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by mandarax8 82 days ago
Not OP: how would you handle the second interrupt during the interrupt handler here then? I can see how you could use two separate ring buffers for different contexts, but I don't see how to handle the nested interrupt. Also indeed they just drop all these samples that get deadlocked.

Actually, as long as you use different ring buffers for interrupt/non-interrupt context, it should be fine to just drop if you encounter a deadlock due to interrupting an already running interrupt handler.
1 comments
Veserv 82 days ago
The code described is not nested interrupt handlers. It is eBPF code executing during a context switch which is interrupted by the sampling NMI which is also configured to execute eBPF code.

NMIs will not nest, so there is no risk of arbitrary nesting. So, there should be at most three nesting levels: regular, interrupt (I suspect they do not do logging during interrupts so this may not even exist in their use case), non-maskable interrupt.

Off the top of my head I can think of at least 5 unique ways to not drop the sample with your idea of separate ring buffers being one of them.

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mandarax8 80 days ago
Indeed I misread. I figured the NMI must have been nested or otherwise they wouldn't have gone through all this trouble just to drop samples :)
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