linux-kernel - Re: [PATCH 0/4] memcg: nmi-safe kmem charging

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Message-ID: <20250514071144.GB24938@noisy.programming.kicks-ass.net>
Date: Wed, 14 May 2025 09:11:44 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Alexei Starovoitov <ast@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	npiggin@...il.com
Subject: Re: [PATCH 0/4] memcg: nmi-safe kmem charging

On Tue, May 13, 2025 at 03:17:00PM -0700, Shakeel Butt wrote:

> > IIRC Power64 has issues here, 'funnily' their local_t is NMI safe.
> > Perhaps we could do the same for their this_cpu_*(), but ideally someone
> > with actual power hardware should do this ;-)
> > 
> 
> Is CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS the right config to
> differentiate between such archs? I see Power64 does not have that
> enabled.

> > There is no config symbol for this presently.
> 
> Hmm what about CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS?

Hmm, I didn't know about that one, and it escaped my grep yesterday.

Anyway, PPC is fixable, just not sure its worth it for them.

> > 
> > > - (if the above leaves any 64bit arch) its 64bit atomics implementation is safe
> > 
> > True, only because HPPA does not in fact have NMIs.
> 
> What is HPPA?

arch/parisc the worst 64bit arch ever.

They saw sparc32-smp and thought that was a great idea, or something
along those lines. Both are quite terrible. Sparc64 realized the mistake
and fixed it -- it has cmpxchg.


Nick, is this something that's useful for you guys?

---
diff --git a/arch/powerpc/include/asm/percpu.h b/arch/powerpc/include/asm/percpu.h
index ecf5ac70cfae..aa188db68ef5 100644
--- a/arch/powerpc/include/asm/percpu.h
+++ b/arch/powerpc/include/asm/percpu.h
@@ -25,6 +25,11 @@ DECLARE_STATIC_KEY_FALSE(__percpu_first_chunk_is_paged);
 #define percpu_first_chunk_is_paged	false
 #endif
 
+#ifdef CONFIG_PPC_BOOK3S_64
+#define __pcpu_local_irq_save(f) powerpc_local_irq_pmu_save(f)
+#define __pcpu_local_irq_restore(s) powerpc_local_irq_pmu_restore(f)
+#endif
+
 #include <asm-generic/percpu.h>
 
 #include <asm/paca.h>
diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
index 02aeca21479a..5c8376588dfb 100644
--- a/include/asm-generic/percpu.h
+++ b/include/asm-generic/percpu.h
@@ -75,6 +75,11 @@ extern void setup_per_cpu_areas(void);
 #define PER_CPU_ATTRIBUTES
 #endif
 
+#ifndef __pcpu_local_irq_save
+#define __pcpu_local_irq_save(x) raw_local_irq_save(x)
+#define __pcpu_local_irq_restore(x) raw_local_irq_restore(x)
+#endif
+
 #define raw_cpu_generic_read(pcp)					\
 ({									\
 	*raw_cpu_ptr(&(pcp));						\
@@ -146,9 +151,9 @@ do {									\
 ({									\
 	TYPEOF_UNQUAL(pcp) ___ret;					\
 	unsigned long ___flags;						\
-	raw_local_irq_save(___flags);					\
+	__pcpu_local_irq_save(___flags);					\
 	___ret = raw_cpu_generic_read(pcp);				\
-	raw_local_irq_restore(___flags);				\
+	__pcpu_local_irq_restore(___flags);				\
 	___ret;								\
 })
 
@@ -165,9 +170,9 @@ do {									\
 #define this_cpu_generic_to_op(pcp, val, op)				\
 do {									\
 	unsigned long __flags;						\
-	raw_local_irq_save(__flags);					\
+	__pcpu_local_irq_save(__flags);					\
 	raw_cpu_generic_to_op(pcp, val, op);				\
-	raw_local_irq_restore(__flags);					\
+	__pcpu_local_irq_restore(__flags);					\
 } while (0)
 
 
@@ -175,9 +180,9 @@ do {									\
 ({									\
 	TYPEOF_UNQUAL(pcp) __ret;					\
 	unsigned long __flags;						\
-	raw_local_irq_save(__flags);					\
+	__pcpu_local_irq_save(__flags);					\
 	__ret = raw_cpu_generic_add_return(pcp, val);			\
-	raw_local_irq_restore(__flags);					\
+	__pcpu_local_irq_restore(__flags);					\
 	__ret;								\
 })
 
@@ -185,9 +190,9 @@ do {									\
 ({									\
 	TYPEOF_UNQUAL(pcp) __ret;					\
 	unsigned long __flags;						\
-	raw_local_irq_save(__flags);					\
+	__pcpu_local_irq_save(__flags);					\
 	__ret = raw_cpu_generic_xchg(pcp, nval);			\
-	raw_local_irq_restore(__flags);					\
+	__pcpu_local_irq_restore(__flags);					\
 	__ret;								\
 })
 
@@ -195,9 +200,9 @@ do {									\
 ({									\
 	bool __ret;							\
 	unsigned long __flags;						\
-	raw_local_irq_save(__flags);					\
+	__pcpu_local_irq_save(__flags);					\
 	__ret = raw_cpu_generic_try_cmpxchg(pcp, ovalp, nval);		\
-	raw_local_irq_restore(__flags);					\
+	__pcpu_local_irq_restore(__flags);					\
 	__ret;								\
 })
 
@@ -205,9 +210,9 @@ do {									\
 ({									\
 	TYPEOF_UNQUAL(pcp) __ret;					\
 	unsigned long __flags;						\
-	raw_local_irq_save(__flags);					\
+	__pcpu_local_irq_save(__flags);					\
 	__ret = raw_cpu_generic_cmpxchg(pcp, oval, nval);		\
-	raw_local_irq_restore(__flags);					\
+	__pcpu_local_irq_restore(__flags);					\
 	__ret;								\
 })