On Fri, May 21, 2021 at 03:00:42PM +0200, Daniel Bristot de Oliveira wrote: > On 5/21/21 12:37 PM, Will Deacon wrote: > > Interesting, thanks. Thinking about this some more, it strikes me that with > > these silly asymmetric systems there could be an interesting additional > > problem with hotplug and deadline tasks. Imagine the following sequence of > > events: > > > > 1. All online CPUs are 32-bit-capable > > 2. sched_setattr() admits a 32-bit deadline task > > 3. A 64-bit-only CPU is onlined > > At the point 3, the global scheduler assumption is broken. For instance, in a > system with four CPUs and five ready 32-bit-capable tasks, when the fifth CPU as > added, the working conserving rule is violated because the five highest priority > thread are not running (only four are) :-(. > > So, at this point, for us to keep to the current behavior, the addition should > be.. blocked? :-(( > > > 4. Some of the 32-bit-capable CPUs are offlined > > Assuming that point 3 does not exist (i.e., all CPUs are 32-bit-capable). At > this point, we will have an increase in the pressure on the 32-bit-capable CPUs. > > This can also create bad effects for 64-bit tasks, as the "contended" 32-bit > tasks will still be "queued" in a future time where they were supposed to be > done (leaving time for the 64-bit tasks). That's a really interesting point that I hadn't previously considered. It means that the effects of 32-bit tasks with forced affinity are far reaching when it comes to deadline tasks. > > I wonder if we can get into a situation where we think we have enough > > bandwidth available, but in reality the 32-bit task is in trouble because > > it can't make use of the 64-bit-only CPU. > > I would have to think more, but there might be a case where this contended > 32-bit tasks could cause deadline misses for the 64-bit too. > > > If so, then it seems to me that admission control is really just > > "best-effort" for 32-bit deadline tasks on these systems because it's based > > on a snapshot in time of the available resources. > > The admission test as is now is "best-effort" in the sense that it allows a > workload higher than it could handle (it is necessary, but not sufficient AC). > But it should not be considered "best-effort" because of violations in the > working conserving property as a result of arbitrary affinities among tasks. > Overall, we have been trying to close any "exception left" to this later case. > > I know, it is a complex situation, I am just trying to illustrate our concerns, > because, in the near future we might have a scheduler that handles arbitrary > affinity correctly. But that might require us to stick to an AC. The AC is > something precious for us. I've implemented AC on execve() of a 32-bit program so we'll fail that system call with -ENOEXEC if the root domain contains 64-bit-only CPUs. As expected, the failure mode is awful because it seems as though the ELF binary is then treated like a shell script by userspace and passed to /bin/sh: $ sudo chrt -d -T 5000000 -P 16666666 0 ./hello32 ./hello32: 1: Syntax error: word unexpected (expecting ")") Anyway, I'll include this in v7. Cheers, Will
