Re: [PATCH 6/7] drm/i915/pmu: Lazy unregister

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 23, 2024 at 10:30:08AM -0500, Lucas De Marchi wrote:
> On Tue, Jul 23, 2024 at 09:03:25AM GMT, Tvrtko Ursulin wrote:
> > 
> > On 22/07/2024 22:06, Lucas De Marchi wrote:
> > > Instead of calling perf_pmu_unregister() when unbinding, defer that to
> > > the destruction of i915 object. Since perf itself holds a reference in
> > > the event, this only happens when all events are gone, which guarantees
> > > i915 is not unregistering the pmu with live events.
> > > 
> > > Previously, running the following sequence would crash the system after
> > > ~2 tries:
> > > 
> > > 	1) bind device to i915
> > > 	2) wait events to show up on sysfs
> > > 	3) start perf  stat -I 1000 -e i915/rcs0-busy/
> > > 	4) unbind driver
> > > 	5) kill perf
> > > 
> > > Most of the time this crashes in perf_pmu_disable() while accessing the
> > > percpu pmu_disable_count. This happens because perf_pmu_unregister()
> > > destroys it with free_percpu(pmu->pmu_disable_count).
> > > 
> > > With a lazy unbind, the pmu is only unregistered after (5) as opposed to
> > > after (4). The downside is that if a new bind operation is attempted for
> > > the same device/driver without killing the perf process, i915 will fail
> > > to register the pmu (but still load successfully). This seems better
> > > than completely crashing the system.
> > 
> > So effectively allows unbind to succeed without fully unbinding the
> > driver from the device? That sounds like a significant drawback and if
> > so, I wonder if a more complicated solution wouldn't be better after
> > all. Or is there precedence for allowing userspace keeping their paws on
> > unbound devices in this way?
> 
> keeping the resources alive but "unplunged" while the hardware
> disappeared is a common thing to do... it's the whole point of the
> drmm-managed resource for example. If you bind the driver and then
> unbind it while userspace is holding a ref, next time you try to bind it
> will come up with a different card number. A similar thing that could be
> done is to adjust the name of the event - currently we add the mangled
> pci slot.
> 
> That said, I agree a better approach would be to allow
> perf_pmu_unregister() to do its job even when there are open events. On
> top of that (or as a way to help achieve that), make perf core replace
> the callbacks with stubs when pmu is unregistered - that would even kill
> the need for i915's checks on pmu->closed (and fix the lack thereof in
> other drivers).
> 
> It can be a can of worms though and may be pushed back by perf core
> maintainers, so it'd be good have their feedback.

I don't think I understand the problem. I also don't understand drivers
much -- so that might be the problem.

So the problem appears to be that the device just disappears without
warning? How can a GPU go away like that?

Since you have a notion of this device, can't you do this stubbing you
talk about? That is, if your internal device reference becomes NULL, let
the PMU methods preserve the state like no-ops.

And then when the last event goes away, tear down the whole thing.

Again, I'm not sure I'm following.