Read additional performance counters.

[vext01]

[This issue is the outcome of a discussion at a softdev thrash]

The Idea

On modern Intel CPUs there are 8 programmable performance counters which can be used to measure things like:

• Number of cache hit/misses (at varying L-levels).
• Number of branch predilections.
• ...

Krun could collect some of this stuff and it may help in diagnosing odd behaviours.

How

The programmable performance counters can be read from userspace if a flag in an MSR is set (and I think there's a Linux /proc or /sys node to enable it too). However in order to be able to read the counter for the specified core, you would need a way to force the instruction execute on a given core. This can be done with:

• cpu_set_affinity() from user space.
• smp_call_function_single() from the kernel.

[We already have code in the kernel to do the latter for a couple of fixed-function performance counters (albeit via MSRs, just because we could), but also to read APERF and MPERF.]