Hi,
Interesting patch! I know this patch already merged.
While study this patch, I have a few questions.
On 2024/2/8 17:51, Jocelyn Falempe wrote:
We found a regression in v5.10 on real-time server, using the
rt-kernel and the mgag200 driver. It's some really specialized
workload, with <10us latency expectation on isolated core.
After the v5.10, the real time tasks missed their <10us latency
when something prints on the screen (fbcon or printk)
The regression has been bisected to 2 commits:
commit 0b34d58b6c32 ("drm/mgag200: Enable caching for SHMEM pages")
commit 4862ffaec523 ("drm/mgag200: Move vmap out of commit tail")
The first one changed the system memory framebuffer from Write-Combine
to the default caching.
Before the second commit, the mgag200 driver used to unmap the
framebuffer after each frame, which implicitly does a cache flush.
I don't know why it need to do a cache flush, where is the code.
I'm asking because I want to study this technique.
Generally speaking, X86-64 platform's default page caching is cached.
And I think the cached mapping is fastest for software rendering. And
the platform guaranteed the coherency for us, right?
Because X86-64 platform(or CPU)'s write buffer is implemented on the
top of cache? I'm means that for ARM(or other) CPU, when using Write-combine
the data will has nothing to do with cache.
Both regressions are fixed by this commit, which restore WC mapping
for the framebuffer in system memory, and add a cache flush.
So switch back to WC probably will decrease overall performance, I think.
And the cache flush operation should not have a impact. Except X86-64's
Write-Combine is different other platform's Write-Combine?
This is only needed on x86_64, for low-latency workload,
so the new kconfig DRM_MGAG200_IOBURST_WORKAROUND depends on
PREEMPT_RT and X86.
For more context, the whole thread can be found here [1]
Signed-off-by: Jocelyn Falempe <jfalempe@xxxxxxxxxx>
Link: https://lore.kernel.org/dri-devel/20231019135655.313759-1-jfalempe@xxxxxxxxxx/ # 1
Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
