linux-kernel - RCU vs NOHZ

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Message-ID: <YyLksEr05QTNo05Q@hirez.programming.kicks-ass.net>
Date:   Thu, 15 Sep 2022 10:39:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>,
        Paul McKenney <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: RCU vs NOHZ

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.