linux-kernel - Re: [PATCH v2 2/4] mm/sparse: Optimize sparse_add_one

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Message-ID: <20190326141803.GX3659@MiWiFi-R3L-srv>
Date:   Tue, 26 Mar 2019 22:18:03 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, rppt@...ux.ibm.com, osalvador@...e.de,
        willy@...radead.org, william.kucharski@...cle.com
Subject: Re: [PATCH v2 2/4] mm/sparse: Optimize sparse_add_one_section()

On 03/26/19 at 03:03pm, Michal Hocko wrote:
> On Tue 26-03-19 21:45:22, Baoquan He wrote:
> > On 03/26/19 at 11:17am, Michal Hocko wrote:
> > > On Tue 26-03-19 18:08:17, Baoquan He wrote:
> > > > On 03/26/19 at 10:29am, Michal Hocko wrote:
> > > > > On Tue 26-03-19 17:02:25, Baoquan He wrote:
> > > > > > Reorder the allocation of usemap and memmap since usemap allocation
> > > > > > is much simpler and easier. Otherwise hard work is done to make
> > > > > > memmap ready, then have to rollback just because of usemap allocation
> > > > > > failure.
> > > > > 
> > > > > Is this really worth it? I can see that !VMEMMAP is doing memmap size
> > > > > allocation which would be 2MB aka costly allocation but we do not do
> > > > > __GFP_RETRY_MAYFAIL so the allocator backs off early.
> > > > 
> > > > In !VMEMMAP case, it truly does simple allocation directly. surely
> > > > usemap which size is 32 is smaller. So it doesn't matter that much who's
> > > > ahead or who's behind. However, this benefit a little in VMEMMAP case.
> > > 
> > > How does it help there? The failure should be even much less probable
> > > there because we simply fall back to a small 4kB pages and those
> > > essentially never fail.
> > 
> > OK, I am fine to drop it. Or only put the section existence checking
> > earlier to avoid unnecessary usemap/memmap allocation?
> 
> DO you have any data on how often that happens? Should basically never
> happening, right?

Oh, you think about it in this aspect. Yes, it rarely happens.
Always allocating firstly can increase efficiency. Then I will just drop
it.