linux-kernel - Re: [PATCH 2/2] mm: Create non-atomic version of SetPageReserved for init use

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Message-ID: <20180906054157.GI14951@dhcp22.suse.cz>
Date:   Thu, 6 Sep 2018 07:41:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        pavel.tatashin@...rosoft.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/2] mm: Create non-atomic version of SetPageReserved for
 init use

On Wed 05-09-18 13:18:24, Alexander Duyck wrote:
> On Tue, Sep 4, 2018 at 11:24 PM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > On Tue 04-09-18 11:33:45, Alexander Duyck wrote:
> > > From: Alexander Duyck <alexander.h.duyck@...el.com>
> > >
> > > It doesn't make much sense to use the atomic SetPageReserved at init time
> > > when we are using memset to clear the memory and manipulating the page
> > > flags via simple "&=" and "|=" operations in __init_single_page.
> > >
> > > This patch adds a non-atomic version __SetPageReserved that can be used
> > > during page init and shows about a 10% improvement in initialization times
> > > on the systems I have available for testing.
> >
> > I agree with Dave about a comment is due. I am also quite surprised that
> > this leads to such a large improvement. Could you be more specific about
> > your test and machines you were testing on?
> 
> So my test case has been just initializing 4 3TB blocks of persistent
> memory with a few trace_printk values added to track total time in
> move_pfn_range_to_zone.
> 
> What I have been seeing is that the time needed for the call drops on
> average from 35-36 seconds down to around 31-32.

This information belongs to the changelog.

> 
> > Other than that the patch makes sense to me.
> >
> > > Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> >
> > With the above addressed, feel free to add
> > Acked-by: Michal Hocko <mhocko@...e.com>
> >
> > Thanks!
> 
> As far as adding a comment are we just talking about why it is
> reserved, or do we need a description of the __SetPageReserved versus
> SetPageReserved. For now I was looking at adding a comment like:

the later. The reason why we make it reserved should be quite clear. A
comment wouldn't hurt of course and what you have is a good start. But
it is usually atomic vs. non-atomic SetPage$Foo which needs some
clarification.

> @@ -5517,8 +5517,13 @@ void __meminit memmap_init_zone(unsigned long
> size, int nid, unsigned long zone,
>  not_early:
>                 page = pfn_to_page(pfn);
>                 __init_single_page(page, pfn, zone, nid);
> +
> +               /*
> +                * Mark page reserved as it will need to wait for onlining
> +                * phase for it to be fully associated with a zone.
> +                */
>                 if (context == MEMMAP_HOTPLUG)
> -                       SetPageReserved(page);
> +                       __SetPageReserved(page);
> 
>                 /*
>                  * Mark the block movable so that blocks are reserved for
> 
> Any thoughts on this?
> 
> Thanks.
> 
> - Alex

-- 
Michal Hocko
SUSE Labs