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

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Date:   Tue, 28 Nov 2023 10:30:53 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     paulmck@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, 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


Thomas Gleixner <tglx@...utronix.de> writes:

> Paul!
>
> On Tue, Nov 21 2023 at 07:19, Paul E. McKenney wrote:
>> On Tue, Nov 21, 2023 at 10:00:59AM -0500, Steven Rostedt wrote:
>>> 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.
>>
>> Which is quite true, but that whack-a-mole game can be ended without
>> getting rid of build-time selection of the preemption model.  Also,
>> that whack-a-mole game can be ended without eliminating all calls to
>> cond_resched().
>
> Which calls to cond_resched() should not be eliminated?
>
> They all suck and keeping some of them is just counterproductive as
> again people will sprinkle them all over the place for the very wrong
> reasons.

And, as Thomas alludes to here, cond_resched() is not always cost free.
Needing to call cond_resched() forces us to restructure hot paths in
ways that results in worse performance/complex code.

One example is clear_huge_page(), where removing the need to call
cond_resched() every once in a while allows the processor to optimize
differently.

  *Milan*     mm/clear_huge_page   x86/clear_huge_page   change
                          (GB/s)                (GB/s)

  pg-sz=2MB                14.55                 19.29    +32.5%
  pg-sz=1GB                19.34                 49.60   +156.4%

(See https://lore.kernel.org/all/20230830184958.2333078-1-ankur.a.arora@oracle.com/)

And, that's one of the simpler examples from mm. We do this kind of arbitrary
batching all over the place.

Or see the filemap_read() example that Linus gives here:
 https://lore.kernel.org/lkml/CAHk-=whpYjm_AizQij6XEfTd7xvGjrVCx5gzHcHm=2Xijt+Kyg@mail.gmail.com/#t

Thanks
--
ankur