Hi, On Mon, 2024-08-19 at 17:26 +0200, Christian König wrote:Am 19.08.24 um 16:14 schrieb Daniel Vetter:On Mon, Aug 19, 2024 at 01:38:56PM +0200, Thomas Hellström wrote:Hi, Christian, On Mon, 2024-08-19 at 13:03 +0200, Christian König wrote:Am 06.08.24 um 10:29 schrieb Thomas Hellström:Hi, Christian. On Thu, 2024-07-11 at 14:01 +0200, Christian König wrote:Am 10.07.24 um 20:19 schrieb Matthew Brost:On Wed, Jul 10, 2024 at 02:42:58PM +0200, Christian König wrote:That is something drivers really shouldn't mess with.Thomas uses this in Xe to implement a shrinker [1]. Seems to need to remain available for drivers.No, that is exactly what I try to prevent with that. This is an internally thing of TTM and drivers should never use it directly.That driver-facing LRU walker is a direct response/solution to this comment that you made in the first shrinker series: https://lore.kernel.org/linux-mm/b7491378-defd-4f1c-31e2-29e4c77e2d67@xxxxxxx/T/#ma918844aa8a6efe8768fdcda0c6590d5c93850c9Ah, yeah that was about how we are should be avoiding middle layer design. But a function which returns the next candidate for eviction and a function which calls a callback for eviction is exactly the opposite.That is also mentioned in the cover letter of the recent shrinker series, and this walker has been around in that shrinker series for more than half a year now so if you think it's not the correct driver facing API IMHO that should be addressed by a review comment in that series rather than in posting a conflicting patch?I actually outlined that in the review comments for the patch series. E.g. a walker function with a callback is basically a middle layer. What outlined in the link above is that a function which returns the next eviction candidate is a better approach than a callback.So assuming that we still want the driver to register the shrinker, IMO that helper abstracts away all the nasty locking and pitfalls for a driver-registered shrinker, and is similar in structure to for example the pagewalk helper in mm/pagewalk.c. An alternative that could be tried as a driver-facing API is to provide a for_each_bo_in_lru_lock() macro where the driver open-codes "process_bo()" inside the for loop but I tried this and found it quite fragile since the driver might exit the loop without performing the necessary cleanup.The point is that the shrinker should *never* need to have context. E.g. a walker which allows going over multiple BOs for eviction is exactly the wrong approach for that. The shrinker should evict always only exactly one BO and the next invocation of a shrinker should not depend on the result of the previous one. Or am I missing something vital here?A couple of things, 1) I'd like to think of the middle-layer vs helper like the helper has its own ops, and can be used optionally from the driver. IIRC, the atomic modesetting / pageflip ops are implemented in exactly this way. Sometimes a certain loop operation can't be easily or at least robustly implemented using a for_each_.. approach. Like for example mm/pagewalk.c. In this shrinking case I think it's probably possible using the scoped_guard() in cleanup.h. This way we could get rid of this middle layer discussion by turning the interface inside-out: for_each_bo_on_lru_locked(xxx) driver_shrink(); But I do think the currently suggested approach is less fragile and prone to driver error. FWIW in addition to the above examples, also drm_gem_lru_scan works like this.a iteration state structure (like drm_connector_iter) plus then a macro for the common case that uses cleanup.h should get the job done.Yeah, completely agree. It basically boils down to which one needs to pack a state bag. In a mid-layer design it's the driver or the caller of functions, e.g. the much hated init callback in the DRM layer was a perfect example of that. In a non mid-layer approach it's the framework or the called function, examples are how the fence iterators in the dma_resv or the drm_connector, plane etc.. work. One big difference between the two approach is who and where things like preparation and cleanup are done, e.g. who takes locks for example.So what in your opinion is an acceptable solution here? The "get next object to shrink" approach won't work, since it's subject to the old TTM swap problem now removed: If shrinking fails we will get the same object upon subsequent calls.
But and that is expected behavior. If shrinking fails just going to the next object is invalid as far as I can see.
That's why I was so puzzled why you tried to apply the walker to the TTM shrinker.
Or why exactly should shrinking fail?
If we bump LRU we could end up with infinite loops. So IMO we need to be able to loop. I don't really care wether we do this as an explicit loop or whether we use the LRU walker, but I think from a maintainability point-of-view it is better to keep LRU walking in a single place. If we return an unlocked object, we'd need to refcount and drop the lru lock, but maybe that's not a bad thing. But what's the main drawback of exporting the existing helper.
Well that we re-creates exactly the mid-layer mess I worked so hard to remove from TTM.
2) The shrinkers suggest to shrink a number of pages, not a single bo, again drm_gem_lru_scan works like this. i915 works like this. I think we should align with those.Yeah that's how shrinkers work, so if we demidlayer then it realls needs to be a loop in the driver, not a "here's the next bo to nuke" I think.Hui? Well that's not how I understand shrinkers. The shrinker gives you the maximum number of objects to scan and not how many pages to free. In return you give the actual number of objects to scanned and pages freed. As far as I know the number of objects are in the sense of SLUBs or rather different LRU lists. So for BO shrinking we should probably only free or rather unpin a single BO per callback otherwise we mess up the fairness between shrinkers in the MM layer.Hm. AFAICT all drm shrinkers use pages as objects, and the docs of nr_to_scan says it's the number of objects to scan and try to reclaim. I think this strategy has had a fair amount of testing with the i915 driver. In any case, I don't think TTM should enforce a different way of shrinking by the means of a severely restricted helper?
Well, as far as I can see that is exactly what TTM should do.
I mean the main advantage to make a common component is to enforce correct behavior.
Regards,
Christian.
/Thomas
