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

From: Shile Zhang <shile.zhang@linux.alibaba.com>
To: David Laight <David.Laight@ACULAB.COM>,
	'Josh Poimboeuf' <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"x86@kernel.org" <x86@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-kbuild@vger.kernel.org" <linux-kbuild@vger.kernel.org>
Subject: Re: [RFC PATCH v3 7/7] x86/unwind/orc: remove run-time ORC unwind tables sort
Date: Mon, 18 Nov 2019 18:50:19 +0800	[thread overview]
Message-ID: <c15ef0d0-b4a2-0c8c-8868-2441d09f7891@linux.alibaba.com> (raw)
In-Reply-To: <5fe9024bc69c4a4eb115b3c2f3f9bcd1@AcuMS.aculab.com>

On 2019/11/18 18:05, David Laight wrote:
> From: Josh Poimboeuf <jpoimboe@redhat.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.
>
> 	David

Hi, David,

Thanks for your review!
Due to the sort inside kernel is heap-sort, so it cost almost the same 
time for sorted inputs.
I wondered if we can add error handling in the link script, exit with 
error if sort encountered any errors.

Thanks!

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