linux-kernel - Re: [PATCH/RFC 3/3] s390: query dynamic DEBUG

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Date:	Thu, 28 Jan 2016 13:25:10 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	David Rientjes <rientjes@...gle.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH/RFC 3/3] s390: query dynamic DEBUG_PAGEALLOC setting

On Wed, Jan 27, 2016 at 02:03:36PM +0100, Vlastimil Babka wrote:
> On 01/27/2016 01:47 PM, Christian Borntraeger wrote:
> > On 01/27/2016 01:41 PM, Vlastimil Babka wrote:
> >> On 01/27/2016 01:59 AM, Joonsoo Kim wrote:
> >> 
> >> I think it might be worth also to convert debug_pagealloc_enabled() to be based
> >> on static key, like I did for page_owner [1]. That should help make it possible
> >> to have virtually no overhead when compiling kernel with CONFIG_DEBUG_PAGEALLOC
> >> without enabling it boot-time. I assume it's one of the goals here?
> > 
> > We could do something like that but dump_stack and setup of the initial identity
> > mapping of the kernel as well as the initial page protection are not hot path
> > as far as I can tell. Any other places?
> 
> Well, mostly kernel_map_pages() which is used in page allocation hotpaths.

We cannot just convert it because setup_arch() is called before
jump_label_init(). We can do it by introducing _early variant but
I'm not sure it's worth doing. I think that just make it unlikely
works well. Recently, I tested a micro benchmark on slab alloc/free
on CONFIG_SLUB with applying static branch for debug option which
currently is using unlikely branch and can't find any noticeble
performance difference.

Thanks.