Am 18.03.24 um 15:24 schrieb Maíra Canal:
Not that the CC list wasn't big enough, but I'm adding MM folks
in the CC list.
On 3/18/24 11:04, Christian König wrote:
Am 18.03.24 um 14:28 schrieb Maíra Canal:
Hi Christian,
On 3/18/24 10:10, Christian König wrote:
Am 18.03.24 um 13:42 schrieb Maíra Canal:
Hi Christian,
On 3/12/24 10:48, Christian König wrote:
Am 12.03.24 um 14:09 schrieb Tvrtko Ursulin:
On 12/03/2024 10:37, Christian König wrote:
Am 12.03.24 um 11:31 schrieb Tvrtko Ursulin:
On 12/03/2024 10:23, Christian König wrote:
Am 12.03.24 um 10:30 schrieb Tvrtko Ursulin:
On 12/03/2024 08:59, Christian König wrote:
Am 12.03.24 um 09:51 schrieb Tvrtko Ursulin:
Hi Maira,
On 11/03/2024 10:05, Maíra Canal wrote:
For some applications, such as using huge pages, we might
want to have a
different mountpoint, for which we pass in mount flags
that better match
our usecase.
Therefore, add a new parameter to drm_gem_object_init()
that allow us to
define the tmpfs mountpoint where the GEM object will be
created. If
this parameter is NULL, then we fallback to
shmem_file_setup().
One strategy for reducing churn, and so the number of
drivers this patch touches, could be to add a lower level
drm_gem_object_init() (which takes vfsmount, call it
__drm_gem_object_init(), or drm__gem_object_init_mnt(),
and make drm_gem_object_init() call that one with a NULL
argument.
I would even go a step further into the other direction.
The shmem backed GEM object is just some special handling
as far as I can see.
So I would rather suggest to rename all drm_gem_* function
which only deal with the shmem backed GEM object into
drm_gem_shmem_*.
That makes sense although it would be very churny. I at
least would be on the fence regarding the cost vs benefit.
Yeah, it should clearly not be part of this patch here.
Also the explanation why a different mount point helps with
something isn't very satisfying.
Not satisfying as you think it is not detailed enough to say
driver wants to use huge pages for performance? Or not
satisying as you question why huge pages would help?
That huge pages are beneficial is clear to me, but I'm
missing the connection why a different mount point helps with
using huge pages.
Ah right, same as in i915, one needs to mount a tmpfs instance
passing huge=within_size or huge=always option. Default is
'never', see man 5 tmpfs.
Thanks for the explanation, I wasn't aware of that.
Mhm, shouldn't we always use huge pages? Is there a reason for
a DRM device to not use huge pages with the shmem backend?
AFAIU, according to b901bb89324a ("drm/i915/gemfs: enable THP"),
back then the understanding was within_size may overallocate,
meaning there would be some space wastage, until the memory
pressure makes the thp code split the trailing huge page. I
haven't checked if that still applies.
Other than that I don't know if some drivers/platforms could
have problems if they have some limitations or hardcoded
assumptions when they iterate the sg list.
Yeah, that was the whole point behind my question. As far as I
can see this isn't driver specific, but platform specific.
I might be wrong here, but I think we should then probably not
have that handling in each individual driver, but rather
centralized in the DRM code.
I don't see a point in enabling THP for all shmem drivers. A huge
page
is only useful if the driver is going to use it. On V3D, for example,
I only need huge pages because I need the memory contiguously
allocated
to implement Super Pages. Otherwise, if we don't have the Super Pages
support implemented in the driver, I would be creating memory
pressure
without any performance gain.
Well that's the point I'm disagreeing with. THP doesn't seem to
create much extra memory pressure for this use case.
As far as I can see background for the option is that files in
tmpfs usually have a varying size, so it usually isn't beneficial
to allocate a huge page just to find that the shmem file is much
smaller than what's needed.
But GEM objects have a fixed size. So we of hand knew if we need
4KiB or 1GiB and can therefore directly allocate huge pages if they
are available and object large enough to back them with.
If the memory pressure is so high that we don't have huge pages
available the shmem code falls back to standard pages anyway.
The matter is: how do we define the point where the memory pressure
is high?
Well as driver developers/maintainers we simply don't do that. This
is the job of the shmem code.
For example, notice that in this implementation of Super Pages
for the V3D driver, I only use a Super Page if the BO is bigger than
2MB. I'm doing that because the Raspberry Pi only has 4GB of RAM
available for the GPU. If I created huge pages for every BO
allocation (and initially, I tried that), I would end up with hangs
in some applications.
Yeah, that is what I meant with the trivial optimisation to the shmem
code. Essentially when you have 2MiB+4KiB as BO size then the shmem
code should use a 2MiB and a 4KiB page to back them, but what it
currently does is to use two 2MiB pages and then split up the second
page when it find that it isn't needed.
That is wasteful and leads to fragmentation, but as soon as we stop
doing that we should be fine to enable it unconditionally for all
drivers.
I see your point, but I believe that it would be tangent to the goal of
this series. As you mentioned, currently, we have a lot of memory
fragmentation when using THP and while we don't solve that (at the tmpfs
level), I believe we could move on with the current implementation (with
improvements proposed by Tvrtko).
Oh, I seriously don't want to block this patch set here. Just asking if
it's the right approach.
Point is we might need to revert the driver changes again when THP is
further optimized and the options aren't needed any more.
But if and how that should happen is perfectly up to Tvrtko.
Regards,
Christian.
Best Regards,
- Maíra
TTM does essentially the same thing for years.
Regards,
Christian.
At least, for V3D, I wouldn't like to see THP being used for all the
allocations. But, we have maintainers of other drivers in the CC.
Best Regards,
- Maíra
So THP is almost always beneficial for GEM even if the driver
doesn't actually need it. The only potential case I can think of
which might not be handled gracefully is the tail pages, e.g. huge
+ 4kib.
But that is trivial to optimize in the shmem code when the final
size of the file is known beforehand.
Regards,
Christian.
Best Regards,
- Maíra
Regards,
Christian.
Te Cc is plenty large so perhaps someone else will have
additional information. :)
Regards,
Tvrtko
I mean it would make this patch here even smaller.
Regards,
Christian.
Regards,
Tvrtko
