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

[Tvrtko Ursulin]

On 29/07/2021 15:06, Daniel Vetter wrote:
> 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>

Eero's testing results at 
https://gitlab.freedesktop.org/drm/intel/-/issues/430 are looking good - 
seem to show this to be a net win for at least Gen9 and Gen12 platforms.

Is the ack enough to merge in this case or I should look for an r-b as well?

Regards,

Tvrtko

>> ---
>>   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
>>
> 
>