On 20/05/21 19:01, Will Deacon wrote: > 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. Completely understand your perplexity. But how can the kernel still give guarantees to "pure" 64-bit applications if there are 32-bit applications around that essentially broke admission control when they were restricted to a subset of cores? > 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, for hotplug we currently have a check that would make hotplug operations fail if removing a CPU would mean not enough bandwidth to run the currently admitted set of DEADLINE tasks. > 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. OK, this is interesting and I guess a very valid alternative. That would force users to create exclusive domains for 32-bit tasks, right? > 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? Partly! :) Thanks a lot for the discussion so far. Juri
