Re: [RFC PATCH v1] perf trace: Mitigate failures in parallel perf trace instances

[Namhyung Kim]

Hello,

Sorry for the late reply.

On Mon, Jun 09, 2025 at 11:38:00AM -0700, Howard Chu wrote:
> Hi Jiri,
> 
> On Wed, Jun 4, 2025 at 3:25 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> >
> > On Mon, Jun 02, 2025 at 06:17:43PM -0400, Steven Rostedt wrote:
> > > On Fri, 30 May 2025 17:00:38 -0700
> > > Howard Chu <howardchu95@xxxxxxxxx> wrote:
> > >
> > > > Hello Namhyung,
> > > >
> > > > On Fri, May 30, 2025 at 4:37 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > > > > On Wed, May 28, 2025 at 11:55:36PM -0700, Howard Chu wrote:
> > > > > > perf trace utilizes the tracepoint utility, the only filter in perf
> > > > > > trace is a filter on syscall type. For example, if perf traces only
> > > > > > openat, then it filters all the other syscalls, such as readlinkat,
> > > > > > readv, etc.
> > > > > >
> > > > > > This filtering is flawed. Consider this case: two perf trace
> > > > > > instances are running at the same time, trace instance A tracing
> > > > > > readlinkat, trace instance B tracing openat. When an openat syscall
> > > > > > enters, it triggers both BPF programs (sys_enter) in both perf trace
> > > > > > instances, these kernel functions will be executed:
> > > > > >
> > > > > > perf_syscall_enter
> > > > > >   perf_call_bpf_enter
> > > > > >     trace_call_bpf
> > > > > >       bpf_prog_run_array
> > > > > >
> > > > > > In bpf_prog_run_array:
> > > > > > ~~~
> > > > > > while ((prog = READ_ONCE(item->prog))) {
> > > > > >       run_ctx.bpf_cookie = item->bpf_cookie;
> > > > > >       ret &= run_prog(prog, ctx);
> > > > > >       item++;
> > > > > > }
> > > > > > ~~~
> > > > > >
> > > > > > I'm not a BPF expert, but by tinkering I found that if one of the BPF
> > > > > > programs returns 0, there will be no tracepoint sample. That is,
> > > > > >
> > > > > > (Is there a sample?) = ProgRetA & ProgRetB & ProgRetC
> > > > > >
> > > > > > Where ProgRetA is the return value of one of the BPF programs in the BPF
> > > > > > program array.
> > > > > >
> > > > > > Go back to the case, when two perf trace instances are tracing two
> > > > > > different syscalls, again, A is tracing readlinkat, B is tracing openat,
> > > > > > when an openat syscall enters, it triggers the sys_enter program in
> > > > > > instance A, call it ProgA, and the sys_enter program in instance B,
> > > > > > ProgB, now ProgA will return 0 because ProgA cares about readlinkat only,
> > > > > > even though ProgB returns 1; (Is there a sample?) = ProgRetA (0) &
> > > > > > ProgRetB (1) = 0. So there won't be a tracepoint sample in B's output,
> > > > > > when there really should be one.
> > > > >
> > > > > Sounds like a bug.  I think it should run bpf programs attached to the
> > > > > current perf_event only.  Isn't it the case for tracepoint + perf + bpf?
> > > >
> > > > I really can't answer that question.
> >
> > bpf programs for tracepoint are executed before the perf event specific
> > check/trigger in perf_trace_run_bpf_submit
> >
> > bpf programs array is part of struct trace_event_call so it's global per
> > tracepoint, not per perf event

Right, I think we need a way to attach a BPF program to perf_event (as
an overflow handler), not to the trace_event_call, when it comes to a
tracepoint event.  So that it can only affect behaviors of the calling
thread.  It would access the trace data as sample raw data from ctx.

Maybe it needs new link_create flags and requires BPF_PROG_TYPE_PERF_EVENT.

Wdyt?

Thanks,
Namhyung