Messages in this thread Patches in this message | | | Date | Wed, 8 Oct 2014 10:03:36 +0100 | From | Chris Wilson <> | Subject | i915.ko WC writes are slow after ea8596bb2d8d379 |
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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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