linux-kernel - Re: slow boot with 7fef431be9c9 ("mm/page_alloc: place pages to tail in __free_pages

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Date:   Tue, 16 Mar 2021 12:30:00 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     "Liang, Liang (Leo)" <Liang.Liang@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ray" <Ray.Huang@....com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        George Kennedy <george.kennedy@...cle.com>
Subject: Re: slow boot with 7fef431be9c9 ("mm/page_alloc: place pages to tail
 in __free_pages_core()")

On Tue, Mar 16, 2021 at 10:08:10AM +0100, David Hildenbrand wrote:
> On 16.03.21 09:58, Liang, Liang (Leo) wrote:
> > [AMD Public Use]
> > 
> > Hi David,
> > 
> > root@...u-Chachani:~# cat /proc/mtrr
> > reg00: base=0x000000000 (    0MB), size= 2048MB, count=1: write-back
> > reg01: base=0x0ffe00000 ( 4094MB), size=    2MB, count=1: write-protect
> > reg02: base=0x100000000 ( 4096MB), size=   16MB, count=1: write-protect
> 
> ^ there it is
> 
> https://wiki.osdev.org/MTRR
> 
> "Reads allocate cache lines on a cache miss. All writes update main memory.
> 
> Cache lines are not allocated on a write miss. Write hits invalidate the
> cache line and update main memory. "
> 
> AFAIU, writes completely bypass caches and store directly to main mamory. If
> there are cache lines from a previous read, they are invalidated. So I think
> especially slow will be read(addr), write(addr), read(addr), ... which is
> what we have in the kstream benchmark.
> 
> 
> The question is:
> 
> who sets this up without owning the memory?
> Is the memory actually special/slow or is that setting wrong?

I really doubt that 16M at 0x100000000 in a system with 8G RAM would
*physically* differ from the neighbouring memory.

> Buggy firmware/BIOS?
> Buggy device driver?

[    0.000027] MTRR default type: uncachable
[    0.000028] MTRR fixed ranges enabled:
[    0.000030]   00000-9FFFF write-back
[    0.000031]   A0000-BFFFF uncachable
[    0.000032]   C0000-FFFFF write-through
[    0.000033] MTRR variable ranges enabled:
[    0.000034]   0 base 000000000000 mask FFFF80000000 write-back
[    0.000036]   1 base 0000FFE00000 mask FFFFFFE00000 write-protect
[    0.000037]   2 base 000100000000 mask FFFFFF000000 write-protect

As we have the range at 0x100000000 write-protected reported that early in
boot I'd say it's BIOS.

The question is how to reliably detect that this is a bogus setting...

[    0.000038]   3 base 0000FFDE0000 mask FFFFFFFE0000 write-protect
[    0.000039]   4 base 0000FF000000 mask FFFFFFF80000 write-protect
[    0.000040]   5 disabled
[    0.000041]   6 disabled
[    0.000042]   7 disabled
[    0.000042] TOM2: 0000000280000000 aka 10240M


-- 
Sincerely yours,
Mike.