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Migrate back to the little core if the thread hasn't used AVX in a while.

Linux already tracks how long ago a task used AVX-512. I assume the same mechanism could be used to track AVX as well.

https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/...

It's not quite relevant to your point, but generally you wouldn't want to have one AVX2 instruction as they are only really useful if you have a bunch of data.

Yes, even if the process has 1 avx2 instruction, it will be scheduled to run on the main CPU. I mean you could switch the process back to the small CPU when you detect a long stretch of opcode without the avx2 instruction, but the constant switching between the main CPU and small CPU probably is not good for performance.

Thread migration only costs on the order of 100 microseconds, including the effect of cold caches. If you keep the AVX thread on the big core for at least 100 milliseconds at a time, you only lose ~0.2% performance.

Good to know the switching cost is low. Runtime profiling would be key to provide insight into when to switch back.