On Wed, 2024-10-09 at 15:39 +0200, Thomas Hellström wrote:On Mon, 2024-10-07 at 11:08 +0200, Christian König wrote:Hi Thomas, I'm on sick leave, but I will try to answer questions and share some thoughts on this to unblock you. Am 18.09.24 um 14:57 schrieb Thomas Hellström: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=9Yeah that looks like a big step in the right direction.(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.I think that is actually not correct. TTM already knows about purgeable objects. E.g. when TTM asks the driver what to do with evicted objects it is perfectly valid to return an empty placement list indicating that the backing store can just be thrown away. We use this for things like temporary buffers for example. That this doesn't apply to swapping looks like a design bug in the driver/TTM interface to me.Yes we can do that with TTM, but for shrinking there's no point in trying to shrink when there is no swap-space left, other than purgeable object. The number of shrinkable objects returned in shrinker::count needs to reflect that, and *then* only those objects should be selected for shrinking. If we were to announce that to TTM using a callback, we're actually back to version 1 of this series which was rejected by you exactly since it was using callbacks a year or so ago????
Yeah that indeed doesn't sound like a good idea.
On the other hand the decision that only purgeable objects should be selected when there is no swap space left sounds like something TTM should do and not the driver.
2) Devices who need the blitter during shrinking would want to punt runtime_pm_get() to kswapd to avoid sleeping direct reclaim.I think the outcome of the discussion is that runtime PM should never be mixed into TTM swapping. You can power up blocks to enable a HW blitter for swapping, but this then can't be driven by the runtime PM framework.Still that power-on might be sleeping, so what's the difference using runtime-pm or not? Why should the driver implement yet another power interface, just because TTM refuses to publish a sane LRU walk interface?
See the discussion with Sima around the PM functions.
My understanding might be wrong, but I now think that with local memory you can't do the i915 approach where the PM functions are so low level that they can also be called inside the shrinker.
So you basically have PM functions for your whole device and PM functions for only the HW blocks necessary for the shrinker.
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.Mhm, what do you mean with that?When we fired the blitter from direct reclaim, we get a fence. If we wait for it in direct reclaim we will be sleeping waiting for gpu, which is bad form. We'd like return a failure so the object is retried when idle, or from kswapd.
Oh, that is a really good point. So basically you want to avoid dependencies on the dma_fence.
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.Why would a folio trylock succeed with one BO and not another?Good point. We'd fail anyway but would perhaps need to call SHRINK_STOP..And why would that trylock something the device specific driver should handle?It happens in the TTM shrinker helper called from the driver in the spirit of demidlayering.Arguments against: None really. I thought the idea of demidlayering would be to allow the driver more freedom.Well that is a huge argument against it. Giving drivers more freedom is absolutely not something which turned out to be valuable in the past.So then what's the point of demidlayering?
So that drivers can prepare the environment for TTM to work with instead of TTM asking for it.
In your case for example that means powering up HW blocks so that BOs could be moved.
Instead we should put device drivers in a very strict corset of validated approaches to do things. Background is that in my experience driver developers are perfectly willing to do unclean approaches which only work in like 99% of all cases just to get a bit more performance or simpler driver implementation. Those approaches are not legal and in my opinion as subsystem maintainer I think we need to be more strict and push back much harder on stuff like that.Still, historically that has made developers abandon common components for driver-specific solutions. And the question is still not answered. Exactly *why* can't the driver fail and continue traversing the LRU, because all our argumentation revolves around this and you have yet to provide an objective reason why.And in the end, while I think there definitely things worthy of discussion in this series, I don't think there is a point in trying to land a LNL shrinker operating against a crippled TTM LRU walk interface, nor do I think anyone would want to attempt to port i915 over, and reworking it three times I'm now pretty familiar with what works and what doesn't. So question becomes, with proper reviews can I merge the series here as is, *with* the de-midlayered LRU walk and noting your advise against it, or not?
More or less yes, I still have a bad feeling about it but we probably need to see the whole thing getting used to judge if it really works or not.
I mean it's not UAPI we are talking about, so even if we find in 5years from now that it was a bad idea we can still roll it back and try something else.
So yeah, feel free to go ahead.
Regards,
Christian.
Thanks, Thomas/ThomasRegards, Christian.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
