Message-ID: <43db286-7c8f-5c4-3f11-b8c103092c@inria.fr>
Date:   Wed, 8 Nov 2023 04:49:52 -0500 (EST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Ankur Arora <ankur.a.arora@...cle.com>
cc:     Julia Lawall <julia.lawall@...ia.fr>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, peterz@...radead.org,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
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        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        Nicolas Palix <nicolas.palix@...g.fr>
Subject: Re: [RFC PATCH 57/86] coccinelle: script to remove cond_resched()



On Wed, 8 Nov 2023, Ankur Arora wrote:

>
> Julia Lawall <julia.lawall@...ia.fr> writes:
>
> > On Tue, 7 Nov 2023, Ankur Arora wrote:
> >
> >> Rudimentary script to remove the straight-forward subset of
> >> cond_resched() and allies:
> >>
> >> 1)  if (need_resched())
> >> 	  cond_resched()
> >>
> >> 2)  expression*;
> >>     cond_resched();  /* or in the reverse order */
> >>
> >> 3)  if (expression)
> >> 	statement
> >>     cond_resched();  /* or in the reverse order */
> >>
> >> The last two patterns depend on the control flow level to ensure
> >> that the complex cond_resched() patterns (ex. conditioned ones)
> >> are left alone and we only pick up ones which are only minimally
> >> related the neighbouring code.
> >>
> >> Cc: Julia Lawall <Julia.Lawall@...ia.fr>
> >> Cc: Nicolas Palix <nicolas.palix@...g.fr>
> >> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> >> ---
> >>  scripts/coccinelle/api/cond_resched.cocci | 53 +++++++++++++++++++++++
> >>  1 file changed, 53 insertions(+)
> >>  create mode 100644 scripts/coccinelle/api/cond_resched.cocci
> >>
> >> diff --git a/scripts/coccinelle/api/cond_resched.cocci b/scripts/coccinelle/api/cond_resched.cocci
> >> new file mode 100644
> >> index 000000000000..bf43768a8f8c
> >> --- /dev/null
> >> +++ b/scripts/coccinelle/api/cond_resched.cocci
> >> @@ -0,0 +1,53 @@
> >> +// SPDX-License-Identifier: GPL-2.0-only
> >> +/// Remove naked cond_resched() statements
> >> +///
> >> +//# Remove cond_resched() statements when:
> >> +//#   - executing at the same control flow level as the previous or the
> >> +//#     next statement (this lets us avoid complicated conditionals in
> >> +//#     the neighbourhood.)
> >> +//#   - they are of the form "if (need_resched()) cond_resched()" which
> >> +//#     is always safe.
> >> +//#
> >> +//# Coccinelle generally takes care of comments in the immediate neighbourhood
> >> +//# but might need to handle other comments alluding to rescheduling.
> >> +//#
> >> +virtual patch
> >> +virtual context
> >> +
> >> +@ r1 @
> >> +identifier r;
> >> +@@
> >> +
> >> +(
> >> + r = cond_resched();
> >> +|
> >> +-if (need_resched())
> >> +-	cond_resched();
> >> +)
> >
> > This rule doesn't make sense.  The first branch of the disjunction will
> > never match a a place where the second branch matches.  Anyway, in the
> > second branch there is no assignment, so I don't see what the first branch
> > is protecting against.
> >
> > The disjunction is just useless.  Whether it is there or or whether only
> > the second brancha is there, doesn't have any impact on the result.
> >
> >> +
> >> +@ r2 @
> >> +expression E;
> >> +statement S,T;
> >> +@@
> >> +(
> >> + E;
> >> +|
> >> + if (E) S
> >
> > This case is not needed.  It will be matched by the next case.
> >
> >> +|
> >> + if (E) S else T
> >> +|
> >> +)
> >> +-cond_resched();
> >> +
> >> +@ r3 @
> >> +expression E;
> >> +statement S,T;
> >> +@@
> >> +-cond_resched();
> >> +(
> >> + E;
> >> +|
> >> + if (E) S
> >
> > As above.
> >
> >> +|
> >> + if (E) S else T
> >> +)
> >
> > I have the impression that you are trying to retain some cond_rescheds.
> > Could you send an example of one that you are trying to keep?  Overall,
> > the above rules seem a bit ad hoc.  You may be keeping some cases you
> > don't want to, or removing some cases that you want to keep.
>
> Right. I was trying to ensure that the script only handled the cases
> that didn't have any "interesting" connections to the surrounding code.
>
> Just to give you an example of the kind of constructs that I wanted
> to avoid:
>
> mm/memoy.c::zap_pmd_range():
>
>                 if (addr != next)
>                         pmd--;
>         } while (pmd++, cond_resched(), addr != end);
>
> mm/backing-dev.c::cleanup_offline_cgwbs_workfn()
>
>                 while (cleanup_offline_cgwb(wb))
>                         cond_resched();
>
>
>                 while (cleanup_offline_cgwb(wb))
>                         cond_resched();
>
> But from a quick check the simplest coccinelle script does a much
> better job than my overly complex (and incorrect) one:
>
> @r1@
> @@
> -       cond_resched();
>
> It avoids the first one. And transforms the second to:
>
>                 while (cleanup_offline_cgwb(wb))
>                         {}
>
> which is exactly what I wanted.

Perfect!

It could be good to run both scripts and compare the results.

julia

>
> > Of course, if you are confident that the job is done with this semantic
> > patch as it is, then that's fine too.
>
> Not at all. Thanks for pointing out the mistakes.
>
>
>
> --
> ankur
>

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