On Wed, 2011-06-15 at 12:11 -0700, Linus Torvalds wrote:

And it results in real problems. For example, if you use "perf record"
to see what the hell is up, the use of kernel threads for RCU
callbacks means that the RCU cost is never even seen. I don't know how
Tim did his profiling to figure out the costs, and I don't know how he
decided that the spinlock to semaphore conversion was the culprit, but
it is entirely possible that Tim didn't actually bisect the problem,
but instead used "perf record" on the exim task, saw that the
semaphore costs had gone up, and decided that it must be the
conversion.

Yes, I was using perf to do the profiling. I thought that the mutex
conversion was the most likely culprit based on the change in profile.

Tim

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