Re: [External] Re: [PATCH v18 9/9] mm: hugetlb: optimize the code with the help of the compiler

From: Michal Hocko <mhocko@suse.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: "Jonathan Corbet" <corbet@lwn.net>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
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	dave.hansen@linux.intel.com, luto@kernel.org,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	paulmck@kernel.org, mchehab+huawei@kernel.org,
	pawan.kumar.gupta@linux.intel.com,
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	"HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>,
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	"Chen Huang" <chenhuang5@huawei.com>,
	"Bodeddula Balasubramaniam" <bodeddub@amazon.com>
Subject: Re: [External] Re: [PATCH v18 9/9] mm: hugetlb: optimize the code with the help of the compiler
Date: Thu, 11 Mar 2021 10:39:18 +0100	[thread overview]
Message-ID: <YEnlRlLJD1bK/Dup@dhcp22.suse.cz> (raw)
In-Reply-To: <CAMZfGtW5uHYiA_1an3W-jEmemsoN3Org7JwieeE2V271wh9X-A@mail.gmail.com>

On Thu 11-03-21 17:08:34, Muchun Song wrote:
> On Thu, Mar 11, 2021 at 4:55 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 11-03-21 15:33:20, Muchun Song wrote:
> > > On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Mon 08-03-21 18:28:07, Muchun Song wrote:
> > > > > When the "struct page size" crosses page boundaries we cannot
> > > > > make use of this feature. Let free_vmemmap_pages_per_hpage()
> > > > > return zero if that is the case, most of the functions can be
> > > > > optimized away.
> > > >
> > > > I am confused. Don't you check for this in early_hugetlb_free_vmemmap_param already?
> > >
> > > Right.
> > >
> > > > Why do we need any runtime checks?
> > >
> > > If the size of the struct page is not power of 2, compiler can think
> > > is_hugetlb_free_vmemmap_enabled() always return false. So
> > > the code snippet of this user can be optimized away.
> > >
> > > E.g.
> > >
> > > if (is_hugetlb_free_vmemmap_enabled())
> > >         /* do something */
> > >
> > > The compiler can drop "/* do something */" directly, because
> > > it knows is_hugetlb_free_vmemmap_enabled() always returns
> > > false.
> >
> > OK, so this is a micro-optimization to generate a better code?
> 
> Right.
> 
> > Is this measurable to warrant more code?
> 
> I have disassembled the code to confirm this behavior.
> I know this is not the hot path. But it actually can decrease
> the code size.

struct page which is not power of 2 is not a common case. Are you sure
it makes sense to micro optimize for an outliar. If you really want to
microptimize then do that for a common case - the feature being
disabled - via static key.
-- 
Michal Hocko
SUSE Labs