Re: [PATCH] tracing/user_events: Add eBPF interface for user_event created events

[Beau Belgrave <beaub@linux.microsoft.com>]

On Wed, Mar 30, 2022 at 11:22:32AM -0700, Alexei Starovoitov wrote:
> On Wed, Mar 30, 2022 at 9:34 AM Beau Belgrave <beaub@linux.microsoft.com> wrote:
> > > >
> > > > But you are fine with uprobe costs? uprobes appear to be much more costly
> > > > than a syscall approach on the hardware I've run on.
> 
> Care to share the numbers?
> uprobe over USDT is a single trap.
> Not much slower compared to syscall with kpti.
> 

Sure, these are the numbers we have from a production device.

They are captured via perf via PERF_COUNT_HW_CPU_CYCLES.
It's running a 20K loop emitting 4 bytes of data out.
Each 4 byte event time is recorded via perf.
At the end we have the total time and the max seen.

null numbers represent a 20K loop with just perf start/stop ioctl costs.

null: min=2863, avg=2953, max=30815
uprobe: min=10994, avg=11376, max=146682
uevent: min=7043, avg=7320, max=95396
lttng: min=6270, avg=6508, max=41951

These costs include the data getting into a buffer, so they represent
what we would see in production vs the trap cost alone. For uprobe this
means we created a uprobe and attached it via tracefs to get the above
numbers.

There also seems to be some thinking around this as well from Song Liu.
Link: https://lore.kernel.org/lkml/20200801084721.1812607-1-songliubraving@fb.com/

From the link:
1. User programs are faster. The new selftest added in 5/5, shows that a
   simple uprobe program takes 1400 nanoseconds, while user program only
      takes 300 nanoseconds.

> > >
> > > Can we achieve the same/similar performance with sys_bpf(BPF_PROG_RUN)?
> > >
> >
> > I think so, the tough part is how do you let the user-space know which
> > program is attached to run? In the current code this is done by the BPF
> > program attaching to the event via perf and we run the one there if
> > any when data is emitted out via write calls.
> >
> > I would want to make sure that operators can decide where the user-space
> > data goes (perf/ftrace/eBPF) after the code has been written. With the
> > current code this is done via the tracepoint callbacks that perf/ftrace
> > hook up when operators enable recording via perf, tracefs, libbpf, etc.
> >
> > We have managed code (C#/Java) where we cannot utilize stubs or traps
> > easily due to code movement. So we are limited in how we can approach
> > this problem. Having the interface be mmap/write has enabled this
> > for us, since it's easy to interact with in most languages and gives us
> > lifetime management of the trace objects between user-space and the
> > kernel.
> 
> Then you should probably invest into making USDT work inside
> java applications instead of reinventing the wheel.
> 
> As an alternative you can do a dummy write or any other syscall
> and attach bpf on the kernel side.
> No kernel changes are necessary.

We only want syscall/tracing overheads for the specific events that are
hooked. I don't see how we could hook up a dummy write that is unique
per-event without having a way to know when the event is being traced.

Thanks,
-Beau