linux-mips
[Top] [All Lists]

Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around the

To: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around the fault address
From: "Kirill A. Shutemov" <kirill@shutemov.name>
Date: Thu, 16 Aug 2012 19:43:56 +0300
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org, Andi Kleen <ak@linux.intel.com>, Tim Chen <tim.c.chen@linux.intel.com>, Alex Shi <alex.shu@intel.com>, Jan Beulich <jbeulich@novell.com>, Robert Richter <robert.richter@amd.com>, Andy Lutomirski <luto@amacapital.net>, Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mips@linux-mips.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org
In-reply-to: <20120816161647.GM11188@redhat.com>
List-archive: <http://www.linux-mips.org/archives/linux-mips/>
List-help: <mailto:ecartis@linux-mips.org?Subject=help>
List-id: linux-mips <linux-mips.eddie.linux-mips.org>
List-owner: <mailto:ralf@linux-mips.org>
List-post: <mailto:linux-mips@linux-mips.org>
List-software: Ecartis version 1.0.0
List-subscribe: <mailto:ecartis@linux-mips.org?subject=subscribe%20linux-mips>
List-unsubscribe: <mailto:ecartis@linux-mips.org?subject=unsubscribe%20linux-mips>
References: <1345130154-9602-1-git-send-email-kirill.shutemov@linux.intel.com> <1345130154-9602-7-git-send-email-kirill.shutemov@linux.intel.com> <20120816161647.GM11188@redhat.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.21 (2010-09-15)
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

<Prev in Thread] Current Thread [Next in Thread>