[PATCH 2/2] drm/i915: Use Transparent Hugepages when IOMMU is enabled

[Daniel Vetter]

On Thu, Jul 29, 2021 at 3:34 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 just do that.
>
> 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% (IOMMU on versus
> IOMMU off) to ~2% (same comparison but with THP on).
>
> v2:
>  * Add Kconfig dependency to transparent hugepages and some help text.
>  * Move to helper for easier handling of kernel build options.
>
> v3:
>  * Drop Kconfig. (Daniel)
>
> 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>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Reviewed-by: Rodrigo Vivi <rodrigo.vivi at intel.com> # v1

On both patches: Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  drivers/gpu/drm/i915/gem/i915_gemfs.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gemfs.c b/drivers/gpu/drm/i915/gem/i915_gemfs.c
> index 5e6e8c91ab38..dbdbdc344d87 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gemfs.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gemfs.c
> @@ -6,7 +6,6 @@
>
>  #include <linux/fs.h>
>  #include <linux/mount.h>
> -#include <linux/pagemap.h>
>
>  #include "i915_drv.h"
>  #include "i915_gemfs.h"
> @@ -15,6 +14,7 @@ int i915_gemfs_init(struct drm_i915_private *i915)
>  {
>         struct file_system_type *type;
>         struct vfsmount *gemfs;
> +       char *opts;
>
>         type = get_fs_type("tmpfs");
>         if (!type)
> @@ -26,10 +26,26 @@ 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);
> +       opts = NULL;
> +       if (intel_vtd_active()) {
> +               if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> +                       static char huge_opt[] = "huge=within_size"; /* r/w */
> +
> +                       opts = huge_opt;
> +                       drm_info(&i915->drm,
> +                                "Transparent Hugepage mode '%s'\n",
> +                                opts);
> +               } else {
> +                       drm_notice(&i915->drm,
> +                                  "Transparent Hugepage support is recommended for optimal performance when IOMMU is enabled!\n");
> +               }
> +       }
> +
> +       gemfs = vfs_kern_mount(type, SB_KERNMOUNT, type->name, opts);
>         if (IS_ERR(gemfs))
>                 return PTR_ERR(gemfs);
>
> --
> 2.30.2
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch