linux-kernel - [PATCH v3 5/5] arm64: barrier: Handle waiting in smp_cond_load_relaxed

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Message-Id: <20250627044805.945491-6-ankur.a.arora@oracle.com>
Date: Thu, 26 Jun 2025 21:48:05 -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 v3 5/5] arm64: barrier: Handle waiting in smp_cond_load_relaxed_timewait()

smp_cond_load_{relaxed,acquire}_timewait() wait on a condition variable
until a timeout expires. This waiting is some mix of spinning while
dereferencing an address, and waiting in a WFE for a store event or
periodic events from the event-stream.

Handle the waiting part of the policy in ___smp_cond_timewait() while
offloading the spinning to the generic ___smp_cond_spinwait().

To minimize time spent spinning when the user can tolerate a large
overshoot, choose SMP_TIMEWAIT_DEFAULT_US to be the event-stream
period.

This would result in a worst case delay of ARCH_TIMER_EVT_STREAM_PERIOD_US.

Cc: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 arch/arm64/include/asm/barrier.h | 48 ++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 7c56e2621c7d..a1367f2901f0 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -10,6 +10,7 @@
 #ifndef __ASSEMBLY__
 
 #include <linux/kasan-checks.h>
+#include <linux/minmax.h>
 
 #include <asm/alternative-macros.h>
 
@@ -222,6 +223,53 @@ do {									\
 #define __smp_timewait_store(ptr, val)					\
 		__cmpwait_relaxed(ptr, val)
 
+/*
+ * Redefine ARCH_TIMER_EVT_STREAM_PERIOD_US locally to avoid include hell.
+ */
+#define __ARCH_TIMER_EVT_STREAM_PERIOD_US 100UL
+extern bool arch_timer_evtstrm_available(void);
+
+static inline u64 ___smp_cond_spinwait(u64 now, u64 prev, u64 end,
+				       u32 *spin, bool *wait, u64 slack);
+/*
+ * To minimize time spent spinning, we want to allow a large overshoot.
+ * So, choose a default slack value of the event-stream period.
+ */
+#define SMP_TIMEWAIT_DEFAULT_US __ARCH_TIMER_EVT_STREAM_PERIOD_US
+
+static inline u64 ___smp_cond_timewait(u64 now, u64 prev, u64 end,
+				       u32 *spin, bool *wait, u64 slack)
+{
+	bool wfet = alternative_has_cap_unlikely(ARM64_HAS_WFXT);
+	bool wfe, ev = arch_timer_evtstrm_available();
+	u64 evt_period = __ARCH_TIMER_EVT_STREAM_PERIOD_US;
+	u64 remaining = end - now;
+
+	if (now >= end)
+		return 0;
+	/*
+	 * Use WFE if there's enough slack to get an event-stream wakeup even
+	 * if we don't come out of the WFE due to natural causes.
+	 */
+	wfe = ev && ((remaining + slack) > evt_period);
+
+	if (wfe || wfet) {
+		*wait = true;
+		*spin = 0;
+		return now;
+	}
+
+	/*
+	 * The time remaining is shorter than our wait granularity. Let
+	 * the generic spinwait policy determine how to spin.
+	 */
+	return ___smp_cond_spinwait(now, prev, end, spin, wait, slack);
+}
+
+#ifndef __smp_cond_policy
+#define __smp_cond_policy ___smp_cond_timewait
+#endif
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLY__ */
-- 
2.43.5