perf/core: Reduce context switch overhead

Skip most of the PMU context switching overhead when ctx->nr_events is 0.

50% performance overhead was observed under an extreme testcase.

Signed-off-by: leilei.lin <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
[ Rewrote the changelog. ]
Signed-off-by: Ingo Molnar <[email protected]>

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