linux-kernel - Re: [PATCH 09/19] arm64: Exclude nohz

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Message-ID: <Z3nB0LTdxAp2pPGU@pavilion.home>
Date: Sun, 5 Jan 2025 00:18:40 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 09/19] arm64: Exclude nohz_full CPUs from 32bits el0
 support

Le Fri, Jan 03, 2025 at 03:20:44PM +0000, Will Deacon a écrit :
> On Wed, Dec 11, 2024 at 04:40:22PM +0100, Frederic Weisbecker wrote:
> > +NOHZ FULL
> > +--------
> > +
> > +Nohz full CPUs are not a desirable fallback target to run 32bits el0
> > +applications. If present, a set of housekeeping CPUs that can do
> > +the job instead is preferred. Otherwise 32-bit EL0 is not supported.
> > +Should the need arise, appropriate support can be introduced in the
> > +future.
> 
> I think we can probably word this slightly better. How about something
> more along these lines?
> 
>   To avoid perturbing an adaptive-ticks CPU (specified using
>   ``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
>   are treated as 64-bit-only when support for asymmetric 32-bit systems
>   is enabled.

Indeed!

> 
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 6ce71f444ed8..7ce1b8ab417f 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -75,6 +75,7 @@
> >  #include <linux/cpu.h>
> >  #include <linux/kasan.h>
> >  #include <linux/percpu.h>
> > +#include <linux/sched/isolation.h>
> >  
> >  #include <asm/cpu.h>
> >  #include <asm/cpufeature.h>
> > @@ -3742,7 +3743,10 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
> >  	static int lucky_winner = -1;
> >  
> >  	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> > -	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> > +	bool cpu_32bit = false;
> > +
> > +	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0) && housekeeping_cpu(cpu, HK_TYPE_TICK))
> > +		cpu_32bit = true;
> 
> I think it would be helpful to emit a diagnostic when a 32-bit CPU is
> ignored because of the housekeeping check. e.g.
> 
> 
> 	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> 		if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> 			pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> 		else
> 			cpu_32bit = true;
> 	}
> 
> 
> It's a bit of a bummer that this will fire on hardware that isn't
> asymmetric, but I suppose that's easily resolved by not passing the
> 'allow_mismatched_32bit_el0' option in that case.

Right! How is the following updated version?

---
From: Frederic Weisbecker <frederic@...nel.org>
Date: Fri, 25 Oct 2024 00:12:59 +0200
Subject: [PATCH 1/2] arm64: Exclude nohz_full CPUs from 32bits el0 support

Nohz full CPUs are not a desirable fallback target to run 32bits el0
applications. If present, prefer a set of housekeeping CPUs that can do
the job instead. Otherwise just don't support el0 32 bits. Should the
need arise, appropriate support can be introduced in the future.

Suggested-by: Will Deacon <will@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 Documentation/arch/arm64/asymmetric-32bit.rst |  8 ++++++++
 arch/arm64/kernel/cpufeature.c                | 10 +++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
index 64a0b505da7d..1ca2b359a907 100644
--- a/Documentation/arch/arm64/asymmetric-32bit.rst
+++ b/Documentation/arch/arm64/asymmetric-32bit.rst
@@ -153,3 +153,11 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
 mode will return to host userspace with an ``exit_reason`` of
 ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
 re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
+
+NOHZ FULL
+---------
+
+To avoid perturbing an adaptive-ticks CPU (specified using
+``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
+are treated as 64-bit-only when support for asymmetric 32-bit systems
+is enabled.
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6ce71f444ed8..3c87659c14db 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -75,6 +75,7 @@
 #include <linux/cpu.h>
 #include <linux/kasan.h>
 #include <linux/percpu.h>
+#include <linux/sched/isolation.h>
 
 #include <asm/cpu.h>
 #include <asm/cpufeature.h>
@@ -3742,7 +3743,14 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
 	static int lucky_winner = -1;
 
 	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
-	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
+	bool cpu_32bit = false;
+
+	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
+		if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
+			pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
+		else
+			cpu_32bit = true;
+	}
 
 	if (cpu_32bit) {
 		cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
-- 
2.46.0