Comming to the end of my CONFIG_PROVE_LOCKING / "max latency" story:
It is not a spurious interrupt...
The source of problem is the WiFi driver (brcmfmac via SDIO) which
produces up to 6k interrupts per second, which when
CONFIG_PROOVE_LOCKING is enabled, results sporadically in very long
latencies (up to 10ms). (Possible because pending interrupts were queued).
Further after exactly 1600001 interrupts, the kernel disables this mmc
interrupt with "__report_bad_irq"
(The mmc driver is not only used for SD-cards but also for the SDIO
attached devices.)
Or with other words: never use (this) WiFi on a realtime system...
Regards and sorry for the noise I produced on this list
Matthias
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