Messages in this thread | | | Date | Thu, 15 Sep 2022 10:39:12 +0200 | From | Peter Zijlstra <> | Subject | RCU vs NOHZ |
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Hi,
After watching Joel's talk about RCU and idle ticks I was wondering about why RCU doesn't have NOHZ hooks -- that is regular NOHZ, not the NOHZ_FULL stuff.
These deep idle states are only feasible during NOHZ idle, and the NOHZ path is already relatively expensive (which is offset by then mostly staying idle for a long while).
Specifically my thinking was that when a CPU goes NOHZ it can splice it's callback list onto a global list (cmpxchg), and then the jiffy-updater CPU can look at and consume this global list (xchg).
Before you say... but globals suck (they do), NOHZ already has a fair amount of global state, and as said before, it's offset by the CPU then staying idle for a fair while. If there is heavy contention on the NOHZ data, the idle governor is doing a bad job by selecting deep idle states whilst we're not actually idle for long.
The above would remove the reason for RCU to inhibit NOHZ.
Additionally; when the very last CPU goes idle (I think we know this somewhere, but I can't reaily remember where) we can insta-advance the QS machinery and run the callbacks before going (NOHZ) idle.
Is there a reason this couldn't work? To me this seems like a much simpler solution than the whole rcu-cb thing.
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