Messages in this thread | | | Date | Wed, 4 Sep 2019 09:06:28 -0400 | From | Joel Fernandes <> | Subject | Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint |
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On Wed, Sep 04, 2019 at 11:43:33AM +0100, Qais Yousef wrote: > On 09/04/19 00:23, 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. 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 :) > > It is actually true. > > But you need to make the distinction between a tracepoint > and a trace event first.
I know this distinction well.
> What Valentin is talking about here is the *bare* > tracepoint without any event associated with them like the one I added to the > scheduler recently. These ones are not accessible via eBPF, unless something > has changed since I last tried.
Can this tracepoint be registered on with tracepoint_probe_register()? Quickly looking at these new tracepoint, they can be otherwise how would they even work right? If so, then eBPF can very well access it. Look at __bpf_probe_register() and bpf_raw_tracepoint_open() which implement the BPF_RAW_TRACEPOINT_OPEN.
> The current infrastructure needs to be expanded to allow eBPF to attach these > bare tracepoints. Something similar to what I have in [1] is needed - but > instead of creating a new macro it needs to expand the current macro. [2] might > give full context of when I was trying to come up with alternatives to using > trace events. > > [1] https://github.com/qais-yousef/linux/commit/fb9fea29edb8af327e6b2bf3bc41469a8e66df8b > [2] https://lore.kernel.org/lkml/20190415144945.tumeop4djyj45v6k@e107158-lin.cambridge.arm.com/
As I was mentioning, tracepoints, not "trace events" can already be opened directly with BPF. I don't see how these new tracepoints are different.
I wonder if this distinction of "tracepoint" being non-ABI can be documented somewhere. I would be happy to do that if there is a place for the same. I really want some general "policy" in the kernel on where we draw a line in the sand with respect to tracepoints and ABI :).
For instance, perhaps VFS can also start having non-ABI tracepoints for the benefit of people tracing the VFS.
thanks,
- Joel
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