linux-kernel - Re: [RFC PATCH v3 7/7] x86/unwind/orc: remove run-time ORC unwind tables sort

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Message-ID: <20191118144137.ibxel2hmp57jvspc@treble>
Date:   Mon, 18 Nov 2019 08:41:37 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Shile Zhang' <shile.zhang@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>
Subject: Re: [RFC PATCH v3 7/7] x86/unwind/orc: remove run-time ORC unwind
 tables sort

On Mon, Nov 18, 2019 at 10:05:02AM +0000, David Laight wrote:
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> > Sent: 15 November 2019 17:47
> > On Fri, Nov 15, 2019 at 04:51:24PM +0000, David Laight wrote:
> > > From: Shile Zhang
> > > > Sent: 15 November 2019 06:48
> > > ...
> > > >  arch/x86/kernel/unwind_orc.c | 8 +++++---
> > > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
> > > > index 332ae6530fa8..280da6fa9922 100644
> > > > --- a/arch/x86/kernel/unwind_orc.c
> > > > +++ b/arch/x86/kernel/unwind_orc.c
> > > > @@ -273,9 +273,11 @@ void __init unwind_init(void)
> > > >  		return;
> > > >  	}
> > > >
> > > > -	/* Sort the .orc_unwind and .orc_unwind_ip tables: */
> > > > -	sort(__start_orc_unwind_ip, num_entries, sizeof(int), orc_sort_cmp,
> > > > -	     orc_sort_swap);
> > > > +	/*
> > > > +	 * Note, orc_unwind and orc_unwind_ip tables has been sorted in
> > > > +	 * vmlinux link phase by sorttable tool at build time.
> > > > +	 * Its ready for binary search now.
> > > > +	 */
> > >
> > > How fast is sort() if the table is sorted?
> > > Relying on the kernel sources and build scripts always being in sync seems dangerous.
> > > Probably better to leave the sort in for a release of two.
> > 
> > This patch comes after the build script changes, so they'd be in sync.
> > What would the concern be?
> 
> Mostly that if, for any reason, the build script changes are missing nothing
> will detect the error - but the results will be very confusing.
> If the sort is fast for sorted inputs (some algorithms aren't) then leaving
> it in won't take that long.

But why would the build script changes be missing...

And it should fail gracefully for oopses anyway: stack traces will just
have a bunch of question marks.

-- 
Josh