Messages in this thread | | | Date | Wed, 4 Sep 2019 10:14:48 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint |
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On Wed, Sep 04, 2019 at 12:23:10AM -0400, Joel Fernandes wrote: > 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.
Quite; I hate tracepoints for the API constraints they impose. Been bitten by that, not want to ever have to deal with that again.
> > Qais did something very similar recently: > > > > https://lore.kernel.org/lkml/20190604111459.2862-1-qais.yousef@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 :)
eBPF can access all sorts of kernel internals; if we were to deem eBPF and API we'd be fscked.
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