linux-kernel - Re: [RFC PATCH 47/86] rcu: select PREEMPT

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Date:   Tue, 21 Nov 2023 10:00:59 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, David.Laight@...lab.com,
        richard@....at, mjguzik@...il.com,
        Simon Horman <horms@...ge.net.au>,
        Julian Anastasov <ja@....bg>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [RFC PATCH 47/86] rcu: select PREEMPT_RCU if PREEMPT

On Mon, 20 Nov 2023 21:04:28 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> How about like this, where "Y" means allowed and "N" means not allowed:
> 
> 			Non-Preemptible RCU	Preemptible RCU
> 
> NONE:				Y			Y
> 
> VOLUNTARY:			Y			Y
> 
> PREEMPT:			N			Y
> 
> PREEMPT_RT:			N			Y
> 
> 
> We need preemptible RCU for NONE and VOLUNTARY, as you say,
> to allow CONFIG_PREEMPT_DYNAMIC to continue to work.  (OK, OK,
> CONFIG_PREEMPT_DYNAMIC is no longer, but appears to be unconditional.)
> But again, I don't see why anyone would want (much less need)
> non-preemptible RCU in the PREEMPT and PREEMPT_RT cases.  And if it is
> neither wanted nor needed, there is no point in enabling it, much less
> testing it.
> 
> Or am I missing a use case in there somewhere?

As Ankur replied, this is just an RFC, not the main goal. I'm talking about
the end product which will get rid of the PREEMPT_NONE, PREEMPT_VOLUNTARY
and PREEMPT conifgs, and there will *only* be the PREEMPT_DYNAMIC and
PREEMPT_RT.

And yes, this is going to be a slow and long processes, to find and fix all
regressions. I too am concerned about the latency that this may add. I'm
thinking we could have NEED_RESCHED_LAZY preempt when there is no mutex or
other semi critical section held (like migrate_disable()).

Right now, the use of cond_resched() is basically a whack-a-mole game where
we need to whack all the mole loops with the cond_resched() hammer. As
Thomas said, this is backwards. It makes more sense to just not preempt in
areas that can cause pain (like holding a mutex or in an RCU critical
section), but still have the general kernel be fully preemptable.

-- Steve