linux-kernel - Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around the fault address

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Date:	Thu, 16 Aug 2012 19:43:56 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Alex Shi <alex.shu@...el.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Robert Richter <robert.richter@....com>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mips@...ux-mips.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around
 the fault address

On Thu, Aug 16, 2012 at 06:16:47PM +0200, Andrea Arcangeli wrote:
> Hi Kirill,
> 
> On Thu, Aug 16, 2012 at 06:15:53PM +0300, Kirill A. Shutemov wrote:
> >  	for (i = 0; i < pages_per_huge_page;
> >  	     i++, p = mem_map_next(p, page, i)) {
> 
> It may be more optimal to avoid a multiplication/shiftleft before the
> add, and to do:
> 
>   	for (i = 0, vaddr = haddr; i < pages_per_huge_page;
>   	     i++, p = mem_map_next(p, page, i), vaddr += PAGE_SIZE) {
> 

Makes sense. I'll update it.

> >  		cond_resched();
> > -		clear_user_highpage(p, addr + i * PAGE_SIZE);
> > +		vaddr = haddr + i*PAGE_SIZE;
> 
> Not sure if gcc can optimize it away because of the external calls.
> 
> > +		if (!ARCH_HAS_USER_NOCACHE || i == target)
> > +			clear_user_highpage(page + i, vaddr);
> > +		else
> > +			clear_user_highpage_nocache(page + i, vaddr);
> >  	}
> 
> 
> My only worry overall is if there can be some workload where this may
> actually slow down userland if the CPU cache is very large and
> userland would access most of the faulted in memory after the first
> fault.
> 
> So I wouldn't mind to add one more check in addition of
> !ARCH_HAS_USER_NOCACHE above to check a runtime sysctl variable. It'll
> waste a cacheline yes but I doubt it's measurable compared to the time
> it takes to do a >=2M hugepage copy.

Hm.. I think with static_key we can avoid cache overhead here. I'll try.
 
> Furthermore it would allow people to benchmark its effect without
> having to rebuild the kernel themself.
> 
> All other patches looks fine to me.

Thanks, for review. Could you take a look at huge zero page patchset? ;)

-- 
 Kirill A. Shutemov
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