Re: [RFC][PATCH v6 1/7] drm: Add a sharable drm page-pool implementation
From: Christian König
Date: Wed Feb 10 2021 - 13:45:38 EST
Am 10.02.21 um 17:39 schrieb Suren Baghdasaryan:
On Wed, Feb 10, 2021 at 5:06 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
On Tue, Feb 09, 2021 at 12:16:51PM -0800, Suren Baghdasaryan wrote:
On Tue, Feb 9, 2021 at 12:03 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
On Tue, Feb 9, 2021 at 6:46 PM Christian König <christian.koenig@xxxxxxx> wrote:
Am 09.02.21 um 18:33 schrieb Suren Baghdasaryan:
On Tue, Feb 9, 2021 at 4:57 AM Christian König <christian.koenig@xxxxxxx> wrote:
Am 09.02.21 um 13:11 schrieb Christian König:
[SNIP]
+void drm_page_pool_add(struct drm_page_pool *pool, struct page *page)
+{
+ spin_lock(&pool->lock);
+ list_add_tail(&page->lru, &pool->items);
+ pool->count++;
+ atomic_long_add(1 << pool->order, &total_pages);
+ spin_unlock(&pool->lock);
+
+ mod_node_page_state(page_pgdat(page),
NR_KERNEL_MISC_RECLAIMABLE,
+ 1 << pool->order);
Hui what? What should that be good for?
This is a carryover from the ION page pool implementation:
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My sense is it helps with the vmstat/meminfo accounting so folks can
see the cached pages are shrinkable/freeable. This maybe falls under
other dmabuf accounting/stats discussions, so I'm happy to remove it
for now, or let the drivers using the shared page pool logic handle
the accounting themselves?
Intentionally separated the discussion for that here.
As far as I can see this is just bluntly incorrect.
Either the page is reclaimable or it is part of our pool and freeable
through the shrinker, but never ever both.
IIRC the original motivation for counting ION pooled pages as
reclaimable was to include them into /proc/meminfo's MemAvailable
calculations. NR_KERNEL_MISC_RECLAIMABLE defined as "reclaimable
non-slab kernel pages" seems like a good place to account for them but
I might be wrong.
Yeah, that's what I see here as well. But exactly that is utterly nonsense.
Those pages are not "free" in the sense that get_free_page could return
them directly.
Well on Android that is kinda true, because Android has it's
oom-killer (way back it was just a shrinker callback, not sure how it
works now), which just shot down all the background apps. So at least
some of that (everything used by background apps) is indeed
reclaimable on Android.
But that doesn't hold on Linux in general, so we can't really do this
for common code.
Also I had a long meeting with Suren, John and other googles
yesterday, and the aim is now to try and support all the Android gpu
memory accounting needs with cgroups. That should work, and it will
allow Android to handle all the Android-ism in a clean way in upstream
code. Or that's at least the plan.
I think the only thing we identified that Android still needs on top
is the dma-buf sysfs stuff, so that shared buffers (which on Android
are always dma-buf, and always stay around as dma-buf fd throughout
their lifetime) can be listed/analyzed with full detail.
But aside from this the plan for all the per-process or per-heap
account, oom-killer integration and everything else is planned to be
done with cgroups.
Until cgroups are ready we probably will need to add a sysfs node to
report the total dmabuf pool size and I think that would cover our
current accounting need here.
As I mentioned, not including dmabuf pools into MemAvailable would
affect that stat and I'm wondering if pools should be considered as
part of MemAvailable or not. Since MemAvailable includes SReclaimable
I think it makes sense to include them but maybe there are other
considerations that I'm missing?
On Android, yes, on upstream, not so much. Because upstream doesn't have
the android low memory killer cleanup up all the apps, so effectively we
can't reclaim that memory, and we shouldn't report it as such.
-Daniel
Hmm. Sorry, I fail to see why Android's low memory killer makes a
difference here. In my mind, the pages in the pools are not used but
kept there in case heaps need them (maybe that's the part I'm wrong?).
These pages can be freed by the shrinker if memory pressure rises.
And exactly that's the difference. They *can* be freed is not the same
thing as they *are* free.
In that sense I think it's very similar to reclaimable slabs which are
already accounted as part of MemAvailable. So it seems logical to me
to include unused pages in the pools here as well. What am I missing?
See the shrinkers are there because you need to do some action before
you can re-use the memory.
In the case of the TTM/DRM pool for example you need to change the
caching attributes which might cause sleeping for a TLB flush to finish.
By accounting those pages as free you mess up (for example) the handling
which makes sure that there are enough emergency reserves. I can only
strongly recommend to not do that.
What you could do is to add a sysfs interface to expose the different
shrinkers and the amount of pages in them to userspace. Similar to what
/proc/slabinfo is doing.
Regards,
Christian.
Android (for now) only needs to account overall gpu
memory since none of it is swappable on android drivers anyway, plus
no vram, so not much needed.
Cheers, Daniel
Regards,
Christian.
In the best case this just messes up the accounting, in the worst case
it can cause memory corruption.
Christian.
--
Daniel Vetter
Software Engineer, Intel Corporation
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Daniel Vetter
Software Engineer, Intel Corporation
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