Re: i915.ko WC writes are slow after ea8596bb2d8d379

From: Masami Hiramatsu
Date: Wed Oct 08 2014 - 21:45:44 EST


(2014/10/09 2:47), Chuck Ebbert wrote:
> On Wed, 8 Oct 2014 10:03:36 +0100
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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.

I guess most of them expects stop_machine() is not a configurable
feature...
If some of them requires stop_machine(), it should enable it on its
kconfig entry (including ftrace, kprobes).

> $ 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
>

BTW, as I sent a series of patches, the last two can be removed.
https://lkml.org/lkml/2014/8/25/142

Thank you,

--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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