| Message ID | 20190806233826.2478-1-dxu@dxuuu.xyz |
|---|---|
| Headers | show |
| Series | Add PERF_EVENT_IOC_QUERY_KPROBE ioctl | expand |
On 8/6/19 4:38 PM, Daniel Xu wrote: > It's useful to know kprobe's nmissed and nhit stats. For example with > tracing tools, it's important to know when events may have been lost. > There is currently no way to get that information from the perf API. > This patch adds a new ioctl that lets users query this information. Probably worthwhile to further elaborate the use case. The debugfs does have a way to count kprobe nmissed and nhit through /sys/kernel/debug/tracing/kprobe_profile. But there is no uapi to count the nmissed and nhit for FD-based kprobe interface. bpf program itself might be able to count nhit, but still no uapi to get the nmissed counters. > > Daniel Xu (3): > tracing/kprobe: Add PERF_EVENT_IOC_QUERY_KPROBE ioctl > libbpf: Add helper to extract perf fd from bpf_link > tracing/kprobe: Add self test for PERF_EVENT_IOC_QUERY_KPROBE > > include/linux/trace_events.h | 6 +++ > include/uapi/linux/perf_event.h | 23 ++++++++++ > kernel/events/core.c | 11 +++++ > kernel/trace/trace_kprobe.c | 25 +++++++++++ > tools/include/uapi/linux/perf_event.h | 23 ++++++++++ > tools/lib/bpf/libbpf.c | 13 ++++++ > tools/lib/bpf/libbpf.h | 1 + > tools/lib/bpf/libbpf.map | 5 +++ > .../selftests/bpf/prog_tests/attach_probe.c | 43 +++++++++++++++++++ > 9 files changed, 150 insertions(+) >
On Tue, Aug 6, 2019 at 4:39 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > It's useful to know kprobe's nmissed and nhit stats. For example with > tracing tools, it's important to know when events may have been lost. > There is currently no way to get that information from the perf API. > This patch adds a new ioctl that lets users query this information. > Part of the change is on perf side, so please include linux-kernel and perf mantainers and reviewers: M: Peter Zijlstra <peterz@infradead.org> M: Ingo Molnar <mingo@redhat.com> M: Arnaldo Carvalho de Melo <acme@kernel.org> R: Alexander Shishkin <alexander.shishkin@linux.intel.com> R: Jiri Olsa <jolsa@redhat.com> R: Namhyung Kim <namhyung@kernel.org> L: linux-kernel@vger.kernel.org I guess this is based on bpf-next tree. So please use subject perfix [PATCH bpf-next] ([PATCH v2 bpf-next] when resend). Thanks, Song
On Tue, Aug 6, 2019 at 4:39 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > It's useful to know kprobe's nmissed and nhit stats. For example with Is nmissed/nhit kprobe-specific? What about tracepoints and raw tracepoints, do they have something similar or they can never be missed? At least nhit still seems useful, so would be nice to have ability to get that with the same API, is it possible? > tracing tools, it's important to know when events may have been lost. > There is currently no way to get that information from the perf API. > This patch adds a new ioctl that lets users query this information. > > Daniel Xu (3): > tracing/kprobe: Add PERF_EVENT_IOC_QUERY_KPROBE ioctl > libbpf: Add helper to extract perf fd from bpf_link > tracing/kprobe: Add self test for PERF_EVENT_IOC_QUERY_KPROBE > > include/linux/trace_events.h | 6 +++ > include/uapi/linux/perf_event.h | 23 ++++++++++ > kernel/events/core.c | 11 +++++ > kernel/trace/trace_kprobe.c | 25 +++++++++++ > tools/include/uapi/linux/perf_event.h | 23 ++++++++++ > tools/lib/bpf/libbpf.c | 13 ++++++ > tools/lib/bpf/libbpf.h | 1 + > tools/lib/bpf/libbpf.map | 5 +++ > .../selftests/bpf/prog_tests/attach_probe.c | 43 +++++++++++++++++++ > 9 files changed, 150 insertions(+) > > -- > 2.20.1 >
On Wed, Aug 7, 2019, at 12:03 PM, Andrii Nakryiko wrote: > On Tue, Aug 6, 2019 at 4:39 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > > > It's useful to know kprobe's nmissed and nhit stats. For example with > > Is nmissed/nhit kprobe-specific? What about tracepoints and raw > tracepoints, do they have something similar or they can never be > missed? At least nhit still seems useful, so would be nice to have > ability to get that with the same API, is it possible? > I'm still trying to grok all the tracepoint/ftrace machinery, but it appears to me like it is kprobe/uprobe specific. My guess is that b/c tracepoints are inline (and don't require trapping interrupts), it cannot really "miss". This brings up a good point, though. I think we want the same querying functionality for uprobes so it might be worthwhile to make this API generic. Something like PERF_EVENT_IOC_QUERY_PROBE so we can later add in uprobe stats. And maybe tracepoint if it makes sense. Thoughts?
On Thu, Aug 8, 2019 at 3:32 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > On Wed, Aug 7, 2019, at 12:03 PM, Andrii Nakryiko wrote: > > On Tue, Aug 6, 2019 at 4:39 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > > > > > It's useful to know kprobe's nmissed and nhit stats. For example with > > > > Is nmissed/nhit kprobe-specific? What about tracepoints and raw > > tracepoints, do they have something similar or they can never be > > missed? At least nhit still seems useful, so would be nice to have > > ability to get that with the same API, is it possible? > > > > I'm still trying to grok all the tracepoint/ftrace machinery, but it appears > to me like it is kprobe/uprobe specific. My guess is that b/c tracepoints are > inline (and don't require trapping interrupts), it cannot really "miss". > > This brings up a good point, though. I think we want the same querying > functionality for uprobes so it might be worthwhile to make this API generic. > Something like PERF_EVENT_IOC_QUERY_PROBE so we can later add in > uprobe stats. And maybe tracepoint if it makes sense. > > Thoughts? Yeah, we should definitely unify uprobes, if possible. Having nhit would be great for tracepoints/raw_tracepoints, but we need to look whether it's possible and whether it hurts performance. With bpf_stats we trigger them only from time to time, so counting overhead might be noticeable for some use cases.