Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Oct 09, 2020 at 09:31:47AM +0100, Will Deacon wrote:
> On Fri, Oct 09, 2020 at 10:13:12AM +0200, Morten Rasmussen wrote:
> > On Fri, Oct 09, 2020 at 09:29:43AM +0200, Peter Zijlstra wrote:
> > > On Thu, Oct 08, 2020 at 07:16:41PM +0100, Qais Yousef wrote:
> > > > diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> > > > index cf94cc248fbe..7e97f1589f33 100644
> > > > --- a/arch/arm64/kernel/signal.c
> > > > +++ b/arch/arm64/kernel/signal.c
> > > > @@ -908,13 +908,28 @@ static void do_signal(struct pt_regs *regs)
> > > >  	restore_saved_sigmask();
> > > >  }
> > > >  
> > > > +static void set_32bit_cpus_allowed(void)
> > > >  {
> > > > +	cpumask_var_t cpus_allowed;
> > > > +	int ret = 0;
> > > > +
> > > > +	if (cpumask_subset(current->cpus_ptr, &aarch32_el0_mask))
> > > > +		return;
> > > > +
> > > >  	/*
> > > > +	 * On asym aarch32 systems, if the task has invalid cpus in its mask,
> > > > +	 * we try to fix it by removing the invalid ones.
> > > >  	 */
> > > > +	if (!alloc_cpumask_var(&cpus_allowed, GFP_ATOMIC)) {
> > > > +		ret = -ENOMEM;
> > > > +	} else {
> > > > +		cpumask_and(cpus_allowed, current->cpus_ptr, &aarch32_el0_mask);
> > > > +		ret = set_cpus_allowed_ptr(current, cpus_allowed);
> > > > +		free_cpumask_var(cpus_allowed);
> > > > +	}
> > > > +
> > > > +	if (ret) {
> > > > +		pr_warn_once("Failed to fixup affinity of running 32-bit task\n");
> > > >  		force_sig(SIGKILL);
> > > >  	}
> > > >  }
> > > 
> > > Yeah, no. Not going to happen.
> > > 
> > > Fundamentally, you're not supposed to change the userspace provided
> > > affinity mask. If we want to do something like this, we'll have to teach
> > > the scheduler about this second mask such that it can compute an
> > > effective mask as the intersection between the 'feature' and user mask.
> > 
> > I agree that we shouldn't mess wit the user-space mask directly. Would it
> > be unthinkable to go down the route of maintaining a new mask which is
> > the intersection of the feature mask (controlled and updated by arch
> > code) and the user-space mask?
> > 
> > It shouldn't add overhead in the scheduler as it would use the
> > intersection mask instead of the user-space mask, the main complexity
> > would be around making sure the intersection mask is updated correctly
> > (cpusets, hotplug, ...).
> > 
> > Like the above tweak, this won't help if the intersection mask is empty,
> > task will still get killed but it will allow tasks to survive
> > user-space masks including some non-compatible CPUs. If we want to
> > prevent task killing in all cases (ignoring hotplug) it gets more ugly
> > as we would have to ignore the user-space mask in some cases.
> 
> Honestly, I don't understand why we're trying to hide this asymmetry from
> userspace by playing games with affinity masks in the kernel. Userspace
> is likely to want to move things about _anyway_ because even amongst the
> 32-bit capable cores, you may well have different clock frequencies to
> contend with.
> 
> So I'd be *much* happier to let the schesduler do its thing, and if one
> of these 32-bit tasks ends up on a core that can't deal with it, then
> tough, it gets killed. Give userspace the information it needs to avoid
> that happening in the first place, rather than implicitly limit the mask.
> 
> That way, the kernel support really boils down to two parts:
> 
>   1. Remove the sanity checks we have to prevent 32-bit applications running
>      on asymmetric systems
> 
>   2. Tell userspace about the problem

This works for me as well as long as it is default off with a knob to
turn it on. I'd prefer a sysctl (which can be driven from the command
line in recent kernels IIRC) so that one can play with it a run-time.
This way it's also a userspace choice and not an admin or whoever
controls the cmdline (well, that's rather theoretical since the target
is Android).

-- 
Catalin