Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Apr 16, 2021 at 7:28 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Apr 16, 2021 at 11:29:30AM +0200, Peter Zijlstra wrote:
>
> > > So I think we've had proposals for being able to close fds in the past;
> > > while preserving groups etc. We've always pushed back on that because of
> > > the resource limit issue. By having each counter be a filedesc we get a
> > > natural limit on the amount of resources you can consume. And in that
> > > respect, having to use 400k fds is things working as designed.
> > >
> > > Anyway, there might be a way around this..
>
> So how about we flip the whole thing sideways, instead of doing one
> event for multiple cgroups, do an event for multiple-cpus.
>
> Basically, allow:
>
>         perf_event_open(.pid=fd, cpu=-1, .flag=PID_CGROUP);
>
> Which would have the kernel create nr_cpus events [the corrolary is that
> we'd probably also allow: (.pid=-1, cpu=-1) ].

Do you mean it'd have separate perf_events per cpu internally?
From a cpu's perspective, there's nothing changed, right?
Then it will have the same performance problem as of now.

>
> Output could be done by adding FORMAT_PERCPU, which takes the current
> read() format and writes a copy for each CPU event. (p)read(v)() could
> be used to explode or partial read that.

Yeah, I think it's good for read.  But what about mmap?
I don't think we can use file offset since it's taken for auxtrace.
Maybe we can simply disallow that..

>
> This gets rid of the nasty variadic nature of the
> 'get-me-these-n-cgroups'. While still getting rid of the n*m fd issue
> you're facing.

As I said, it's not just a file descriptor problem.  In fact, performance
is more concerning.

Thanks,
Namhyung