linux-kernel - Re: [RFC PATCH 71/86] treewide: lib: remove cond

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Date:   Wed, 8 Nov 2023 17:15:11 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        paulmck@...nel.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, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Thomas Graf <tgraf@...g.ch>
Subject: Re: [RFC PATCH 71/86] treewide: lib: remove cond_resched()

On Tue, Nov 07, 2023 at 03:08:07PM -0800, Ankur Arora wrote:
> There are broadly three sets of uses of cond_resched():
> 
> 1.  Calls to cond_resched() out of the goodness of our heart,
>     otherwise known as avoiding lockup splats.
> 
> 2.  Open coded variants of cond_resched_lock() which call
>     cond_resched().
> 
> 3.  Retry or error handling loops, where cond_resched() is used as a
>     quick alternative to spinning in a tight-loop.
> 
> When running under a full preemption model, the cond_resched() reduces
> to a NOP (not even a barrier) so removing it obviously cannot matter.
> 
> But considering only voluntary preemption models (for say code that
> has been mostly tested under those), for set-1 and set-2 the
> scheduler can now preempt kernel tasks running beyond their time
> quanta anywhere they are preemptible() [1]. Which removes any need
> for these explicitly placed scheduling points.
> 
> The cond_resched() calls in set-3 are a little more difficult.
> To start with, given it's NOP character under full preemption, it
> never actually saved us from a tight loop.
> With voluntary preemption, it's not a NOP, but it might as well be --
> for most workloads the scheduler does not have an interminable supply
> of runnable tasks on the runqueue.
> 
> So, cond_resched() is useful to not get softlockup splats, but not
> terribly good for error handling. Ideally, these should be replaced
> with some kind of timed or event wait.
> For now we use cond_resched_stall(), which tries to schedule if
> possible, and executes a cpu_relax() if not.
> 
> Almost all the cond_resched() calls are from set-1. Remove them.
> 
> [1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@oracle.com/
> 
> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> Cc: "David S. Miller" <davem@...emloft.net> 
> Cc: Kees Cook <keescook@...omium.org> 
> Cc: Eric Dumazet <edumazet@...gle.com> 
> Cc: Jakub Kicinski <kuba@...nel.org> 
> Cc: Paolo Abeni <pabeni@...hat.com> 
> Cc: Thomas Graf <tgraf@...g.ch>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  lib/crc32test.c          |  2 --
>  lib/crypto/mpi/mpi-pow.c |  1 -
>  lib/memcpy_kunit.c       |  5 -----
>  lib/random32.c           |  1 -
>  lib/rhashtable.c         |  2 --
>  lib/test_bpf.c           |  3 ---
>  lib/test_lockup.c        |  2 +-
>  lib/test_maple_tree.c    |  8 --------
>  lib/test_rhashtable.c    | 10 ----------
>  9 files changed, 1 insertion(+), 33 deletions(-)

Nack.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt