On 24-Jan 16:31, Peter Zijlstra wrote: > On Thu, Jan 24, 2019 at 12:30:09PM +0000, Patrick Bellasi wrote: > > > So I'll have to go over the code again, but I'm wondering why you're > > > changing uclamp_se::bucket_id on a runnable task. > > > > We change only the "requested" value, not the "effective" one. > > > > > Ideally you keep bucket_id invariant between enqueue and dequeue; then > > > dequeue knows where we put it. > > > > Right, that's what we do for the "effective" value. > > So the problem I have is that you first introduce uclamp_se::value and > use that all over the code, and then introduce effective and change all > the usage sites. Right, because the moment we introduce the combination/aggregation mechanism is the moment "effective" value makes sense to have. That's when the code show that: a task cannot always get what it "request", an "effective" value is computed by aggregation and that's the value we use now for actual clamp enforcing. > That seems daft. Why not keep all the code as-is and add orig_value. If you prefer, I can use effective values since the beginning and then add the "requested" values later... but I fear the patchset will not be more clear to parse. > > > Now I suppose actually determining bucket_id is 'expensive' (it > > > certainly is with the whole mapping scheme, but even that integer > > > division is not nice), so we'd like to precompute the bucket_id. > > > > Yes, although the complexity is mostly in the composition logic > > described above not on mapping at all. We have "mapping" overheads > > only when we change a "request" value and that's from slow-paths. > > It's weird though. Esp. when combined with that mapping logic, because > then you get to use additional maps that are not in fact ever used. Mmm... don't get this point... AFAICS "mapping" and "effective" are two different concepts, that's why I can probably get rid of the first by I would prefer to keep the second. > > > We can update uclamp_se::value and set uclamp_se::changed, and then the > > > next enqueue will (unlikely) test-and-clear changed and recompute the > > > bucket_id. > > > > This mean will lazy update the "requested" bucket_id by deferring its > > computation at enqueue time. Which saves us a copy of the bucket_id, > > i.e. we will have only the "effective" value updated at enqueue time. > > > > But... > > > > > Would that not be simpler? > > > > ... although being simpler it does not fully exploit the slow-path, > > a syscall which is usually running from a different process context > > (system management software). > > > > It also fits better for lazy updates but, in the cgroup case, where we > > wanna enforce an update ASAP for RUNNABLE tasks, we will still have to > > do the updates from the slow-path. > > > > Will look better into this simplification while working on v7, perhaps > > the linear mapping can really help in that too. > > OK. So mostly my complaint is that it seems to do things odd for ill > explained reasons. :( I'm really sorry I'm not able to convey the overall design idea. TBH however, despite being a quite "limited" feature it has many different viewpoints: task-specific, cgroups and system defaults. I've really tried my best to come up with something reasonable but I understand that, looking at the single patches, the overall design could be difficult to grasp... without considering that optimizations are always possible of course. If you like better, I can try to give a respin by just removing the mapping part and then we go back and see if the reaming bits makes more sense ? -- #include <best/regards.h> Patrick Bellasi
