On Mon, Aug 12, 2019 at 5:39 PM Daniel Xu <d...@dxuuu.xyz> wrote: > > On Mon, Aug 12, 2019, at 8:57 AM, Andrii Nakryiko wrote: > > On Fri, Aug 9, 2019 at 2:47 PM Daniel Xu <d...@dxuuu.xyz> wrote: > > > > > > It's useful to know [uk]probe's nmissed and nhit stats. For example with > > > tracing tools, it's important to know when events may have been lost. > > > debugfs currently exposes a control file to get this information, but > > > it is not compatible with probes registered with the perf API. > > > > > > While bpf programs may be able to manually count nhit, there is no way > > > to gather nmissed. In other words, it is currently not possible to > > > retrieve information about FD-based probes. > > > > > > This patch adds a new ioctl that lets users query nmissed (as well as > > > nhit for completeness). We currently only add support for [uk]probes > > > but leave the possibility open for other probes like tracepoint. > > > > > > Signed-off-by: Daniel Xu <d...@dxuuu.xyz> > > > --- > > > include/linux/trace_events.h | 12 ++++++++++++ > > > include/uapi/linux/perf_event.h | 19 +++++++++++++++++++ > > > kernel/events/core.c | 20 ++++++++++++++++++++ > > > kernel/trace/trace_kprobe.c | 23 +++++++++++++++++++++++ > > > kernel/trace/trace_uprobe.c | 23 +++++++++++++++++++++++ > > > 5 files changed, 97 insertions(+) > > > > [...] > > > + struct trace_kprobe *tk = (struct trace_kprobe *)call->data; > > > + u64 nmissed, nhit; > > > + > > > + if (!capable(CAP_SYS_ADMIN)) > > > + return -EPERM; > > > + if (copy_from_user(&query, uquery, sizeof(query)))
Not sure why we are reading that struct in, if we never use that? With size as a first argument (see below about compatiblity), I'd also read just first 4 or 8 bytes only. > > > > what about forward/backward compatibility? Didn't you have a size > > field for perf_event_query_probe? > > I initially did, yes. But after thinking about it more, I'm not convinced it > is necessary. It seems the last change to the debugfs counterpart was in > the initial comit cd7e7bd5e4, ~10 years ago. I cannot think of any other > information that would be useful off the top of my head, so I figured it'd > be best if we didn't make more complicated something that doesn't seem > likely to change. If we really needed something else, I figured adding > another ioctl is pretty cheap. > > If you (or anyone) feels strongly about adding it back, I can make it a > u64 so there's no holes. Debugfs is not stable API, so I guess that matters less. I think we should support this forward/backward compatibility mechanism that kernel implements for a lot of other stable APIs. > > > > > > + return -EFAULT; > > > + > > > + nhit = trace_kprobe_nhit(tk); > > > + nmissed = tk->rp.kp.nmissed; > > > + > > > + if (put_user(nmissed, &uquery->nmissed) || > > > + put_user(nhit, &uquery->nhit)) > > > > Wouldn't it be nicer to just do one user put for entire struct (or at > > least relevant part of it with backward/forward compatibility?). > > Not sure how that didn't occur to me. Thanks. Once you add back size field for compatibility, doing it with one call will make it easier to write only first N requested bytes.
