On Tue, Sep 03, 2019 at 05:05:47PM +0100, Valentin Schneider wrote: > On 03/09/2019 16:43, Radim Krčmář wrote: > > The paper "The Linux Scheduler: a Decade of Wasted Cores" used several > > custom data gathering points to better understand what was going on in > > the scheduler. > > Red Hat adapted one of them for the tracepoint framework and created a > > tool to plot a heatmap of nr_running, where the sched_update_nr_running > > tracepoint is being used for fine grained monitoring of scheduling > > imbalance. > > The tool is available from https://github.com/jirvoz/plot-nr-running. > > > > The best place for the tracepoints is inside the add/sub_nr_running, > > which requires some shenanigans to make it work as they are defined > > inside sched.h. > > The tracepoints have to be included from sched.h, which means that > > CREATE_TRACE_POINTS has to be defined for the whole header and this > > might cause problems if tree-wide headers expose tracepoints in sched.h > > dependencies, but I'd argue it's the other side's misuse of tracepoints. > > > > Moving the import sched.h line lower would require fixes in s390 and ppc > > headers, because they don't include dependecies properly and expect > > sched.h to do it, so it is simpler to keep sched.h there and > > preventively undefine CREATE_TRACE_POINTS right after. > > > > Exports of the pelt tracepoints remain because they don't need to be > > protected by CREATE_TRACE_POINTS and moving them closer would be > > unsightly. > > > > Pure trace events are frowned upon in scheduler world, try going with > trace points. Qais did something very similar recently: > > https://lore.kernel.org/lkml/20190604111459.2862-1-qais.you...@arm.com/ > > You'll have to implement the associated trace events in a module, which > lets you define your own event format and doesn't form an ABI :).
Is that really true? eBPF programs loaded from userspace can access tracepoints through BPF_RAW_TRACEPOINT_OPEN, which is UAPI: https://github.com/torvalds/linux/blob/master/include/uapi/linux/bpf.h#L103 I don't have a strong opinion about considering tracepoints as ABI / API or not, but just want to get the facts straight :) thanks, - Joel