Re: [PATCH v2 10/15] tools/rv: Add BPF monitors

On 2026-10-19, Tomas Glozar [off-list ref] wrote:

Do I understand it correctly that the idea is to modify existing
tracefs instances when enabling an RV monitor to filter our RV-related
bpf_trace_printk events?

Not necessarily. But does the ordering matter? If I run:

$ sudo trace-cmd record -e bpf_trace:bpf_trace_printk \
                        -f 'bpf_string ~ "rv: violation: *"'

it does not matter if the instance is running before or after starting
the monitor.

quoted

quoted

I can imagine an RV-specific trace event for that, however, it would
require a separate kfunc to generate it, and ideally also distinct
labeling of a BPF program eligible to call that kfunc. The "rv"
userspace tool could then iterate over these labels so the user knows
which violation events can be expected to occur on the system.

That'd work too. It's what I initially envisioned but I'm not sure a kfunc
producing a tracepoint is something BPF folks would really like (and technically
any other unrelated BPF program could call that, unless there's some access
control mechanism I'm not aware of).

Not sure how much it matters in practice against the glob filter, it probably
really depends on the other bpf_printk users (and a mildly better usability).

bpf_trace_printk is already a BPF helper producing a tracepoint, this
would be equivalent. Of course, that does not mean we want it, but it
is something we already have in at least one instance. I know of at
least one quirk: you can have a BPF program attached to a tracepoint
that will call bpf_trace_printk(), and a tracefs instance tracing both
bpf_trace_printk() and the original tracepoint. That leads to a new
tracepoint triggering while another tracepoint is already in progress.
I don't know how it is currently handled; from what I recall when
testing RTLA's BPF actions, the bpf_trace_printk() event might get
dropped by the tracefs instance, so I had to use a different way to
signal to userspace in BPF actions test.

This is interesting because it is very likely that the RV monitor is
attaching to and generating the violation from the same tracepoints that
I may be additionally tracing. For example, the sleep RV monitor
attaches to sched_waking, which I would also use while investigating my
broken real-time system.

However, I just ran a test with a simple eBPF program that attaches to a
tracepoint and calls bpf_trace_printk(). I started trace-cmd to trace
both the attach tracepoint and also bpf_trace_printk using a glob. After
starting trace-cmd, I loaded/attached the eBPF program. The running
trace-cmd was able to capture both events (with about 15us difference in
timestamp).

As I said, to prevent unrelated BPF programs from calling the new
tracepoint, you'd need to recognize and track RV BPF programs as a
special type of program, similarly to what your original RFC did. I'm
not sure if that's what we want, but I see some advantages. In
addition to better filtering of reactor events, you could separate
reactors from the monitors entirely: you could trigger the tracepoint
unconditionally, and attach reactors as tracepoint triggers to the new
tracepoint.

Thank you for making this suggestion. This is the direction I would
prefer we take.

All of that adds complexity, though, and requires separate discussion,
so it's probably not something relevant for the initial
implementation. I just wanted to mention it so that it doesn't get
lost.

Since relocating RV to userspace requires an enormous amount of effort
and coordination (mainlining, packaging, distributions, documentation),
I do not think it makes sense to have an "initial implementation" that
is already not what we want in the long term.

John