process to monitor, currently active cpuset, or cores selected by the -c option of pmcstat) support it. It might be a good idea to somehow report the supported PMCs to the userland by means of dev.cpu sysctl variables. These could either identify the core architecture or just provide a list of supported PMC register numbers as a string (e.g. in the style of "1,5-10" or perhaps as a list of register names). That would make it possible to list the core numbers that allow some specific measurement, for example, without the user remembering all details of the CPU. -- You are receiving this mail because: You are the assignee for the bug.