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Date: Thu, 6 Jun 2024 06:38:57 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>, linux-kernel@vger.kernel.org,
	tglx@linutronix.de, torvalds@linux-foundation.org,
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	joel@joelfernandes.org, raghavendra.kt@amd.com,
	sshegde@linux.ibm.com, boris.ostrovsky@oracle.com,
	konrad.wilk@oracle.com, Ingo Molnar <mingo@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH v2 16/35] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full
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On Thu, Jun 06, 2024 at 01:53:25PM +0200, Peter Zijlstra wrote:
> On Thu, May 30, 2024 at 04:20:26PM -0700, Paul E. McKenney wrote:
> 
> > My selfish motivation here is to avoid testing this combination unless
> > and until someone actually has a good use for it.
> 
> That doesn't make sense, the whole LAZY thing is fundamentally identical
> to FULL, except it sometimes delays the preemption a wee bit. But all
> the preemption scenarios from FULL are possible.

As noted earlier in this thread, this is not the case for non-preemptible
RCU, which disables preemption across its read-side critical sections.
In addition, from a performance/throughput viewpoint, it is not just
the possibility of preemption that matters, but also the probability.

> As such, it makes far more sense to only test FULL.

You have considerable work left to do in order to convince me of this one.

							Thanx, Paul