[PATCH 0/2] qcom: add l2 cache perf events driver

[Mark Rutland]

On Wed, Jun 08, 2016 at 11:21:16AM -0400, Neil Leeder wrote:
> 
> 
> On 6/6/2016 05:04 AM, Mark Rutland wrote:
> > On Fri, Jun 03, 2016 at 05:03:30PM -0400, Neil Leeder wrote:
> >> This adds a new dynamic PMU to the Perf Events framework to program
> >> and control the L2 cache PMUs in some Qualcomm Technologies SOCs.
> >>
> >> The driver exports formatting and event information to sysfs so it can
> >> be used by the perf user space tools with the syntax:
> >> perf stat -e l2cache/event=0x42/
> >>
> >> One point to note is that there are certain combinations of events
> >> which are invalid, and which are detected in event_add().
> > 
> > Which combinations of events are invalid?
> > 
> > Please elaborate.
> > 
> >> Simply having event_add() fail would result in event_sched_in() making
> >> it Inactive, treating it as over-allocation of counters, leading to
> >> repeated attempts to allocate the events and ending up with a
> >> statistical count.  A solution for this situation is to turn the
> >> conflicting event off in event_add(). This allows a single error
> >> message to be generated, and no recurring attempts to re-add the
> >> invalid event. In order for this to work, event_sched_in()
> >> needs to detect that event_add() changed the state, and not override it
> >> and force it to Inactive.
> > 
> > For heterogeneous PMUs, we added the pmu::filter_match(event) callback
> > for a similar purpose: preventing an event from being scheduled on a
> > core which does not support that event, while allowing other events to
> > be scheduled.
> > 
> > So if you truly need to filter events, the infrastructure for doing so
> > already exists.
> > 
> > However, you will need to elaborate on "there are certain combinations
> > of events which are invalid".
> > 
> 
> Qualcomm PMUs have events arranged in a matrix of rows and columns.
> Only one event can be enabled from each column at once. So this isn't a
> heterogeneous CPU issue, and it doesn't seem to fit into filter_match()
> because it is not an absolute restriction that this event can't be
> enabled on this cpu, it's related to the other events which have 
> already been enabled.

The above is useful context. Please add (something like) it to the cover
and relevant patches in future postings!

Ok. So if I understand correctly, each counter can only count certain
events (and therefore each event can only go into some counters), rather
than all counters being identical?

So the issue is that there is no _suitable_ counter available for an
event, but there are still counters available for events in general.

This case is somewhat different to the heterogeneous PMU case.

Unfortunately, trying to filter events in this manner can be very
expensive, and allows a malicious user to DoS the system, as Peter
pointed out when I tried to do similar things in this area. Take a look
at [1] and associated replies.

If you can test the availability of a relevant counter very cheaply,
then having a specific return code for the case of no relevant counter
may be more palatable.

> >> This patchset requires:
> >> [PATCH] soc: qcom: provide mechanism for drivers to access L2 registers
> > 
> > A link would be remarkably helpful.
> 
> http://archive.arm.linux.org.uk/lurker/message/20160603.205900.1970f20d.en.html
> 
> > 
> > Better would be to fold that patch into this series, as it's the only
> > user, and both are helpful review context for the other.
> > 
> 
> The L2 PMU driver is the first user of the L2-accessors patch
> but it won't be the only one, which is why I kept it separate.

If other users aren't going to appear in the same merge window, IMO it
would be better to place them in the same series for now. Otherwise,
please have a link in the cover in future postings.

Thanks,
Mark.

[1] http://lkml.kernel.org/r/1392054264-23570-5-git-send-email-mark.rutland@arm.com