On Thu, May 20, 2021 at 02:38:55PM +0200, Daniel Bristot de Oliveira wrote: > On 5/20/21 12:33 PM, Quentin Perret wrote: > > On Thursday 20 May 2021 at 11:16:41 (+0100), Will Deacon wrote: > >> Ok, thanks for the insight. In which case, I'll go with what we discussed: > >> require admission control to be disabled for sched_setattr() but allow > >> execve() to a 32-bit task from a 64-bit deadline task with a warning (this > >> is probably similar to CPU hotplug?). > > > > Still not sure that we can let execve go through ... It will break AC > > all the same, so it should probably fail as well if AC is on IMO > > > > If the cpumask of the 32-bit task is != of the 64-bit task that is executing it, > the admission control needs to be re-executed, and it could fail. So I see this > operation equivalent to sched_setaffinity(). This will likely be true for future > schedulers that will allow arbitrary affinities (AC should run on affinity > change, and could fail). > > I would vote with Juri: "I'd go with fail hard if AC is on, let it > pass if AC is off (supposedly the user knows what to do)," (also hope nobody > complains until we add better support for affinity, and use this as a motivation > to get back on this front). I can have a go at implementing it, but I don't think it's a great solution and here's why: Failing an execve() is _very_ likely to be fatal to the application. It's also very likely that the task calling execve() doesn't know whether the program it's trying to execute is 32-bit or not. Consequently, if we go with failing execve() then all that will happen is that people will disable admission control altogether. That has a negative impact on "pure" 64-bit applications and so I think we end up with the tail wagging the dog because admission control will be disabled for everybody just because there is a handful of 32-bit programs which may get executed. I understand that it also means that RT throttling would be disabled. Allowing the execve() to continue with a warning is very similar to the case in which all the 64-bit CPUs are hot-unplugged at the point of execve(), and this is much closer to the illusion that this patch series intends to provide. So, personally speaking, I would prefer the behaviour where we refuse to admit 32-bit tasks vioa sched_set_attr() if the root domain contains 64-bit CPUs, but we _don't_ fail execve() of a 32-bit program from a 64-bit deadline task. However, you're the deadline experts so ultimately I'll implement what you prefer. I just wanted to explain why I think it's a poor interface. Have I changed anybody's mind? Will
