linux-kernel - Re: i915.ko WC writes are slow after ea8596bb2d8d379

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Date:	Wed, 8 Oct 2014 12:47:05 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jiri Kosina <jkosina@...e.cz>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...mai.com>, yrl.pp-manager.tt@...achi.com,
	Borislav Petkov <bpetkov@...e.de>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: i915.ko WC writes are slow after ea8596bb2d8d379

On Wed, 8 Oct 2014 10:03:36 +0100
Chris Wilson <chris@...is-wilson.co.uk> wrote:

> and adding that back into the current build, e.g.
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 3632743..48a8a69 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -87,6 +87,7 @@ config X86
>         select HAVE_USER_RETURN_NOTIFIER
>         select ARCH_BINFMT_ELF_RANDOMIZE_PIE
>         select HAVE_ARCH_JUMP_LABEL
> +       select STOP_MACHINE
>         select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
>         select SPARSE_IRQ
>         select GENERIC_FIND_FIRST_BIT
> 
> fixes the regression.
> 

Looking closer at this, it seems most configs work by accident,
because they have MOD_UNLOAD and/or HOTPLUG_CPU enabled. I take it
you disabled both of those? stop_machine() is called from all kinds
of places and almost none of them make sure STOP_MACHINE is selected.

$ find -name Kconf\* | xargs grep STOP_MACHINE
./init/Kconfig:config STOP_MACHINE

All these places use stop_machine():

    mm/page_alloc.c, line 3886
    drivers/xen/manage.c, line 130
    drivers/char/hw_random/intel-rng.c, line 373
    arch/powerpc/mm/numa.c:
        line 1616
        line 1623 
    arch/powerpc/platforms/powernv/subcore.c, line 324
    arch/arm/kernel/kprobes.c, line 165
    arch/arm/kernel/patch.c:
        line 64
        line 71 
    arch/s390/kernel/jump_label.c, line 61
    arch/s390/kernel/kprobes.c:
        line 311
        line 320 
    arch/s390/kernel/time.c:
        line 820
        line 1590 
    arch/x86/kernel/cpu/mtrr/main.c, line 231
    arch/arm64/kernel/insn.c, line 181
    kernel/time/timekeeping.c, line 892
    kernel/trace/ftrace.c, line 2219
    kernel/module.c:
        line 770
        line 1861 
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