Re: [PATCH 5/6] perf/core: Use ioctl to communicate driver configuration to kernel

[Mathieu Poirier]

On Wed, 4 Jul 2018 at 04:35, Alexander Shishkin
<alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
>
> Mathieu Poirier <mathieu.poirier@xxxxxxxxxx> writes:
>
> > On Tue, 3 Jul 2018 at 04:57, Alexander Shishkin
> > <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
> >>
> >> On Tue, Jul 03, 2018 at 01:03:48PM +0300, Alexander Shishkin wrote:
> >> > On Mon, Jul 02, 2018 at 04:33:29PM -0600, Mathieu Poirier wrote:
> >> > > +/*
> >> > > + * PMU driver configuration works the same way as filter management above,
> >> > > + * but without the need to deal with memory mapping.  Driver configuration
> >> > > + * arrives through the SET_DRV_CONFIG ioctl() where it is validated and applied
> >> > > + * to the event.  When the PMU is ready it calls perf_event_drv_config_sync() to
> >> > > + * bring the configuration information within reach of the PMU.
> >> >
> >> > Wait a second. The reason why we dance around with the generations of filters
> >> > is the locking order of ctx::mutex vs mmap_sem. In an mmap path, where we're
> >> > notified about mapping changes, we're called under the latter, and we'd need
> >> > to grab the former to update the event configuration. In your case, the
> >> > update comes in via perf_ioctl(), where we're already holding the ctx::mutex,
> >> > so you can just kick the PMU right there, via an event_function_call() or
> >> > perf_event_stop(restart=1). In the latter case, your pmu::start() would just
> >> > grab the new configuration. Should also be about 90% less code. :)
> >>
> >> Also, since it affects the AUX buffer configuration, it is probably a one
> >> time ioctl command that you issue before you mmap the buffer. If that's
> >> the case, you don't even have to worry about stopping the event, as it
> >> shouldn't be running, because without the buffer perf_aux_output_begin()
> >> should fail and so should the pmu::add() iirc.
> >>
> >
> > The idea behind the current approach was to make the SET_DRV_CONFIG
> > ioctl() usable by other drivers where multiple ioctl() calls could be
> > performed while a session in ongoing.  I also opted to introduce a
> > _sync() function to let the PMU refresh its configuration at the time
> > of its own choosing rather than having to interrupt the session.
>
> Yes, but the times of PMU's own choosing would still be more or less
> limited to ->start()/->stop().
>
> You can also do an event_function_call(), which would call
> ->config_sync(), which would be free to decide what to do with the new
> information, up to and including doing a ->stop()/->start() sequence. My
> guess is that you'd want to do either of the following:
>  * decide to apply the new configuration immediately, and do the
>  start-stop thing,
>  * decide to defer the new configuration until the next ->start().
>
> Both should work via cross call directly from the ioctl() call.

Other than doing a cross call, do you see any advantage of using
event_function_call()?  For what I currently need the cross call is
not necessary.

>
> > But all I need for coresight is to have available the sink information
> > and PMU configuration (in an upcoming patchset) by the time
> > setup_aux() is called.  You are correct, this is a one time
> > configuration and since the event isn't running there is no need for
> > locking - I should be able to access the PMU when the ioctl is called.
> >
> > If you are fine with this bare-bone scenario and don't care much about
> > usability in different situation, I'll do a respin with minimal
> > functionality that cover my needs.
>
> It doesn't have to be bare-bones, what I'm saying is that you shouldn't
> need the event->drv_config, as you can directly call
> pmu::config(new_config) (or config_sync(), but I'm guessing the _sync
> part is redundant if you don't keep the configuration in 2 parts) from
> the ioctl() and it should cover all your bases.

I think I get your point - I will do another respin and we can go from there.

Thanks,
Mathieu

>
> Regards,
> --
> Alex