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Subjecti915.ko WC writes are slow after ea8596bb2d8d379

I ran into a problem on a Sandybridge i5-2500s whilst measuring the
performance of GTT write-combining access. I found subsequent runs were
about 10-40x slower than the first. For example,

igt/gem_gtt_speed:

Time to read 16k through a GTT map: 325.285µs
Time to write 16k through a GTT map: 4.729µs
Time to clear 16k through a GTT map: 4.584µs
Time to clear 16k through a cached GTT map: 1.342µs

on the second run became:

Time to read 16k through a GTT map: 332.148µs
Time to write 16k through a GTT map: 209.411µs
Time to clear 16k through a GTT map: 56.460µs
Time to clear 16k through a cached GTT map: 50.897µs

Naively I would say that we lost the wc on our ioremap.
/sys/kernel/debug/x86/pat_memtype_list remained the same across repeated
runs.

A bisection pointed to

commit ea8596bb2d8d37957f3e92db9511c50801689180
Author: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Date: Thu Jul 18 20:47:53 2013 +0900

kprobes/x86: Remove unused text_poke_smp() and text_poke_smp_batch() functions

of which the active ingredient was just

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b32ebf9..f4001e0 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2334,7 +2334,6 @@ config HAVE_ATOMIC_IOMAP

config HAVE_TEXT_POKE_SMP
bool
- select STOP_MACHINE if SMP

config X86_DEV_DMA_OPS
bool
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.

For the record, this kernel build doesn't use modules, which seems relevant
in light of ea8596bb2 "fixes a Kconfig dependency issue on STOP_MACHINE
in the case of CONFIG_SMP && !CONFIG_MODULE_UNLOAD".
-Chris

--
Chris Wilson, Intel Open Source Technology Centre
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