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

[Peter Zijlstra <peterz@infradead.org>]

Duh.. this is a half-finished email I meant to save for later. Anyway,
I'll reply more.

On Fri, Apr 16, 2021 at 11:26:39AM +0200, Peter Zijlstra wrote:
> 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@infradead.org> 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.
> 
> What?
> 
> > > >  * 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!
> > 
> > > with PERF_SAMPLE_READ|PERF_SAMPLE_CGROUP and PERF_FORMAT_GROUP in a
> > > 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.