Re: [PATCH v2 16/35] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full

Paul E. McKenney
2024-06-06T13:38:57Z

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