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x86/perf/amd: Resolve NMI latency issues for active PMCs
On AMD processors, the detection of an overflowed PMC counter in the NMI handler relies on the current value of the PMC. So, for example, to check for overflow on a 48-bit counter, bit 47 is checked to see if it is 1 (not overflowed) or 0 (overflowed). When the perf NMI handler executes it does not know in advance which PMC counters have overflowed. As such, the NMI handler will process all active PMC counters that have overflowed. NMI latency in newer AMD processors can result in multiple overflowed PMC counters being processed in one NMI and then a subsequent NMI, that does not appear to be a back-to-back NMI, not finding any PMC counters that have overflowed. This may appear to be an unhandled NMI resulting in either a panic or a series of messages, depending on how the kernel was configured. To mitigate this issue, add an AMD handle_irq callback function, amd_pmu_handle_irq(), that will invoke the common x86_pmu_handle_irq() function and upon return perform some additional processing that will indicate if the NMI has been handled or would have been handled had an earlier NMI not handled the overflowed PMC. Using a per-CPU variable, a minimum value of the number of active PMCs or 2 will be set whenever a PMC is active. This is used to indicate the possible number of NMIs that can still occur. The value of 2 is used for when an NMI does not arrive at the LAPIC in time to be collapsed into an already pending NMI. Each time the function is called without having handled an overflowed counter, the per-CPU value is checked. If the value is non-zero, it is decremented and the NMI indicates that it handled the NMI. If the value is zero, then the NMI indicates that it did not handle the NMI. Signed-off-by:Tom Lendacky <thomas.lendacky@amd.com> Signed-off-by: