On 07/01/2016 09:40 PM, Deak, Imre wrote:
Setting a write-back cache policy in the MOCS entry definition also
implies snooping, which has a considerable overhead. This is
unexpected for a few reasons:
- From user-space's point of view since it didn't want a coherent
surface (it didn't set the buffer as such via the set caching IOCTL).
- There is a separate MOCS entry field for snooping (which we never
set).
- This MOCS table is about caching in (e)LLC and there is no (e)LLC on
BXT. There is a separate table for L3 cache control.
Considering the above the current behavior of snooping looks like an
unintentional side-effect of the WB setting. Changing it to be LLC-UC
gets rid of the snooping without any ill-effects. For a coherent
surface the application would use a separate MOCS entry at index 1 and
call the set caching IOCTL to setup the PTE entries for the
corresponding buffer to be snooped. In the future we could also add a
new MOCS entry for coherent surfaces.
This resulted in 70% improvement in synthetic texturing benchmarks.
Kudos to Valtteri Rantala, Eero Tamminen and Michael T Frederick and
Ville who helped to narrow the source of problem to the kernel and to
the snooping behaviour in particular.
With a follow-up change to adjust the 3rd entry value
igt/gem_mocs_settings is passing after this change.
v2:
- Rebase on v2 of patch 1/2.
v3:
- Set the entry as LLC uncached instead of PTE-passthrough. This way
we also keep snooping disabled, but we also make the cacheability/
coherency setting indepent of the PTE which is managed by the
kernel. (Chris)
CC: Rong R Yang<rong.r.yang@xxxxxxxxx>
CC: Yakui Zhao<yakui.zhao@xxxxxxxxx>
CC: Valtteri Rantala<valtteri.rantala@xxxxxxxxx>
CC: Eero Tamminen<eero.t.tamminen@xxxxxxxxx>
CC: Michael T Frederick<michael.t.frederick@xxxxxxxxx>
CC: Ville Syrjälä<ville.syrjala@xxxxxxxxxxxxxxx>
CC: Chris Wilson<chris@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Imre Deak<imre.deak@xxxxxxxxx>
As the BXT has no LLC, setting the WB-policy will add the extra
overhead. In such case the patch looks more reasonable for BXT.
Add: Acked-by: Zhao Yakui <yakui.zhao@xxxxxxxxx>
---
drivers/gpu/drm/i915/intel_mocs.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_mocs.c b/drivers/gpu/drm/i915/intel_mocs.c
index d36e609..927825f 100644
--- a/drivers/gpu/drm/i915/intel_mocs.c
+++ b/drivers/gpu/drm/i915/intel_mocs.c
@@ -149,8 +149,8 @@ static const struct drm_i915_mocs_entry broxton_mocs_table[] = {
.l3cc_value = L3_ESC(0) | L3_SCC(0) | L3_CACHEABILITY(L3_WB),
},
{
- /* 0x0000003b */
- .control_value = LE_CACHEABILITY(LE_WB) |
+ /* 0x00000039 */
+ .control_value = LE_CACHEABILITY(LE_UC) |
LE_TGT_CACHE(LE_TC_LLC_ELLC) |
LE_LRUM(3) | LE_AOM(0) | LE_RSC(0) | LE_SCC(0) |
LE_PFM(0) | LE_SCF(0),
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