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

From: Peter Zijlstra
Date: Fri Apr 16 2021 - 05:27:26 EST

On Fri, Apr 16, 2021 at 08:48:12AM +0900, Namhyung Kim wrote:
> On Thu, Apr 15, 2021 at 11:51 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Tue, Apr 13, 2021 at 08:53:36AM -0700, Namhyung Kim wrote:

> > > cgroup event counting (i.e. perf stat).
> > >
> > > * PERF_EVENT_IOC_ATTACH_CGROUP - it takes a buffer consists of a
> > > 64-bit array to attach given cgroups. The first element is a
> > > number of cgroups in the buffer, and the rest is a list of cgroup
> > > ids to add a cgroup info to the given event.
> >
> > WTH is a cgroup-id? The syscall takes a fd to the path, why have two
> > different ways?
> As you know, we already use cgroup-id for sampling. Yeah we
> can do it with the fd but one of the point in this patch is to reduce
> the number of file descriptors. :)

Well, I found those patches again after I wrote that. But I'm still not
sure what a cgroup-id is from userspace.

How does userspace get one given a cgroup? (I actually mounted cgroupfs
in order to see if there's some new 'id' file to read, there is not)
Does having the cgroup-id ensure the cgroup exists? Can the cgroup-id
get re-used?

I really don't konw what the thing is. I don't use cgroups, like ever,
except when I'm forced to due to some regression or bugreport.

> Also, having cgroup-id is good to match with the result (from read)
> as it contains the cgroup information.


> > > * PERF_EVENT_IOC_READ_CGROUP - it takes a buffer consists of a 64-bit
> > > array to get the event counter values. The first element is size
> > > of the array in byte, and the second element is a cgroup id to
> > > read. The rest is to save the counter value and timings.
> >
> > :-(
> >
> > So basically you're doing a whole seconds cgroup interface, one that
> > violates the one counter per file premise and lives off of ioctl()s.
> Right, but I'm not sure that we really want a separate event for each
> cgroup if underlying hardware events are all the same.

Sure, I see where you're coming from; I just don't much like where it
got you :-)

> > *IF* we're going to do something like this, I feel we should explore the
> > whole vector-per-fd concept before proceeding. Can we make it less yuck
> > (less special ioctl() and more regular file ops. Can we apply the
> > concept to more things?
> Ideally it'd do without keeping file descriptors open. Maybe we can make
> the vector accept various types like vector-per-cgroup_id or so.

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..

> > The second patch extends the ioctl() to be more read() like, instead of
> > doing the sane things and extending read() by adding PERF_FORMAT_VECTOR
> > or whatever. In fact, this whole second ioctl() doesn't make sense to
> > have if we do indeed want to do vector-per-fd.
> One of the upside of the ioctl() is that we can pass cgroup-id to read.
> Probably we can keep the index in the vector and set the file offset
> with it. Or else just read the whole vector, and then it has a cgroup-id
> in the output like PERF_FORMAT_CGROUP?
> >
> > Also, I suppose you can already fake this, by having a
> > SW_CGROUP_SWITCHES (sorry, I though I picked those up, done now) event
> Thanks!
> > group with a bunch of events. Then the buffer will fill with the values
> > you use here.
> Right, I'll do an experiment with it.
> >
> > Yes, I suppose it has higher overhead, but you get the data you want
> > without having to do terrible things like this.
> That's true. And we don't need many things in the perf record like
> synthesizing task/mmap info. Also there's a risk we can miss some
> samples for some reason.
> Another concern is that it'd add huge slow down in the perf event
> open as it creates a mixed sw/hw group. The synchronized_rcu in
> the move_cgroup path caused significant problems in my
> environment as it adds up in proportion to the number of cpus.

Since when is perf_event_open() a performance concern? That thing is
slow in all possible ways.