perf/arm: Special-case hetereogeneous CPUs

Commit:

  26657848502b7847 ("perf/core: Verify we have a single perf_hw_context PMU")

forcefully prevents multiple PMUs from sharing perf_hw_context, as this
generally doesn't make sense. It is a common bug for uncore PMUs to
use perf_hw_context rather than perf_invalid_context, which this detects.

However, systems exist with heterogeneous CPUs (and hence heterogeneous
HW PMUs), for which sharing perf_hw_context is necessary, and possible
in some limited cases.

To make this work we have to perform some gymnastics, as we did in these
commits:

  66eb579e66ecfea5 ("perf: allow for PMU-specific event filtering")
  c904e32a69b7c779 ("arm: perf: filter unschedulable events")

To allow those systems to work, we must allow PMUs for heterogeneous
CPUs to share perf_hw_context, though we must still disallow sharing
otherwise to detect the common misuse of perf_hw_context.

This patch adds a new PERF_PMU_CAP_HETEROGENEOUS_CPUS for this, updates
the core logic to account for this, and makes use of it in the arm_pmu
code that is used for systems with heterogeneous CPUs. Comments are
added to make the rationale clear and hopefully avoid accidental abuse.

Signed-off-by: Mark Rutland <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vince Weaver <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/20160426103346.GA20836@leverpostej
Signed-off-by: Ingo Molnar <[email protected]>

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