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

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Date:	Thu, 09 Oct 2014 10:45:15 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
Cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	linux-kernel@...r.kernel.org, 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

(2014/10/09 2:47), Chuck Ebbert wrote:
> 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.

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@...achi.com


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