linux-kernel - [PATCH v2 7/7] bpf: rqspinlock: add rqspinlock policy handler for arm64

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Message-Id: <20250502085223.1316925-8-ankur.a.arora@oracle.com>
Date: Fri,  2 May 2025 01:52:23 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Cc: arnd@...db.de, catalin.marinas@....com, will@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
        harisokn@...zon.com, cl@...two.org, ast@...nel.org, memxor@...il.com,
        zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: [PATCH v2 7/7] bpf: rqspinlock: add rqspinlock policy handler for arm64

The local copy of smp_cond_load_acquire_timewait() (from [1]) is only
usable for rqspinlock timeout and deadlock checking in a degenerate
fashion by overloading the evaluation of the condvar.

Update smp_cond_load_acquire_timewait(). Move the timeout and deadlock
handlng (partially stubbed) to the wait policy handler.

[1] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com

Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---

Note: This patch is missing all the important bits. Just wanted to check
if the interface is workable before threshing plugging in the deadlock
checking etc.

 arch/arm64/include/asm/rqspinlock.h | 96 ++++++-----------------------
 1 file changed, 19 insertions(+), 77 deletions(-)

diff --git a/arch/arm64/include/asm/rqspinlock.h b/arch/arm64/include/asm/rqspinlock.h
index 9ea0a74e5892..27138b591e31 100644
--- a/arch/arm64/include/asm/rqspinlock.h
+++ b/arch/arm64/include/asm/rqspinlock.h
@@ -4,89 +4,31 @@
 
 #include <asm/barrier.h>
 
-/*
- * Hardcode res_smp_cond_load_acquire implementations for arm64 to a custom
- * version based on [0]. In rqspinlock code, our conditional expression involves
- * checking the value _and_ additionally a timeout. However, on arm64, the
- * WFE-based implementation may never spin again if no stores occur to the
- * locked byte in the lock word. As such, we may be stuck forever if
- * event-stream based unblocking is not available on the platform for WFE spin
- * loops (arch_timer_evtstrm_available).
- *
- * Once support for smp_cond_load_acquire_timewait [0] lands, we can drop this
- * copy-paste.
- *
- * While we rely on the implementation to amortize the cost of sampling
- * cond_expr for us, it will not happen when event stream support is
- * unavailable, time_expr check is amortized. This is not the common case, and
- * it would be difficult to fit our logic in the time_expr_ns >= time_limit_ns
- * comparison, hence just let it be. In case of event-stream, the loop is woken
- * up at microsecond granularity.
- *
- * [0]: https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com
- */
+#define RES_DEF_SPIN_COUNT	(32 * 1024)
 
-#ifndef smp_cond_load_acquire_timewait
-
-#define smp_cond_time_check_count	200
-
-#define __smp_cond_load_relaxed_spinwait(ptr, cond_expr, time_expr_ns,	\
-					 time_limit_ns) ({		\
-	typeof(ptr) __PTR = (ptr);					\
-	__unqual_scalar_typeof(*ptr) VAL;				\
-	unsigned int __count = 0;					\
-	for (;;) {							\
-		VAL = READ_ONCE(*__PTR);				\
-		if (cond_expr)						\
-			break;						\
-		cpu_relax();						\
-		if (__count++ < smp_cond_time_check_count)		\
-			continue;					\
-		if ((time_expr_ns) >= (time_limit_ns))			\
-			break;						\
-		__count = 0;						\
-	}								\
-	(typeof(*ptr))VAL;						\
-})
-
-#define __smp_cond_load_acquire_timewait(ptr, cond_expr,		\
-					 time_expr_ns, time_limit_ns)	\
-({									\
-	typeof(ptr) __PTR = (ptr);					\
-	__unqual_scalar_typeof(*ptr) VAL;				\
-	for (;;) {							\
-		VAL = smp_load_acquire(__PTR);				\
-		if (cond_expr)						\
-			break;						\
-		__cmpwait_relaxed(__PTR, VAL);				\
-		if ((time_expr_ns) >= (time_limit_ns))			\
-			break;						\
-	}								\
-	(typeof(*ptr))VAL;						\
-})
-
-#define smp_cond_load_acquire_timewait(ptr, cond_expr,			\
-				      time_expr_ns, time_limit_ns)	\
-({									\
-	__unqual_scalar_typeof(*ptr) _val;				\
-	int __wfe = arch_timer_evtstrm_available();			\
+#define rqspinlock_cond_timewait(now, prev, end, spin, wait) ({		\
+	bool __ev = arch_timer_evtstrm_available();			\
+	bool __wfet = alternative_has_cap_unlikely(ARM64_HAS_WFXT);	\
+	u64 __ret;							\
 									\
-	if (likely(__wfe)) {						\
-		_val = __smp_cond_load_acquire_timewait(ptr, cond_expr,	\
-							time_expr_ns,	\
-							time_limit_ns);	\
+	*wait = false;							\
+	/* TODO Handle deadlock check. */				\
+	if (end >= now) {						\
+		__ret = 0;						\
 	} else {							\
-		_val = __smp_cond_load_relaxed_spinwait(ptr, cond_expr,	\
-							time_expr_ns,	\
-							time_limit_ns);	\
-		smp_acquire__after_ctrl_dep();				\
+		if (__ev || __wfet)					\
+			*wait = true;					\
+		else							\
+			*spin = RES_DEF_SPIN_COUNT;			\
+		__ret = now;						\
 	}								\
-	(typeof(*ptr))_val;						\
+									\
+	__ret;								\
 })
 
-#endif
-
-#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire_timewait(v, c, 0, 1)
+#define res_smp_cond_load_acquire(v, c)					\
+	smp_cond_load_acquire_timewait(v, c, rqspinlock_cond_timewait,	\
+				       ktime_get_mono_fast_ns(), (u64)RES_DEF_TIMEOUT)
 
 #include <asm-generic/rqspinlock.h>
 
-- 
2.43.5