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SubjectRe: [PATCH V2 1/6] perf,core: allow invalid context events to be part of sw/hw groups
> Even if you ignore the fundamental semantics of groups, there are other
> problems with allowing shared contexts:
> * The *_txn functions only get called on the group leader's PMU. If your
> system PMU has these functions, they are not called.
> * Event rotation is per ctx, but now you could have some events in a CPU
> PMU's context, and some in the uncore PMU's context. So those can race
> with each other.
> * Throttling is also per-context. So those can race with each other too.

There's also a break down of behaviour: events in the uncore context
will get migrated to another CPU in the event of a hot unplug, while
events that are grouped with CPU events (and hence live in the CPU
context) will be destroyed.


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