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McKenney" Cc: Peter Zijlstra , Ankur Arora , linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, frederic@kernel.org, efault@gmx.de Subject: Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations Message-ID: <20241023065808.ck-vPSbO@linutronix.de> References: <20241011144341.mQKXkGkm@linutronix.de> <20241015112224.KdvzKo80@linutronix.de> <2a2a3ae6-ed0b-4afe-b48a-489cf19667a3@paulmck-laptop> <20241017070710.U9bTJFMS@linutronix.de> <0313c8c5-a6a0-4d09-9f85-ac5afa379041@paulmck-laptop> <20241021112755.m7dWNbc0@linutronix.de> <20241022140933.XfxSIpDu@linutronix.de> <88b90ca8-9d73-4691-b391-43891a057c77@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <88b90ca8-9d73-4691-b391-43891a057c77@paulmck-laptop> On 2024-10-22 16:54:11 [-0700], Paul E. McKenney wrote: > > Yes. I have no idea which of those two (PREEMPT_RCU vs !PREEMPT_RCU) is > > to be preferred. Therefore I'm suggesting to make configurable here. > > As Ankur noted, the original intent was to move people from both > PREEMPT_NONE and PREEMPT_VOLUNTARY to lazy preemption. This strongly > suggests setting the value of PREEMPT_RCU to n. Not just the default, > but the value. We need to have a definite non-speculative case for > forcing people to once again worry about RCU preemptibility, and I know > of no such case. okay. > > If you have a benchmark for memory consumption or anything else of > > interest, I could throw it a box or two to get some numbers. I've been > > looking at free memory at boot and this was almost the same (+- noise). > > Unfortunately, the benchmark is the fleet and all of the various > non-public applications that run on it. :-( I see. > > > > Therefore I would suggest to make PREEMPT_RCU > > > > - selectable for LAZY && !PREEMPT_DYNAMIC, default yes > > > > - selected for LAZY && PREEMPT_DYNAMIC > > > > - the current unchanged state for NONE, VOLUNTARY, PREEMPT (with > > > > !PREEMPT_DYNAMIC) > > > > > > > > Does this make sense to you? > > > > > > Not really. At the very least, default no. > > > > > > Unless LAZIEST makes the most sense for us (which will take time to > > > figure out), in which case make PREMPT_RCU: > > > > > > - hard-defined =n for LAZIEST. > > > - selectable for LAZY && !PREEMPT_DYNAMIC, default yes > > > - selected for LAZY && PREEMPT_DYNAMIC > > > - the current unchanged state for NONE, VOLUNTARY, PREEMPT (with > > > !PREEMPT_DYNAMIC) > > > > > > Or am I still missing some aspect of this series? > > > > no, that is perfect. > > And assuming LAZIEST is not to be with us much longer, this becomes: > > - hard-defined to "no" for LAZY && !PREEMPT_DYNAMIC, just like > NONE or VOLUNTARY with !PREEMPT_DYNAMIC. > - selected for LAZY && PREEMPT_DYNAMIC > - the current unchanged state for NONE, VOLUNTARY, PREEMPT (with > !PREEMPT_DYNAMIC) > > Fair enough? Guess this hard no and worry later makes sense. > Thanx, Paul Sebastian