Hi On Wed, 2024-10-02 at 13:32 +0200, Christian König wrote: > Ah, yes sorry totally forgotten about that. > > Give me till Friday to swap everything back into my head again. > > Christian. Thanks! Once we agree on a direction MBrost is ready to do an in-depth review. /Thomas > > Am 02.10.24 um 13:30 schrieb Thomas Hellström: > > Hi, Christian, > > > > Ping? Can i get an ack to proceed with this? > > > > Thanks, > > Thomas > > > > > > > > On Wed, 2024-09-18 at 14:57 +0200, Thomas Hellström wrote: > > > Sima, Christian > > > > > > I've updated the shrinker series now with a guarded for_each > > > macro > > > instead: > > > > > > https://patchwork.freedesktop.org/patch/614514/?series=131815&rev=9 > > > > > > (Note I forgot to remove the export of the previous LRU walker). > > > > > > so the midlayer argument is now not an issue anymore. The > > > cleanup.h > > > guard provides some additional protection against drivers exiting > > > the > > > LRU loop early. > > > > > > So remaining is the question whether the driver is allowed to > > > discard > > > a > > > suggested bo to shrink from TTM. > > > > > > Arguments for: > > > > > > 1) Not allowing that would require teaching TTM about purgeable > > > objects. > > > 2) Devices who need the blitter during shrinking would want to > > > punt > > > runtime_pm_get() to kswapd to avoid sleeping direct reclaim. > > > 3) If those devices end up blitting (LNL) to be able to shrink, > > > they > > > would want to punt waiting for the fence to signal to kswapd to > > > avoid > > > waiting in direct reclaim. > > > 4) It looks like we need to resort to folio_trylock in the shmem > > > backup > > > backend when shrinking is called for gfp_t = GFP_NOFS. A failing > > > trylock will require a new bo. > > > > > > Arguments against: > > > None really. I thought the idea of demidlayering would be to > > > allow > > > the > > > driver more freedom. > > > > > > So any feedback appreciated. If that is found acceptable we can > > > proceed > > > with reviewing this patch and also with the shrinker series. > > > > > > Thanks, > > > Thomas > > > > > > > > > On Mon, 2024-09-02 at 13:07 +0200, Daniel Vetter wrote: > > > > On Wed, Aug 28, 2024 at 02:20:34PM +0200, Christian König > > > > wrote: > > > > > Am 27.08.24 um 19:53 schrieb Daniel Vetter: > > > > > > On Tue, Aug 27, 2024 at 06:52:13PM +0200, Daniel Vetter > > > > > > wrote: > > > > > > > On Thu, Aug 22, 2024 at 03:19:29PM +0200, Christian König > > > > > > > wrote: > > > > > > > > Completely agree that this is complicated, but I still > > > > > > > > don't > > > > > > > > see the need > > > > > > > > for it. > > > > > > > > > > > > > > > > Drivers just need to use pm_runtime_get_if_in_use() > > > > > > > > inside > > > > > > > > the shrinker and > > > > > > > > postpone all hw activity until resume. > > > > > > > Not good enough, at least long term I think. Also > > > > > > > postponing > > > > > > > hw > > > > > > > activity > > > > > > > to resume doesn't solve the deadlock issue, if you still > > > > > > > need > > > > > > > to grab ttm > > > > > > > locks on resume. > > > > > > Pondered this specific aspect some more, and I think you > > > > > > still > > > > > > have a race > > > > > > here (even if you avoid the deadlock): If the condiditional > > > > > > rpm_get call > > > > > > fails there's no guarantee that the device will > > > > > > suspend/resume > > > > > > and clean > > > > > > up the GART mapping. > > > > > Well I think we have a major disconnect here. When the device > > > > > is > > > > > powered > > > > > down there is no GART mapping to clean up any more. > > > > > > > > > > In other words GART is a table in local memory (VRAM) when > > > > > the > > > > > device is > > > > > powered down this table is completely destroyed. Any BO which > > > > > was > > > > > mapped > > > > > inside this table is now not mapped any more. > > > > > > > > > > So when the shrinker wants to evict a BO which is marked as > > > > > mapped > > > > > to GART > > > > > and the device is powered down we just skip the GART > > > > > unmapping > > > > > part > > > > > because > > > > > that has already implicitly happened during power down. > > > > > > > > > > Before mapping any BO into the GART again we power the GPU up > > > > > through the > > > > > runtime PM calls. And while powering it up again the GART is > > > > > restored. > > > > My point is that you can't tell whether the device will power > > > > down > > > > or > > > > not, > > > > you can only tell whether there's a chance it might be powering > > > > down > > > > and > > > > so you can't get at the rpm reference without deadlock issues. > > > > > > > > > > The race gets a bit smaller if you use > > > > > > pm_runtime_get_if_active(), but even then you might catch > > > > > > it > > > > > > right when > > > > > > resume almost finished. > > > > > What race are you talking about? > > > > > > > > > > The worst thing which could happen is that we restore a GART > > > > > entry > > > > > which > > > > > isn't needed any more, but that is pretty much irrelevant > > > > > since > > > > > we > > > > > only > > > > > clear them to avoid some hw bugs. > > > > The race I'm seeing is where you thought the GART entry is not > > > > issue, > > > > tossed an object, but the device didn't suspend, so might still > > > > use > > > > it. > > > > > > > > I guess if we're clearly separating the sw allocation of the > > > > TTM_TT > > > > with > > > > the physical entries in the GART that should all work, but > > > > feels a > > > > bit > > > > tricky. The race I've seen is essentially these two getting out > > > > of > > > > sync. > > > > > > > > So maybe it was me who's stuck. > > > > > > > > What I wonder is whether it works in practice, since on the > > > > restore > > > > side > > > > you need to get some locks to figure out which gart mappings > > > > exist > > > > and > > > > need restoring. And that's the same locks as the shrinker needs > > > > to > > > > figure > > > > out whether it might need to reap a gart mapping. > > > > > > > > Or do you just copy the gart entries over and restore them > > > > exactly > > > > as-is, > > > > so that there's no shared locks? > > > > > > > > > > That means we'll have ttm bo hanging around with GART > > > > > > allocations/mappings > > > > > > which aren't actually valid anymore (since they might > > > > > > escape > > > > > > the > > > > > > cleanup > > > > > > upon resume due to the race). That doesn't feel like a > > > > > > solid > > > > > > design > > > > > > either. > > > > > I'm most likely missing something, but I'm really scratching > > > > > my > > > > > head where > > > > > you see a problem here. > > > > I guess one issue is that at least traditionally, igfx drivers > > > > have > > > > nested > > > > runtime pm within dma_resv lock. And dgpu drivers the other way > > > > round. > > > > Which is a bit awkward if you're trying for common code. > > > > > > > > Cheers, Sima >
