From: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will@kernel.org>
Cc: linux-arch@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Qais Yousef <qais.yousef@arm.com>, Marc Zyngier <maz@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Morten Rasmussen <morten.rasmussen@arm.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu
Date: Fri, 9 Oct 2020 10:33:41 +0100 [thread overview]
Message-ID: <20201009093340.GC23638@gaia> (raw)
In-Reply-To: <20201009083146.GA29594@willie-the-truck>
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
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next prev parent reply other threads:[~2020-10-09 9:35 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-08 18:16 [RFC PATCH 0/3] Add support for Asymmetric AArch32 systems Qais Yousef
2020-10-08 18:16 ` [RFC PATCH 1/3] arm64: kvm: Handle " Qais Yousef
2020-10-09 8:12 ` Marc Zyngier
2020-10-09 9:58 ` Qais Yousef
2020-10-09 12:34 ` Marc Zyngier
2020-10-09 12:48 ` Qais Yousef
2020-10-12 15:32 ` James Morse
2020-10-13 10:32 ` Marc Zyngier
2020-10-13 11:51 ` James Morse
2020-10-13 11:59 ` Qais Yousef
2020-10-13 12:09 ` Marc Zyngier
2020-10-13 12:16 ` Qais Yousef
2020-10-08 18:16 ` [RFC PATCH 2/3] arm64: Add support for asymmetric AArch32 EL0 configurations Qais Yousef
2020-10-08 18:22 ` Randy Dunlap
2020-10-12 10:22 ` Qais Yousef
2020-10-09 6:13 ` Greg Kroah-Hartman
2020-10-09 8:40 ` Will Deacon
2020-10-09 8:50 ` Catalin Marinas
2020-10-09 9:39 ` Catalin Marinas
2020-10-12 12:46 ` Qais Yousef
2020-10-08 18:16 ` [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu Qais Yousef
2020-10-09 7:29 ` Peter Zijlstra
2020-10-09 8:13 ` Morten Rasmussen
2020-10-09 8:31 ` Will Deacon
2020-10-09 8:50 ` Morten Rasmussen
2020-10-09 9:33 ` Catalin Marinas [this message]
2020-10-09 9:42 ` Greg Kroah-Hartman
2020-10-09 11:31 ` Qais Yousef
2020-10-09 12:40 ` Catalin Marinas
2020-10-13 14:23 ` Qais Yousef
2020-10-09 9:25 ` Peter Zijlstra
2020-10-09 9:39 ` Qais Yousef
2020-10-09 9:51 ` Catalin Marinas
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