[Dropping Li Zefan as his mail is bouncing]

On Tue, May 18, 2021 at 10:20:38AM +0000, Quentin Perret wrote:

On Tuesday 18 May 2021 at 10:47:17 (+0100), Will Deacon wrote:

quoted

On asymmetric systems where the affinity of a task is restricted to
contain only the CPUs capable of running it, admission to the deadline
scheduler is likely to fail because the span of the sched domain
contains incompatible CPUs. Although this is arguably the right thing to
do, it is inconsistent with the case where the affinity of a task is
restricted after already having been admitted to the deadline scheduler.

For example, on an arm64 system where not all CPUs support 32-bit
applications, a 64-bit deadline task can exec() a 32-bit image and have
its affinity forcefully restricted.

So I guess the alternative would be to fail exec-ing into 32bit from a
64bit DL task, and then drop this patch?

The nice thing about your approach is that existing applications won't
really notice a difference (execve would still 'work'), but on the cons
side it breaks admission control, which is sad.

Right, with your suggestion here we would forbid any 32-bit deadline tasks
on an asymmetric system, even if you'd gone to the extraordinary effort
to cater for that (e.g. by having a separate root domain).

I don't expect this weird execve-to-32bit pattern from DL to be that
common in practice (at the very least not in Android), so maybe we could
start with the stricter version (fail the execve), and wait to see if
folks complain? Making things stricter later will be harder.

Thoughts?

I don't have strong opinions on this, but I _do_ want the admission via
sched_setattr() to be consistent with execve(). What you're suggesting
ticks that box, but how many applications are prepared to handle a failed
execve()? I suspect it will be fatal.

Probably also worth pointing out that the approach here will at least
warn in the execve() case when the affinity is overridden for a deadline
task.

Will

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