On 20/05/2021 20:03, Will Deacon wrote:

Hi Dietmar,

On Thu, May 20, 2021 at 07:55:27PM +0200, Dietmar Eggemann wrote:

quoted

On 20/05/2021 18:00, Daniel Bristot de Oliveira wrote:

quoted

On 5/20/21 5:06 PM, Dietmar Eggemann wrote:

quoted

(1) # chrt -d -T 5000000 -P 16666666 0 ./32bit_app

(2) # ./32bit_app &

    # chrt -d -T 5000000 -P 16666666 -p 0 pid_of(32bit_app)


Wouldn't the behaviour of (1) and (2) be different w/o this patch?

In (1) __sched_setscheduler() happens before execve so it operates on
p->cpus_ptr equal span.

In (2) span != p->cpus_ptr so DL AC will fail.

As far as I got, the case (1) would be spitted in two steps:

 - __sched_setscheduler() will work, then
 - execv() would fail because (span != p->cpus_ptr)

So... at the end, both (1) and (2) would result in a failure...

am I missing something?

Not sure. Reading this thread I was under the assumption that the only
change would be the drop of this patch. But I assume there is also this
'if DL AC is on then let sched_setattr() fail for this 32bit task'.

IMHO, the current patch-stack w/o this patch should let (1) succeed with
DL AC.

That's what I'm proposing, yes, but others (including Daniel) prefer to
fail the execve(). See my other reply just now for a summary [1].

[...]

Thanks, Will ... Now I understand. Or at least I think I do ;-)

[1] https://lore.kernel.org/lkml/20210520180138.GA10523@willie-the-truck/T/#u (local)