Messages in this thread |  | | From | Namhyung Kim <> | Date | Sun, 9 May 2021 00:13:48 -0700 | Subject | Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups |
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Hi Peter,
Thinking about the interface a bit more...
On Fri, Apr 16, 2021 at 4:59 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Fri, Apr 16, 2021 at 08:22:38PM +0900, Namhyung Kim wrote: > > 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. > > Yes, but we'll not end up in ioctl() hell. The interface is sooo much > better. The performance thing just means we need to think harder.
So I'd like to have vector support for cgroups but it could be extended later. So open with a flag that it'd accept a vector
fd = perf_event_open(.pid=-1, .cpu=N, .flag=VECTOR);
Then it'd still need an additional interface (probably ioctl) to set (or append) the vector.
ioctl(fd, ADD_VECTOR, { .type = VEC_CGROUP, .nr = N, ... });
Maybe we also need to add FORMAT_VECTOR and use read(v) or friends to read the contents for each entry. It'd be nice if it can have a vector-specific info like cgroup-id in this case.
What do you think?
Thanks, Namhyung
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