Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like: - Add a new TTM_PAGE_FLAG_SHMEM flag, or similar. - Skip the ttm_pages_allocated accounting on such objects, similar to how FLAG_SG is already handled. - Skip all the page->mapping and page->index related bits, like in tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific? - Skip calling into ttm_bo_swap_out/in and just have ttm_populate/unpopulate handle this directly for such objects. - Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Regards, Christian.
________________________________ Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri-devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like: - Add a new TTM_PAGE_FLAG_SHMEM flag, or similar. - Skip the ttm_pages_allocated accounting on such objects, similar to how FLAG_SG is already handled. - Skip all the page->mapping and page->index related bits, like in tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific? - Skip calling into ttm_bo_swap_out/in and just have ttm_populate/unpopulate handle this directly for such objects. - Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian Christian.Koenig@amd.com wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you please point out where that is in the TTM code?
Regards, Christian.
Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri-devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar to
how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
Am 09.09.21 um 18:56 schrieb Matthew Auld:
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian Christian.Koenig@amd.com wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you please point out where that is in the TTM code?
When TTM moves a buffer it must make sure that the buffer is not accessed by the CPU while moving it.
For this the standard reverse lockup functionality of the Linux kernel is used to figure out in which page tables a page is mapped and mark those as invalid. Accessing the buffer object will then cause a page fault which in turn waits for the buffer move to finish.
But when you back the pages in a TT object with pages from a shmemfile this reverse lockup functionality doesn't work for some reason. I couldn't figure out what exactly was going wrong here and didn't looked deeper, I assumed it's because of not setting up page->mapping and page->index correctly. Thomas or Daniel might know more.
Apart from that your approach sounds like pretty much what I tried as well.
Regards, Christian.
Regards, Christian.
Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri-devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar to
how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
Perhaps some background and goal is worth mentioning here.
On Thu, 2021-09-09 at 17:56 +0100, Matthew Auld wrote:
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian Christian.Koenig@amd.com wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you please point out where that is in the TTM code?
I think this is in unmap_mapping_range() where, if we use VM_MIXEDMAP, there is a reverse lookup on the PTEs that point to real pages. Now that we move over to VM_PFNMAP, that problem should go away since core vm never has a page to investigate. Probably this is why things works on non-TTM i915 GEM.
@Christian: Some background here: First I think that there might be things like the above that will pose problems, and we may or may not be able to overcome those but more importantly is that we agree with you that *if* we make it work, it is something that you as a maintainer of TTM can accept from a design- and maintainabiltiy point of view.
The approach would be similar to the buddy allocator, we adapt some driver code to TTM in a way that it may be reused with other drivers, and if other drivers are interested, we'd assist in moving to core TTM. In essence it'd be a TTM shmem page pool with full shrinking ability for cached pages only.
What we're really after here is the ability to shrink that doesn't regress much w r t the elaborate shrinker that's in i915 today that is power management aware and is also able to start shmem writebacks to avoid shmem just caching the pages instead of giving them back to the system (IIRC it was partly the lack of this that blocked earlier TTM shrinking efforts).
And since it doesn't really matter whether the shrinker sits in core TTM or in a driver, I think a future goal might be a set of TTM shrinker helpers that makes sure we shrink the right TTM object, and perhaps a simple implementation that is typically used by simple drivers and other drivers can build on that for a more elaborate power- management aware shrinker.
/Thomas
Regards, Christian.
Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri- devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar
to how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
Am 10.09.21 um 10:08 schrieb Thomas Hellström:
Perhaps some background and goal is worth mentioning here.
On Thu, 2021-09-09 at 17:56 +0100, Matthew Auld wrote:
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian Christian.Koenig@amd.com wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you please point out where that is in the TTM code?
I think this is in unmap_mapping_range() where, if we use VM_MIXEDMAP, there is a reverse lookup on the PTEs that point to real pages. Now that we move over to VM_PFNMAP, that problem should go away since core vm never has a page to investigate. Probably this is why things works on non-TTM i915 GEM.
Yeah, that was really likely the root problem. I didn't kept investigating after realizing that my approach wouldn't work.
@Christian: Some background here: First I think that there might be things like the above that will pose problems, and we may or may not be able to overcome those but more importantly is that we agree with you that *if* we make it work, it is something that you as a maintainer of TTM can accept from a design- and maintainabiltiy point of view.
The approach would be similar to the buddy allocator, we adapt some driver code to TTM in a way that it may be reused with other drivers, and if other drivers are interested, we'd assist in moving to core TTM. In essence it'd be a TTM shmem page pool with full shrinking ability for cached pages only.
What we're really after here is the ability to shrink that doesn't regress much w r t the elaborate shrinker that's in i915 today that is power management aware and is also able to start shmem writebacks to avoid shmem just caching the pages instead of giving them back to the system (IIRC it was partly the lack of this that blocked earlier TTM shrinking efforts).
And since it doesn't really matter whether the shrinker sits in core TTM or in a driver, I think a future goal might be a set of TTM shrinker helpers that makes sure we shrink the right TTM object, and perhaps a simple implementation that is typically used by simple drivers and other drivers can build on that for a more elaborate power- management aware shrinker.
That's understandable, but I think not necessary what we should aim for in the long term.
First of all I would really like to move more of the functionality from ttm_pool.c into the core memory management, especially handling of uncached and write combined memory.
That's essentially completely architecture dependent and currently implemented extremely awkward. Either Daniels suggestion of having a GFP_WC or Christophs approach of moving all this into the DMA API is the way to go here.
As long as i915 has no interest in USWC support implementing their own shmemfile backend sounds fine to me, but I have strong doubt that this will be of use to anybody else.
Christian.
/Thomas
Regards, Christian.
Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri- devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar
to how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
On Fri, 2021-09-10 at 10:25 +0200, Christian König wrote:
Am 10.09.21 um 10:08 schrieb Thomas Hellström:
Perhaps some background and goal is worth mentioning here.
On Thu, 2021-09-09 at 17:56 +0100, Matthew Auld wrote:
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian Christian.Koenig@amd.com wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you please point out where that is in the TTM code?
I think this is in unmap_mapping_range() where, if we use VM_MIXEDMAP, there is a reverse lookup on the PTEs that point to real pages. Now that we move over to VM_PFNMAP, that problem should go away since core vm never has a page to investigate. Probably this is why things works on non-TTM i915 GEM.
Yeah, that was really likely the root problem. I didn't kept investigating after realizing that my approach wouldn't work.
@Christian: Some background here: First I think that there might be things like the above that will pose problems, and we may or may not be able to overcome those but more importantly is that we agree with you that *if* we make it work, it is something that you as a maintainer of TTM can accept from a design- and maintainabiltiy point of view.
The approach would be similar to the buddy allocator, we adapt some driver code to TTM in a way that it may be reused with other drivers, and if other drivers are interested, we'd assist in moving to core TTM. In essence it'd be a TTM shmem page pool with full shrinking ability for cached pages only.
What we're really after here is the ability to shrink that doesn't regress much w r t the elaborate shrinker that's in i915 today that is power management aware and is also able to start shmem writebacks to avoid shmem just caching the pages instead of giving them back to the system (IIRC it was partly the lack of this that blocked earlier TTM shrinking efforts).
And since it doesn't really matter whether the shrinker sits in core TTM or in a driver, I think a future goal might be a set of TTM shrinker helpers that makes sure we shrink the right TTM object, and perhaps a simple implementation that is typically used by simple drivers and other drivers can build on that for a more elaborate power- management aware shrinker.
That's understandable, but I think not necessary what we should aim for in the long term.
First of all I would really like to move more of the functionality from ttm_pool.c into the core memory management, especially handling of uncached and write combined memory.
That's essentially completely architecture dependent and currently implemented extremely awkward. Either Daniels suggestion of having a GFP_WC or Christophs approach of moving all this into the DMA API is the way to go here.
As long as i915 has no interest in USWC support implementing their own shmemfile backend sounds fine to me, but I have strong doubt that this will be of use to anybody else.
OK. Sounds fine. In situations where we use WC system memory we will use what's in TTM today. BTW on the shrinking approach for WC pages, does the Christoph's DMA API solution envision some kind of support for this?
/Thomas
Christian.
/Thomas
Regards, Christian.
Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri- devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects,
similar to how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like
in tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
Am 10.09.21 um 10:40 schrieb Thomas Hellström:
On Fri, 2021-09-10 at 10:25 +0200, Christian König wrote:
Am 10.09.21 um 10:08 schrieb Thomas Hellström:
Perhaps some background and goal is worth mentioning here.
On Thu, 2021-09-09 at 17:56 +0100, Matthew Auld wrote:
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian Christian.Koenig@amd.com wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you please point out where that is in the TTM code?
I think this is in unmap_mapping_range() where, if we use VM_MIXEDMAP, there is a reverse lookup on the PTEs that point to real pages. Now that we move over to VM_PFNMAP, that problem should go away since core vm never has a page to investigate. Probably this is why things works on non-TTM i915 GEM.
Yeah, that was really likely the root problem. I didn't kept investigating after realizing that my approach wouldn't work.
@Christian: Some background here: First I think that there might be things like the above that will pose problems, and we may or may not be able to overcome those but more importantly is that we agree with you that *if* we make it work, it is something that you as a maintainer of TTM can accept from a design- and maintainabiltiy point of view.
The approach would be similar to the buddy allocator, we adapt some driver code to TTM in a way that it may be reused with other drivers, and if other drivers are interested, we'd assist in moving to core TTM. In essence it'd be a TTM shmem page pool with full shrinking ability for cached pages only.
What we're really after here is the ability to shrink that doesn't regress much w r t the elaborate shrinker that's in i915 today that is power management aware and is also able to start shmem writebacks to avoid shmem just caching the pages instead of giving them back to the system (IIRC it was partly the lack of this that blocked earlier TTM shrinking efforts).
And since it doesn't really matter whether the shrinker sits in core TTM or in a driver, I think a future goal might be a set of TTM shrinker helpers that makes sure we shrink the right TTM object, and perhaps a simple implementation that is typically used by simple drivers and other drivers can build on that for a more elaborate power- management aware shrinker.
That's understandable, but I think not necessary what we should aim for in the long term.
First of all I would really like to move more of the functionality from ttm_pool.c into the core memory management, especially handling of uncached and write combined memory.
That's essentially completely architecture dependent and currently implemented extremely awkward. Either Daniels suggestion of having a GFP_WC or Christophs approach of moving all this into the DMA API is the way to go here.
As long as i915 has no interest in USWC support implementing their own shmemfile backend sounds fine to me, but I have strong doubt that this will be of use to anybody else.
OK. Sounds fine. In situations where we use WC system memory we will use what's in TTM today. BTW on the shrinking approach for WC pages, does the Christoph's DMA API solution envision some kind of support for this?
Not Christoph DMA API solution, but what I have in mind for the TTM shrinker should work.
Essentially a shmemfile per device should help in solving most of the issues we ran into.
Christian.
/Thomas
Christian.
/Thomas
Regards, Christian.
Von: Matthew Auld matthew.william.auld@gmail.com Gesendet: Donnerstag, 9. September 2021 16:56 An: Christian König ckoenig.leichtzumerken@gmail.com; Koenig, Christian Christian.Koenig@amd.com Cc: Thomas Hellström thomas.hellstrom@linux.intel.com; ML dri- devel dri-devel@lists.freedesktop.org Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects,
similar to how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like
in tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
Hi,
On 9/9/21 4:56 PM, Matthew Auld wrote:
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached system memory objects. We would also like to make such objects visible to the i915-gem shrinker, so that they may be swapped out or discarded when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar to
how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem. Not sure what functionally this might break, but looks like it's maybe only driver specific?
IIrc the page->mapping and index is needed when doing dirty-tracking using mkwrite and by vmwgfx at some point when doing fb_defio on top of TTM buffers. I don't think vmwgfx does that anymore, but it still does dirty-tracking.
/Thomas
dri-devel@lists.freedesktop.org
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Christian König -
Koenig, Christian -
Matthew Auld -
Thomas Hellström