Bug 84794

Summary: [snb/ivb/hsw/bdw] GTT go slow
Product: DRI Reporter: Chris Wilson <chris>
Component: DRM/IntelAssignee: H. Peter Anvin <hpa>
Status: RESOLVED FIXED QA Contact: Intel GFX Bugs mailing list <intel-gfx-bugs>
Severity: critical    
Priority: medium CC: intel-gfx-bugs, przanoni
Version: unspecified   
Hardware: Other   
OS: All   
URL: https://lkml.org/lkml/2014/10/8/124
Whiteboard:
i915 platform: i915 features:
Attachments:
Description Flags
x86 mttr bogon none

Description Chris Wilson 2014-10-08 08:29:28 UTC 
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
    
    Since introducing the text_poke_bp() for all text_poke_smp*()
    callers, text_poke_smp*() are now unused. This patch basically
    reverts:
    
      3d55cc8a058e ("x86: Add text_poke_smp for SMP cross modifying code")
      7deb18dcf047 ("x86: Introduce text_poke_smp_batch() for batch-code modifying")
    
    and related commits.
    
    This patch also fixes a Kconfig dependency issue on STOP_MACHINE
    in the case of CONFIG_SMP && !CONFIG_MODULE_UNLOAD.
    
    Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
    Reviewed-by: Jiri Kosina <jkosina@suse.cz>
    Cc: H. Peter Anvin <hpa@linux.intel.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Jason Baron <jbaron@akamai.com>
    Cc: yrl.pp-manager.tt@hitachi.com
    Cc: Borislav Petkov <bpetkov@suse.de>
    Link: http://lkml.kernel.org/r/20130718114753.26675.18714.stgit@mhiramat-M0-7522
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

(bisected and manually checked)

This patch appears to make repeated access through the GTT extremely slow (roughly equivalent to uc access I would say). This is demonstrated by igt/gem_gtt_speed, and affects everything using X and mesa

???
Comment 1 Chris Wilson 2014-10-08 08:37:23 UTC 
The active ingredient is 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
Comment 2 Chris Wilson 2014-10-08 08:44:33 UTC 
And correspondingly

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 current kernel.
Comment 3 Paulo Zanoni 2014-10-08 18:50:03 UTC 
So what exactly is the action plan here? Report the "regression" of the kprobes patch? Propose changes to the default x86 config parameters? Fix i915.ko somehow?
Comment 4 Chris Wilson 2014-10-08 18:58:29 UTC 
Oh, fdo. I added a link to the upstream report [https://lkml.org/lkml/2014/10/8/124]
Comment 5 Jani Nikula 2015-01-29 13:39:53 UTC 
(In reply to Chris Wilson from comment #4)
> Oh, fdo. I added a link to the upstream report
> [https://lkml.org/lkml/2014/10/8/124]

Seems to have stalled, what next?
Comment 6 Jesse Barnes 2015-03-25 22:11:26 UTC 
Giving this one to Peter as he touched it on lkml. :)  Sounds like a core x86 issue at any rate.
Comment 7 Chris Wilson 2015-07-26 10:35:45 UTC 
Still affecting current kernels (4.2-rc3).
Comment 8 Chris Wilson 2015-07-26 10:37:23 UTC 
Created attachment 117380 [details] [review]
x86 mttr bogon
Comment 9 Chris Wilson 2019-10-14 13:03:32 UTC 
commit 86fffe4a61dd972d5a4e23260d530be6da02f614
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Dec 11 13:40:46 2015 -0800

    kernel: remove stop_machine() Kconfig dependency
    
    Currently the full stop_machine() routine is only enabled on SMP if
    module unloading is enabled, or if the CPUs are hotpluggable.  This
    leads to configurations where stop_machine() is broken as it will then
    only run the callback on the local CPU with irqs disabled, and not stop
    the other CPUs or run the callback on them.
    
    For example, this breaks MTRR setup on x86 in certain configs since
    ea8596bb2d8d379 ("kprobes/x86: Remove unused text_poke_smp() and
    text_poke_smp_batch() functions") as the MTRR is only established on the
    boot CPU.
    
    This patch removes the Kconfig option for STOP_MACHINE and uses the SMP
    and HOTPLUG_CPU config options to compile the correct stop_machine() for
    the architecture, removing the false dependency on MODULE_UNLOAD in the
    process.
    
    Link: https://lkml.org/lkml/2014/10/8/124
    References: https://bugs.freedesktop.org/show_bug.cgi?id=84794

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