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

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Date:   Wed, 5 Sep 2018 13:18:24 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     mhocko@...nel.org
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 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.

> 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:
@@ -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