| Message ID | 20240216131306.101932-1-thomas.hellstrom@linux.intel.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | TTM unlockable restartable LRU list iteration | expand |
Am 16.02.24 um 14:13 schrieb Thomas Hellström: > This patch-set is a prerequisite for a standalone TTM shrinker > and for exhaustive TTM eviction using sleeping dma_resv locks, > which is the motivation it. > > Currently when unlocking the TTM lru list lock, iteration needs > to be restarted from the beginning, rather from the next LRU list > node. This can potentially be a big problem, because if eviction > or shrinking fails for whatever reason after unlock, restarting > is likely to cause the same failure over and over again. Oh, yes please. I have been working on that problem before as well, but wasn't able to come up with something working. > There are various schemes to be able to continue the list > iteration from where we left off. One such scheme used by the > GEM LRU list traversal is to pull items already considered off > the LRU list and reinsert them when iteration is done. > This has the drawback that concurrent list iteration doesn't see > the complete list (which is bad for exhaustive eviction) and also > doesn't lend itself well to bulk-move sublists since these will > be split in the process where items from those lists are > temporarily pulled from the list and moved to the list tail. Completely agree that this is not a desirable solution. > The approach taken here is that list iterators insert themselves > into the list next position using a special list node. Iteration > is then using that list node as starting point when restarting. > Concurrent iterators just skip over the special list nodes. > > This is implemented in patch 1 and 2. > > For bulk move sublist the approach is the same, but when a bulk > move sublist is moved to the tail, the iterator is also moved, > causing us to skip parts of the list. That is undesirable. > Patch 3 deals with that, and when iterator detects it is > traversing a sublist, it inserts a second restarting point just > after the sublist and if the sublist is moved to the tail, > it just uses the second restarting point instead. > > This is implemented in patch 3. Interesting approach, that is probably even better than what I tried. My approach was basically to not only lock the current BO, but also the next one. Since only a locked BO can move on the LRU we effectively created an anchor. Before I dig into the code a couple of questions: 1. How do you distinct BOs and iteration anchors on the LRU? 2. How do you detect that a bulk list moved on the LRU? 3. How do you remove the iteration anchors from the bulk list? Regards, Christian. > > The restartable property is used in patch 4 to restart swapout if > needed, but the main purpose is this paves the way for > shrinker- and exhaustive eviction. > > Cc: Christian König <christian.koenig@amd.com> > Cc: <dri-devel@lists.freedesktop.org> > > Thomas Hellström (4): > drm/ttm: Allow TTM LRU list nodes of different types > drm/ttm: Use LRU hitches > drm/ttm: Consider hitch moves within bulk sublist moves > drm/ttm: Allow continued swapout after -ENOSPC falure > > drivers/gpu/drm/ttm/ttm_bo.c | 1 + > drivers/gpu/drm/ttm/ttm_device.c | 33 +++-- > drivers/gpu/drm/ttm/ttm_resource.c | 202 +++++++++++++++++++++++------ > include/drm/ttm/ttm_resource.h | 91 +++++++++++-- > 4 files changed, 267 insertions(+), 60 deletions(-) >
On Fri, 2024-02-16 at 15:00 +0100, Christian König wrote: > Am 16.02.24 um 14:13 schrieb Thomas Hellström: > > This patch-set is a prerequisite for a standalone TTM shrinker > > and for exhaustive TTM eviction using sleeping dma_resv locks, > > which is the motivation it. > > > > Currently when unlocking the TTM lru list lock, iteration needs > > to be restarted from the beginning, rather from the next LRU list > > node. This can potentially be a big problem, because if eviction > > or shrinking fails for whatever reason after unlock, restarting > > is likely to cause the same failure over and over again. > > Oh, yes please. I have been working on that problem before as well, > but > wasn't able to come up with something working. > > > There are various schemes to be able to continue the list > > iteration from where we left off. One such scheme used by the > > GEM LRU list traversal is to pull items already considered off > > the LRU list and reinsert them when iteration is done. > > This has the drawback that concurrent list iteration doesn't see > > the complete list (which is bad for exhaustive eviction) and also > > doesn't lend itself well to bulk-move sublists since these will > > be split in the process where items from those lists are > > temporarily pulled from the list and moved to the list tail. > > Completely agree that this is not a desirable solution. > > > The approach taken here is that list iterators insert themselves > > into the list next position using a special list node. Iteration > > is then using that list node as starting point when restarting. > > Concurrent iterators just skip over the special list nodes. > > > > This is implemented in patch 1 and 2. > > > > For bulk move sublist the approach is the same, but when a bulk > > move sublist is moved to the tail, the iterator is also moved, > > causing us to skip parts of the list. That is undesirable. > > Patch 3 deals with that, and when iterator detects it is > > traversing a sublist, it inserts a second restarting point just > > after the sublist and if the sublist is moved to the tail, > > it just uses the second restarting point instead. > > > > This is implemented in patch 3. > > Interesting approach, that is probably even better than what I tried. > > My approach was basically to not only lock the current BO, but also > the > next one. Since only a locked BO can move on the LRU we effectively > created an anchor. > > Before I dig into the code a couple of questions: These are described in the patches but brief comments inline. > 1. How do you distinct BOs and iteration anchors on the LRU? Using a struct ttm_lru_item, containing a struct list_head and the type. List nodes embeds this instead of a struct list_head. This is larger than the list head but makes it explicit what we're doing. > 2. How do you detect that a bulk list moved on the LRU? An age u64 counter on the bulk move that we're comparing against. It's updated each time it moves. > 3. How do you remove the iteration anchors from the bulk list? A function call at the end of iteration, that the function iterating is requried to call. /Thomas > > Regards, > Christian. > > > > > The restartable property is used in patch 4 to restart swapout if > > needed, but the main purpose is this paves the way for > > shrinker- and exhaustive eviction. > > > > Cc: Christian König <christian.koenig@amd.com> > > Cc: <dri-devel@lists.freedesktop.org> > > > > Thomas Hellström (4): > > drm/ttm: Allow TTM LRU list nodes of different types > > drm/ttm: Use LRU hitches > > drm/ttm: Consider hitch moves within bulk sublist moves > > drm/ttm: Allow continued swapout after -ENOSPC falure > > > > drivers/gpu/drm/ttm/ttm_bo.c | 1 + > > drivers/gpu/drm/ttm/ttm_device.c | 33 +++-- > > drivers/gpu/drm/ttm/ttm_resource.c | 202 +++++++++++++++++++++++- > > ----- > > include/drm/ttm/ttm_resource.h | 91 +++++++++++-- > > 4 files changed, 267 insertions(+), 60 deletions(-) > > >
Am 16.02.24 um 15:20 schrieb Thomas Hellström: [SNIP] >> My approach was basically to not only lock the current BO, but also >> the >> next one. Since only a locked BO can move on the LRU we effectively >> created an anchor. >> >> Before I dig into the code a couple of questions: > These are described in the patches but brief comments inline. > >> 1. How do you distinct BOs and iteration anchors on the LRU? > Using a struct ttm_lru_item, containing a struct list_head and the > type. List nodes embeds this instead of a struct list_head. This is > larger than the list head but makes it explicit what we're doing. Need to look deeper into the code of this, but it would be nice if we could abstract that better somehow. >> 2. How do you detect that a bulk list moved on the LRU? > An age u64 counter on the bulk move that we're comparing against. It's > updated each time it moves. > > >> 3. How do you remove the iteration anchors from the bulk list? > A function call at the end of iteration, that the function iterating is > requried to call. Thinking quite a bit about that in the last week and I came to the conclusion that this might be overkill. All BOs in a bulk share the same reservation object. So when you acquired one you can just keep the dma-resv locked even after evicting the BO. Since moving BOs requires locking the dma-resv object the whole problem then just boils down to a list_for_each_element_safe(). That's probably a bit simpler than doing the add/remove dance. Regards, Christian. > > > /Thomas > >> Regards, >> Christian. >> >>> The restartable property is used in patch 4 to restart swapout if >>> needed, but the main purpose is this paves the way for >>> shrinker- and exhaustive eviction. >>> >>> Cc: Christian König<christian.koenig@amd.com> >>> Cc:<dri-devel@lists.freedesktop.org> >>> >>> Thomas Hellström (4): >>> drm/ttm: Allow TTM LRU list nodes of different types >>> drm/ttm: Use LRU hitches >>> drm/ttm: Consider hitch moves within bulk sublist moves >>> drm/ttm: Allow continued swapout after -ENOSPC falure >>> >>> drivers/gpu/drm/ttm/ttm_bo.c | 1 + >>> drivers/gpu/drm/ttm/ttm_device.c | 33 +++-- >>> drivers/gpu/drm/ttm/ttm_resource.c | 202 +++++++++++++++++++++++- >>> ----- >>> include/drm/ttm/ttm_resource.h | 91 +++++++++++-- >>> 4 files changed, 267 insertions(+), 60 deletions(-) >>>
Hi, Christian. Thanks for having a look. On Thu, 2024-02-29 at 16:08 +0100, Christian König wrote: > Am 16.02.24 um 15:20 schrieb Thomas Hellström: > [SNIP] > > > My approach was basically to not only lock the current BO, but > > > also > > > the > > > next one. Since only a locked BO can move on the LRU we > > > effectively > > > created an anchor. > > > > > > Before I dig into the code a couple of questions: > > These are described in the patches but brief comments inline. > > > > > 1. How do you distinct BOs and iteration anchors on the LRU? > > Using a struct ttm_lru_item, containing a struct list_head and the > > type. List nodes embeds this instead of a struct list_head. This is > > larger than the list head but makes it explicit what we're doing. > > Need to look deeper into the code of this, but it would be nice if we > could abstract that better somehow. Do you have any specific concerns or improvements in mind? I think patch 1 and 2 are pretty straigthforward. Patch 3 is indeed a bit hairy. > > > > 2. How do you detect that a bulk list moved on the LRU? > > An age u64 counter on the bulk move that we're comparing against. > > It's > > updated each time it moves. > > > > > > > 3. How do you remove the iteration anchors from the bulk list? > > A function call at the end of iteration, that the function > > iterating is > > requried to call. > > Thinking quite a bit about that in the last week and I came to the > conclusion that this might be overkill. > > All BOs in a bulk share the same reservation object. So when you > acquired one you can just keep the dma-resv locked even after > evicting > the BO. > > Since moving BOs requires locking the dma-resv object the whole > problem > then just boils down to a list_for_each_element_safe(). > > That's probably a bit simpler than doing the add/remove dance. I think the problem with the "lock the next object" approach is that there are situations that it might not work. First, where not asserting anywhere that all bulk move resource have the same lock, and after individualization they certainly don't. (I think I had a patch somewhere to try to enforce that, but I don't think it ever got reviewed). I tried to sort out the locking rules at one point for resources switching bos to ghost object but I long since forgot those. I guess it all boils down to the list elements being resources, not bos. Also I'm concerned about keeping a resv held for a huge number of evictions will block out a higher priority ticket for a substantial amount of time. I think while the suggested solution here might be a bit of an overkill, it's simple enough to understand, but the locking implications of resources switching resvs arent. But please let me know how we should proceed here. This is a blocker for other pending work we have. /Thomas > > Regards, > Christian. > > > > > > > /Thomas > > > > > Regards, > > > Christian. > > > > > > > The restartable property is used in patch 4 to restart swapout > > > > if > > > > needed, but the main purpose is this paves the way for > > > > shrinker- and exhaustive eviction. > > > > > > > > Cc: Christian König<christian.koenig@amd.com> > > > > Cc:<dri-devel@lists.freedesktop.org> > > > > > > > > Thomas Hellström (4): > > > > drm/ttm: Allow TTM LRU list nodes of different types > > > > drm/ttm: Use LRU hitches > > > > drm/ttm: Consider hitch moves within bulk sublist moves > > > > drm/ttm: Allow continued swapout after -ENOSPC falure > > > > > > > > drivers/gpu/drm/ttm/ttm_bo.c | 1 + > > > > drivers/gpu/drm/ttm/ttm_device.c | 33 +++-- > > > > drivers/gpu/drm/ttm/ttm_resource.c | 202 > > > > +++++++++++++++++++++++- > > > > ----- > > > > include/drm/ttm/ttm_resource.h | 91 +++++++++++-- > > > > 4 files changed, 267 insertions(+), 60 deletions(-) > > > >
On Thu, 2024-02-29 at 18:34 +0100, Thomas Hellström wrote: > Hi, Christian. > > Thanks for having a look. > > On Thu, 2024-02-29 at 16:08 +0100, Christian König wrote: > > Am 16.02.24 um 15:20 schrieb Thomas Hellström: > > [SNIP] > > > > My approach was basically to not only lock the current BO, but > > > > also > > > > the > > > > next one. Since only a locked BO can move on the LRU we > > > > effectively > > > > created an anchor. > > > > > > > > Before I dig into the code a couple of questions: > > > These are described in the patches but brief comments inline. > > > > > > > 1. How do you distinct BOs and iteration anchors on the LRU? > > > Using a struct ttm_lru_item, containing a struct list_head and > > > the > > > type. List nodes embeds this instead of a struct list_head. This > > > is > > > larger than the list head but makes it explicit what we're doing. > > > > Need to look deeper into the code of this, but it would be nice if > > we > > could abstract that better somehow. > > Do you have any specific concerns or improvements in mind? I think > patch 1 and 2 are pretty straigthforward. Patch 3 is indeed a bit > hairy. > > > > > > > 2. How do you detect that a bulk list moved on the LRU? > > > An age u64 counter on the bulk move that we're comparing against. > > > It's > > > updated each time it moves. > > > > > > > > > > 3. How do you remove the iteration anchors from the bulk list? > > > A function call at the end of iteration, that the function > > > iterating is > > > requried to call. > > > > Thinking quite a bit about that in the last week and I came to the > > conclusion that this might be overkill. > > > > All BOs in a bulk share the same reservation object. So when you > > acquired one you can just keep the dma-resv locked even after > > evicting > > the BO. > > > > Since moving BOs requires locking the dma-resv object the whole > > problem > > then just boils down to a list_for_each_element_safe(). > > > > That's probably a bit simpler than doing the add/remove dance. > > I think the problem with the "lock the next object" approach is that > there are situations that it might not work. First, where not > asserting > anywhere that all bulk move resource have the same lock, and after > individualization they certainly don't. (I think I had a patch > somewhere to try to enforce that, but I don't think it ever got > reviewed). I tried to sort out the locking rules at one point for > resources switching bos to ghost object but I long since forgot > those. > > I guess it all boils down to the list elements being resources, not > bos. > > Also I'm concerned about keeping a resv held for a huge number of > evictions will block out a higher priority ticket for a substantial > amount of time. > > I think while the suggested solution here might be a bit of an > overkill, it's simple enough to understand, but the locking > implications of resources switching resvs arent. > > But please let me know how we should proceed here. This is a blocker > for other pending work we have. Actually some more issues with the locking approach would be with the intended use-cases I was planning to use this for. For example the exhaustive eviction where we regularly unlock the lru_lock to take the bo lock. If we need to do that for the first element of a bulk_move list, we can't have the bo lock of the next element when we unlock the list. For part of the list that is not a bulk sublist, this also doesn't work AFAICT. And finally for the tt shinking that's been pending for quite some time, the last comment that made me temporarily shelf is was that we should expose the lru traversal to the drivers, and the drivers implement the shrinkers with TTM helpers, rather than having TTM being the middle layer. So I think exposing the LRU traversal to drivers will probably end up having pretty weird semantics if it sometimes locks or requiring locking of the next object while traversing. But regardless of how this is solved, since I think we are agreeing that the functionality itself is useful and needed, could we perhaps use this implementation that is easy to verify that it works, and I will i no way stand in the way if it turns out you come up with something nicer. I've been thinking a bit of how to make a better approach out of patch 3, and a possible alternative that I could prototype would be to register list cursors that traverse a bulk sublist with the bulk move structure using a list. At destruction of either list cursors or bulk moves either can unregister, and on bulk list bumping the list is traversed and the cursor is moved to the end of the list. Probably the same amount of code but will look nicer. /Thomas > > /Thomas > > > > > > > Regards, > > Christian. > > > > > > > > > > > /Thomas > > > > > > > Regards, > > > > Christian. > > > > > > > > > The restartable property is used in patch 4 to restart > > > > > swapout > > > > > if > > > > > needed, but the main purpose is this paves the way for > > > > > shrinker- and exhaustive eviction. > > > > > > > > > > Cc: Christian König<christian.koenig@amd.com> > > > > > Cc:<dri-devel@lists.freedesktop.org> > > > > > > > > > > Thomas Hellström (4): > > > > > drm/ttm: Allow TTM LRU list nodes of different types > > > > > drm/ttm: Use LRU hitches > > > > > drm/ttm: Consider hitch moves within bulk sublist moves > > > > > drm/ttm: Allow continued swapout after -ENOSPC falure > > > > > > > > > > drivers/gpu/drm/ttm/ttm_bo.c | 1 + > > > > > drivers/gpu/drm/ttm/ttm_device.c | 33 +++-- > > > > > drivers/gpu/drm/ttm/ttm_resource.c | 202 > > > > > +++++++++++++++++++++++- > > > > > ----- > > > > > include/drm/ttm/ttm_resource.h | 91 +++++++++++-- > > > > > 4 files changed, 267 insertions(+), 60 deletions(-) > > > > > >
Am 01.03.24 um 14:45 schrieb Thomas Hellström: > On Thu, 2024-02-29 at 18:34 +0100, Thomas Hellström wrote: >> Hi, Christian. >> >> Thanks for having a look. >> >> On Thu, 2024-02-29 at 16:08 +0100, Christian König wrote: >>> Am 16.02.24 um 15:20 schrieb Thomas Hellström: >>> [SNIP] >>>>> My approach was basically to not only lock the current BO, but >>>>> also >>>>> the >>>>> next one. Since only a locked BO can move on the LRU we >>>>> effectively >>>>> created an anchor. >>>>> >>>>> Before I dig into the code a couple of questions: >>>> These are described in the patches but brief comments inline. >>>> >>>>> 1. How do you distinct BOs and iteration anchors on the LRU? >>>> Using a struct ttm_lru_item, containing a struct list_head and >>>> the >>>> type. List nodes embeds this instead of a struct list_head. This >>>> is >>>> larger than the list head but makes it explicit what we're doing. >>> Need to look deeper into the code of this, but it would be nice if >>> we >>> could abstract that better somehow. >> Do you have any specific concerns or improvements in mind? I think >> patch 1 and 2 are pretty straigthforward. Patch 3 is indeed a bit >> hairy. Yeah, seen that as well. No idea of hand how to improve. Amar should have time to give the patches a more in deep review, maybe he has an idea. >> >>>>> 2. How do you detect that a bulk list moved on the LRU? >>>> An age u64 counter on the bulk move that we're comparing against. >>>> It's >>>> updated each time it moves. >>>> >>>> >>>>> 3. How do you remove the iteration anchors from the bulk list? >>>> A function call at the end of iteration, that the function >>>> iterating is >>>> requried to call. >>> Thinking quite a bit about that in the last week and I came to the >>> conclusion that this might be overkill. >>> >>> All BOs in a bulk share the same reservation object. So when you >>> acquired one you can just keep the dma-resv locked even after >>> evicting >>> the BO. >>> >>> Since moving BOs requires locking the dma-resv object the whole >>> problem >>> then just boils down to a list_for_each_element_safe(). >>> >>> That's probably a bit simpler than doing the add/remove dance. >> I think the problem with the "lock the next object" approach Stop stop, you misunderstood me. I was not suggesting to use the lock the next object approach, this anchor approach here is certainly better. I just wanted to note that we most likely don't need to insert a second anchor for bulk moves. Basically my idea is that we start to use the drm_exec object to lock BOs and those BOs stay locked until everything is completed. That also removes the problem that a BO might be evicted just to be moved back in again by a concurrent submission. >> is that >> there are situations that it might not work. First, where not >> asserting >> anywhere that all bulk move resource have the same lock, Daniel actually wanted that I add such an assert, I just couldn't find a way to easily do this back then. But since I reworked the bulk move since then it should now be possible. >> and after >> individualization they certainly don't. Actually when they are individualized for freeing they shouldn't be part of any bulk any more. >> (I think I had a patch >> somewhere to try to enforce that, but I don't think it ever got >> reviewed). I tried to sort out the locking rules at one point for >> resources switching bos to ghost object but I long since forgot >> those. >> >> I guess it all boils down to the list elements being resources, not >> bos. >> >> Also I'm concerned about keeping a resv held for a huge number of >> evictions will block out a higher priority ticket for a substantial >> amount of time. >> >> I think while the suggested solution here might be a bit of an >> overkill, it's simple enough to understand, but the locking >> implications of resources switching resvs arent. >> >> But please let me know how we should proceed here. This is a blocker >> for other pending work we have. > Actually some more issues with the locking approach would be with the > intended use-cases I was planning to use this for. > > For example the exhaustive eviction where we regularly unlock the > lru_lock to take the bo lock. If we need to do that for the first > element of a bulk_move list, we can't have the bo lock of the next > element when we unlock the list. For part of the list that is not a > bulk sublist, this also doesn't work AFAICT. Well when we drop the LRU lock we should always have the anchor on the LRU before the element we try to lock. This way we actually don't have to move the anchor unless we found a BO which we don't want to evict. E.g. something like Head -> anchor -> BO1 -> BO2 -> BO3 -> BO4 And we Evict BO1, BO2 and then find that BO3 doesn't match the allocation pattern we need so only then is the anchor moved after BO3: Head -> BO3 -> anchor -> BO4.... And when we moved inside a bulk with an anchor we have already locked at least one BO of the bulk, so locking the next one is a no-op. > And finally for the tt shinking that's been pending for quite some > time, the last comment that made me temporarily shelf is was that we > should expose the lru traversal to the drivers, and the drivers > implement the shrinkers with TTM helpers, rather than having TTM being > the middle layer. So I think exposing the LRU traversal to drivers will > probably end up having pretty weird semantics if it sometimes locks or > requiring locking of the next object while traversing. Yeah, I was just yesterday talking about that with Amar and putting him on the task to look into tt shrinking. And completely agree that providing the necessary toolbox for eviction is a better approach than burying the eviction deep into the TTM logic. > But regardless of how this is solved, since I think we are agreeing > that the functionality itself is useful and needed, could we perhaps > use this implementation that is easy to verify that it works, and I > will i no way stand in the way if it turns out you come up with > something nicer. I've been thinking a bit of how to make a better > approach out of patch 3, and a possible alternative that I could > prototype would be to register list cursors that traverse a bulk > sublist with the bulk move structure using a list. At destruction of > either list cursors or bulk moves either can unregister, and on bulk > list bumping the list is traversed and the cursor is moved to the end > of the list. Probably the same amount of code but will look nicer. Yeah, I'm just not sure if this handling here will be so simple with multiple anchors. That sounds very fragile. Regards, Christian. > > /Thomas > > >> /Thomas >> >> >> >>> Regards, >>> Christian. >>> >>>> >>>> /Thomas >>>> >>>>> Regards, >>>>> Christian. >>>>> >>>>>> The restartable property is used in patch 4 to restart >>>>>> swapout >>>>>> if >>>>>> needed, but the main purpose is this paves the way for >>>>>> shrinker- and exhaustive eviction. >>>>>> >>>>>> Cc: Christian König<christian.koenig@amd.com> >>>>>> Cc:<dri-devel@lists.freedesktop.org> >>>>>> >>>>>> Thomas Hellström (4): >>>>>> drm/ttm: Allow TTM LRU list nodes of different types >>>>>> drm/ttm: Use LRU hitches >>>>>> drm/ttm: Consider hitch moves within bulk sublist moves >>>>>> drm/ttm: Allow continued swapout after -ENOSPC falure >>>>>> >>>>>> drivers/gpu/drm/ttm/ttm_bo.c | 1 + >>>>>> drivers/gpu/drm/ttm/ttm_device.c | 33 +++-- >>>>>> drivers/gpu/drm/ttm/ttm_resource.c | 202 >>>>>> +++++++++++++++++++++++- >>>>>> ----- >>>>>> include/drm/ttm/ttm_resource.h | 91 +++++++++++-- >>>>>> 4 files changed, 267 insertions(+), 60 deletions(-) >>>>>>
On Fri, 2024-03-01 at 15:20 +0100, Christian König wrote: > Am 01.03.24 um 14:45 schrieb Thomas Hellström: > > On Thu, 2024-02-29 at 18:34 +0100, Thomas Hellström wrote: > > > Hi, Christian. > > > > > > Thanks for having a look. > > > > > > On Thu, 2024-02-29 at 16:08 +0100, Christian König wrote: > > > > Am 16.02.24 um 15:20 schrieb Thomas Hellström: > > > > [SNIP] > > > > > > My approach was basically to not only lock the current BO, > > > > > > but > > > > > > also > > > > > > the > > > > > > next one. Since only a locked BO can move on the LRU we > > > > > > effectively > > > > > > created an anchor. > > > > > > > > > > > > Before I dig into the code a couple of questions: > > > > > These are described in the patches but brief comments inline. > > > > > > > > > > > 1. How do you distinct BOs and iteration anchors on the > > > > > > LRU? > > > > > Using a struct ttm_lru_item, containing a struct list_head > > > > > and > > > > > the > > > > > type. List nodes embeds this instead of a struct list_head. > > > > > This > > > > > is > > > > > larger than the list head but makes it explicit what we're > > > > > doing. > > > > Need to look deeper into the code of this, but it would be nice > > > > if > > > > we > > > > could abstract that better somehow. > > > Do you have any specific concerns or improvements in mind? I > > > think > > > patch 1 and 2 are pretty straigthforward. Patch 3 is indeed a bit > > > hairy. > > Yeah, seen that as well. No idea of hand how to improve. > > Amar should have time to give the patches a more in deep review, > maybe > he has an idea. > > > > > > > > > > 2. How do you detect that a bulk list moved on the LRU? > > > > > An age u64 counter on the bulk move that we're comparing > > > > > against. > > > > > It's > > > > > updated each time it moves. > > > > > > > > > > > > > > > > 3. How do you remove the iteration anchors from the bulk > > > > > > list? > > > > > A function call at the end of iteration, that the function > > > > > iterating is > > > > > requried to call. > > > > Thinking quite a bit about that in the last week and I came to > > > > the > > > > conclusion that this might be overkill. > > > > > > > > All BOs in a bulk share the same reservation object. So when > > > > you > > > > acquired one you can just keep the dma-resv locked even after > > > > evicting > > > > the BO. > > > > > > > > Since moving BOs requires locking the dma-resv object the whole > > > > problem > > > > then just boils down to a list_for_each_element_safe(). > > > > > > > > That's probably a bit simpler than doing the add/remove dance. > > > I think the problem with the "lock the next object" approach > > Stop stop, you misunderstood me. I was not suggesting to use the lock > the next object approach, this anchor approach here is certainly > better. > > I just wanted to note that we most likely don't need to insert a > second > anchor for bulk moves. > > Basically my idea is that we start to use the drm_exec object to lock > BOs and those BOs stay locked until everything is completed. > > That also removes the problem that a BO might be evicted just to be > moved back in again by a concurrent submission. Ah, yes then we're on the same page. > > > > is that > > > there are situations that it might not work. First, where not > > > asserting > > > anywhere that all bulk move resource have the same lock, > > Daniel actually wanted that I add such an assert, I just couldn't > find a > way to easily do this back then. > > But since I reworked the bulk move since then it should now be > possible. > > > > and after > > > individualization they certainly don't. > > Actually when they are individualized for freeing they shouldn't be > part > of any bulk any more. > > > > (I think I had a patch > > > somewhere to try to enforce that, but I don't think it ever got > > > reviewed). I tried to sort out the locking rules at one point for > > > resources switching bos to ghost object but I long since forgot > > > those. > > > > > > I guess it all boils down to the list elements being resources, > > > not > > > bos. > > > > > > Also I'm concerned about keeping a resv held for a huge number of > > > evictions will block out a higher priority ticket for a > > > substantial > > > amount of time. > > > > > > I think while the suggested solution here might be a bit of an > > > overkill, it's simple enough to understand, but the locking > > > implications of resources switching resvs arent. > > > > > > But please let me know how we should proceed here. This is a > > > blocker > > > for other pending work we have. > > Actually some more issues with the locking approach would be with > > the > > intended use-cases I was planning to use this for. > > > > For example the exhaustive eviction where we regularly unlock the > > lru_lock to take the bo lock. If we need to do that for the first > > element of a bulk_move list, we can't have the bo lock of the next > > element when we unlock the list. For part of the list that is not a > > bulk sublist, this also doesn't work AFAICT. > > Well when we drop the LRU lock we should always have the anchor on > the > LRU before the element we try to lock. > > This way we actually don't have to move the anchor unless we found a > BO > which we don't want to evict. > > E.g. something like > > Head -> anchor -> BO1 -> BO2 -> BO3 -> BO4 > > And we Evict BO1, BO2 and then find that BO3 doesn't match the > allocation pattern we need so only then is the anchor moved after > BO3: > > Head -> BO3 -> anchor -> BO4.... > > And when we moved inside a bulk with an anchor we have already locked > at > least one BO of the bulk, so locking the next one is a no-op. > > > And finally for the tt shinking that's been pending for quite some > > time, the last comment that made me temporarily shelf is was that > > we > > should expose the lru traversal to the drivers, and the drivers > > implement the shrinkers with TTM helpers, rather than having TTM > > being > > the middle layer. So I think exposing the LRU traversal to drivers > > will > > probably end up having pretty weird semantics if it sometimes locks > > or > > requiring locking of the next object while traversing. > > Yeah, I was just yesterday talking about that with Amar and putting > him > on the task to look into tt shrinking. Oh, we should probably sync on that then so we don't create two separate solutions. That'd be wasted work. I think the key patch in the series I had that made things "work" was this helper patch here: https://patchwork.kernel.org/project/intel-gfx/patch/20230215161405.187368-15-thomas.hellstrom@linux.intel.com/ Making sure that we could shrink on a per-page basis (we either insert the page in the swap cache or it might actually even work with copying to shmem, since pages are immediately released one by one and not after copying the whole tt). Sima has little confidence that we'll find a core mm reviewer to the patch I made to insert the pages directly into the swap cache. https://patchwork.kernel.org/project/intel-gfx/patch/20230215161405.187368-13-thomas.hellstrom@linux.intel.com/ /Thomas