[PATCH 2/2] drm/i915: Use Transparent Hugepages when IOMMU is enabled
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Thu Jul 29 12:21:34 UTC 2021
On 29/07/2021 13:07, Daniel Vetter wrote:
> On Thu, Jul 29, 2021 at 1:19 PM Tvrtko Ursulin
> <tvrtko.ursulin at linux.intel.com> wrote:
>>
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> Usage of Transparent Hugepages was disabled in 9987da4b5dcf
>> ("drm/i915: Disable THP until we have a GPU read BW W/A"), but since it
>> appears majority of performance regressions reported with an enabled IOMMU
>> can be almost eliminated by turning them on, lets do that by adding a
>> couple of Kconfig options.
>>
>> To err on the side of safety we keep the current default in cases where
>> IOMMU is not active, and only when it is default to the "huge=within_size"
>> mode. Although there probably would be wins to enable them throughout,
>> more extensive testing across benchmarks and platforms would need to be
>> done.
>>
>> With the patch and IOMMU enabled my local testing on a small Skylake part
>> shows OglVSTangent regression being reduced from ~14% to ~2%.
>
> I guess the 14% regression is iommu disabled vs iommu enabled? Would
> be good to clarify that.
Should be clear from the first paragraph above - "...majority of
performance regressions reported with an _enabled_ IOMMU can be almost
eliminated...".
>
>> v2:
>> * Add Kconfig dependency to transparent hugepages and some help text.
>
> Uh I'm really not a huge fan of Kconfig for everything, especially for
> tuning stuff. Maybe if there's a need a module param for debugging,
> but otherwise can't we just pick the right default?
Kconfig is picking the right default so I do not see a problem by
allowing override from a deep enough menu. But I also do not feel so
strongly about bikeshedding this to no kconfig, or a module param, or
whatever - there are votes for all three options already, as usual. Main
problem I have is actually..
> And it very much sounds like the right default here is "enable it
> unconditionally if we have iommu support".
.. about this - who knows? I will remind you of a certain VLK-20150
which I thought was very important for going forward but was falling on
deaf ears for years. As such I am waiting for Eero to come back and
improvise some unofficial testing. It's extra bewildering to me given
how we had the facility and then shut it down just like that.
Regards,
Tvrtko
> -Daniel
>
>> * Move to helper for easier handling of kernel build options.
>>
>> References: b901bb89324a ("drm/i915/gemfs: enable THP")
>> References: 9987da4b5dcf ("drm/i915: Disable THP until we have a GPU read BW W/A")
>> References: https://gitlab.freedesktop.org/drm/intel/-/issues/430
>> Co-developed-by: Chris Wilson <chris at chris-wilson.co.uk>
>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
>> Cc: Matthew Auld <matthew.auld at intel.com>
>> Cc: Eero Tamminen <eero.t.tamminen at intel.com>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>> Reviewed-by: Rodrigo Vivi <rodrigo.vivi at intel.com> # v1
>> ---
>> drivers/gpu/drm/i915/Kconfig.profile | 73 +++++++++++++++++++++++++++
>> drivers/gpu/drm/i915/gem/i915_gemfs.c | 27 ++++++++--
>> 2 files changed, 97 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/Kconfig.profile b/drivers/gpu/drm/i915/Kconfig.profile
>> index 39328567c200..d49ee794732f 100644
>> --- a/drivers/gpu/drm/i915/Kconfig.profile
>> +++ b/drivers/gpu/drm/i915/Kconfig.profile
>> @@ -119,3 +119,76 @@ config DRM_I915_TIMESLICE_DURATION
>> /sys/class/drm/card?/engine/*/timeslice_duration_ms
>>
>> May be 0 to disable timeslicing.
>> +
>> +choice
>> + prompt "Transparent Hugepage Support (native)"
>> + default DRM_I915_THP_NATIVE_NEVER
>> + depends on TRANSPARENT_HUGEPAGE
>> + help
>> + Select the preferred method for allocating from Transparent Hugepages
>> + when IOMMU is not enabled.
>> +
>> + config DRM_I915_THP_NATIVE_NEVER
>> + bool "Never"
>> + help
>> + Disable using THP for system memory allocations, individually
>> + allocating each 4K chunk as a separate page. It is unlikely that such
>> + individual allocations will return contiguous memory.
>> +
>> + config DRM_I915_THP_NATIVE_WITHIN
>> + bool "Within size"
>> + help
>> + Allocate whole 2M superpages while those chunks do not exceed the
>> + object size. The remainder of the object will be allocated from 4K
>> + pages. No overallocation.
>> +
>> + config DRM_I915_THP_NATIVE_ALWAYS
>> + bool "Always"
>> + help
>> + Allocate the whole object using 2M superpages, even if the object does
>> + not require an exact number of superpages.
>> +
>> +endchoice
>> +
>> +config DRM_I915_THP_NATIVE
>> + string
>> + default "always" if DRM_I915_THP_NATIVE_ALWAYS
>> + default "within_size" if DRM_I915_THP_NATIVE_WITHIN
>> + default "never" if DRM_I915_THP_NATIVE_NEVER
>> +
>> +choice
>> + prompt "Transparent Hugepage Support (IOMMU)"
>> + default DRM_I915_THP_IOMMU_WITHIN if TRANSPARENT_HUGEPAGE=y
>> + default DRM_I915_THP_IOMMU_NEVER if TRANSPARENT_HUGEPAGE=n
>> + depends on TRANSPARENT_HUGEPAGE
>> + help
>> + Select the preferred method for allocating from Transparent Hugepages
>> + with IOMMU active.
>> +
>> + config DRM_I915_THP_IOMMU_NEVER
>> + bool "Never"
>> + help
>> + Disable using THP for system memory allocations, individually
>> + allocating each 4K chunk as a separate page. It is unlikely that such
>> + individual allocations will return contiguous memory.
>> +
>> + config DRM_I915_THP_IOMMU_WITHIN
>> + bool "Within size"
>> + help
>> + Allocate whole 2M superpages while those chunks do not exceed the
>> + object size. The remainder of the object will be allocated from 4K
>> + pages. No overallocation.
>> +
>> + config DRM_I915_THP_IOMMU_ALWAYS
>> + bool "Always"
>> + help
>> + Allocate the whole object using 2M superpages, even if the object does
>> + not require an exact number of superpages.
>> +
>> +endchoice
>> +
>> +config DRM_I915_THP_IOMMU
>> + string
>> + default "always" if DRM_I915_THP_IOMMU_ALWAYS
>> + default "within_size" if DRM_I915_THP_IOMMU_WITHIN
>> + default "never" if DRM_I915_THP_IOMMU_NEVER
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gemfs.c b/drivers/gpu/drm/i915/gem/i915_gemfs.c
>> index 5e6e8c91ab38..871cbfb02fdf 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gemfs.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gemfs.c
>> @@ -11,6 +11,26 @@
>> #include "i915_drv.h"
>> #include "i915_gemfs.h"
>>
>> +#if defined(CONFIG_DRM_I915_THP_NATIVE) && defined(CONFIG_DRM_I915_THP_IOMMU)
>> +static char *gemfd_mount_opts(struct drm_i915_private *i915)
>> +{
>> + static char thp_native[] = "huge=" CONFIG_DRM_I915_THP_NATIVE;
>> + static char thp_iommu[] = "huge=" CONFIG_DRM_I915_THP_IOMMU;
>> + char *opts;
>> +
>> + opts = intel_vtd_active() ? thp_iommu : thp_native;
>> + drm_info(&i915->drm, "Transparent Hugepage mode '%s'", opts);
>> +
>> + return opts;
>> +}
>> +#else
>> +static char *gemfd_mount_opts(struct drm_i915_private *i915)
>> +{
>> + return NULL;
>> +}
>> +#endif
>> +
>> +
>> int i915_gemfs_init(struct drm_i915_private *i915)
>> {
>> struct file_system_type *type;
>> @@ -26,10 +46,11 @@ int i915_gemfs_init(struct drm_i915_private *i915)
>> *
>> * One example, although it is probably better with a per-file
>> * control, is selecting huge page allocations ("huge=within_size").
>> - * Currently unused due to bandwidth issues (slow reads) on Broadwell+.
>> + * However, we only do so to offset the overhead of iommu lookups
>> + * due to bandwidth issues (slow reads) on Broadwell+.
>> */
>> -
>> - gemfs = kern_mount(type);
>> + gemfs = vfs_kern_mount(type, SB_KERNMOUNT, type->name,
>> + gemfd_mount_opts(i915));
>> if (IS_ERR(gemfs))
>> return PTR_ERR(gemfs);
>>
>> --
>> 2.30.2
>>
>
>
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