linux-kernel - Re: [PATCH] efifb: allow user to disable write combined mapping.

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Date:   Tue, 25 Jul 2017 14:00:00 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Peter Jones <pjones@...hat.com>
Cc:     Dave Airlie <airlied@...hat.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] efifb: allow user to disable write combined mapping.

On 19 July 2017 at 00:34, Peter Jones <pjones@...hat.com> wrote:
> On Tue, Jul 18, 2017 at 04:09:09PM +1000, Dave Airlie wrote:
>> This patch allows the user to disable write combined mapping
>> of the efifb framebuffer console using an nowc option.
>>
>> A customer noticed major slowdowns while logging to the console
>> with write combining enabled, on other tasks running on the same
>> CPU. (10x or greater slow down on all other cores on the same CPU
>> as is doing the logging).
>>
>> I reproduced this on a machine with dual CPUs.
>> Intel(R) Xeon(R) CPU E5-2609 v3 @ 1.90GHz (6 core)
>>
>> I wrote a test that just mmaps the pci bar and writes to it in
>> a loop, while this was running in the background one a single
>> core with (taskset -c 1), building a kernel up to init/version.o
>> (taskset -c 8) went from 13s to 133s or so. I've yet to explain
>> why this occurs or what is going wrong I haven't managed to find
>> a perf command that in any way gives insight into this.
>>
>>     11,885,070,715      instructions              #    1.39  insns per cycle
>> vs
>>     12,082,592,342      instructions              #    0.13  insns per cycle
>>
>> is the only thing I've spotted of interest, I've tried at least:
>> dTLB-stores,dTLB-store-misses,L1-dcache-stores,LLC-store,LLC-store-misses,LLC-load-misses,LLC-loads,\mem-loads,mem-stores,iTLB-loads,iTLB-load-misses,cache-references,cache-misses
>>
>> For now it seems at least a good idea to allow a user to disable write
>> combining if they see this until we can figure it out.
>
> Well, that's kind of amazing, given 3c004b4f7eab239e switched us /to/
> using ioremap_wc() for the exact same reason.  I'm not against letting
> the user force one way or the other if it helps, though it sure would be
> nice to know why.
>
> Anyway,
>
> Acked-By: Peter Jones <pjones@...hat.com>
>
> Bartlomiej, do you want to handle this in your devel tree?

I'm happy to stick this in a drm-fixes pull with this ack.

Dave.