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

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Date:   Tue, 25 Jul 2017 10:56:15 +0200
From:   Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
To:     Dave Airlie <airlied@...il.com>
Cc:     Peter Jones <pjones@...hat.com>, Dave Airlie <airlied@...hat.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 Tuesday, July 25, 2017 02:00:00 PM Dave Airlie wrote:
> 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.

I'll put it into fbdev fixes for 4.13 with other fbdev patches.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics