Re: i915.ko WC writes are slow after ea8596bb2d8d379

[Chuck Ebbert]

On Wed, 8 Oct 2014 10:03:36 +0100
Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:

> 
> 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:             325.285Âs
> Time to write 16k through a GTT map:              4.729Âs
> Time to clear 16k through a GTT map:              4.584Âs
> Time to clear 16k through a cached GTT map:       1.342Âs
> 
> on the second run became:
> 
> Time to read 16k through a GTT map:             332.148Âs
> Time to write 16k through a GTT map:            209.411Âs
> Time to clear 16k through a GTT map:             56.460Âs
> Time to clear 16k through a cached GTT map:      50.897Âs
> 
> 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@xxxxxxxxxxx>
> 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.

Hmm, set_mtrr() uses stop_machine(). I wonder if your MTRRs are out of
sync and your results depend on which CPU the test runs on?

> 
> 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".
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