linux-kernel - [RFC PATCH v2 2/9] riscv: mm: Apply a threshold to the number of active ASIDs on each CPU

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Message-ID: <20251127141117.87420-3-luxu.kernel@bytedance.com>
Date: Thu, 27 Nov 2025 22:11:10 +0800
From: Xu Lu <luxu.kernel@...edance.com>
To: pjw@...nel.org,
	palmer@...belt.com,
	aou@...s.berkeley.edu,
	alex@...ti.fr,
	kees@...nel.org,
	mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	vincent.guittot@...aro.org,
	akpm@...ux-foundation.org,
	david@...hat.com,
	apatel@...tanamicro.com,
	guoren@...nel.org
Cc: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Xu Lu <luxu.kernel@...edance.com>
Subject: [RFC PATCH v2 2/9] riscv: mm: Apply a threshold to the number of active ASIDs on each CPU

Since each CPU has limited TLB entries, there exist limited active ASIDs
in each CPU's TLB at the same time. Thus we apply a threshold here. When
a mm_struct is loaded, we mark its ASID as active. If the number of
active ASIDs exceeds the threshold, we evict the mm_struct that has not
been used for the longest time, flush its TLB entries, mark its ASID
inactive, and clear current CPU in its mm_cpumask.

Signed-off-by: Xu Lu <luxu.kernel@...edance.com>
---
 arch/riscv/include/asm/tlbflush.h | 27 +++++++++++++
 arch/riscv/mm/context.c           |  1 +
 arch/riscv/mm/tlbflush.c          | 66 +++++++++++++++++++++++++++++++
 3 files changed, 94 insertions(+)

diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
index eed0abc405143..3f83fd5ef36db 100644
--- a/arch/riscv/include/asm/tlbflush.h
+++ b/arch/riscv/include/asm/tlbflush.h
@@ -66,6 +66,33 @@ void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
 void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
 
 extern unsigned long tlb_flush_all_threshold;
+
+#ifdef CONFIG_RISCV_LAZY_TLB_FLUSH
+
+#define MAX_LOADED_MM					6
+
+struct tlb_context {
+	struct mm_struct *mm;
+	unsigned int gen;
+};
+
+struct tlb_info {
+	rwlock_t rwlock;
+	struct mm_struct *active_mm;
+	unsigned int next_gen;
+	struct tlb_context contexts[MAX_LOADED_MM];
+};
+
+DECLARE_PER_CPU_SHARED_ALIGNED(struct tlb_info, tlbinfo);
+
+void local_load_tlb_mm(struct mm_struct *mm);
+
+#else /* CONFIG_RISCV_LAZY_TLB_FLUSH */
+
+static inline void local_load_tlb_mm(struct mm_struct *mm) {}
+
+#endif /* CONFIG_RISCV_LAZY_TLB_FLUSH */
+
 #else /* CONFIG_MMU */
 #define local_flush_tlb_all()			do { } while (0)
 #endif /* CONFIG_MMU */
diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 55c20ad1f7444..a7cf36ad34678 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -217,6 +217,7 @@ static inline void set_mm(struct mm_struct *prev,
 	 */
 	cpumask_set_cpu(cpu, mm_cpumask(next));
 	if (static_branch_unlikely(&use_asid_allocator)) {
+		local_load_tlb_mm(next);
 		set_mm_asid(next, cpu);
 	} else {
 		cpumask_clear_cpu(cpu, mm_cpumask(prev));
diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index 8404530ec00f9..0b1c21c7aafb8 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -103,6 +103,15 @@ struct flush_tlb_range_data {
 	unsigned long stride;
 };
 
+#ifdef CONFIG_RISCV_LAZY_TLB_FLUSH
+DEFINE_PER_CPU_SHARED_ALIGNED(struct tlb_info, tlbinfo) = {
+	.rwlock = __RW_LOCK_UNLOCKED(tlbinfo.rwlock),
+	.active_mm = NULL,
+	.next_gen = 1,
+	.contexts = { { NULL, 0, }, },
+};
+#endif /* CONFIG_RISCV_LAZY_TLB_FLUSH */
+
 static void __ipi_flush_tlb_range_asid(void *info)
 {
 	struct flush_tlb_range_data *d = info;
@@ -240,3 +249,60 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 			  0, FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
 	cpumask_clear(&batch->cpumask);
 }
+
+#ifdef CONFIG_RISCV_LAZY_TLB_FLUSH
+
+static inline unsigned int new_tlb_gen(struct tlb_info *info)
+{
+	unsigned int gen = info->next_gen++;
+	unsigned int i;
+
+	if (unlikely(!info->next_gen)) {
+		for (i = 0; i < MAX_LOADED_MM; i++) {
+			if (info->contexts[i].gen)
+				info->contexts[i].gen = 1;
+		}
+		info->next_gen = 1;
+		gen = info->next_gen++;
+	}
+
+	return gen;
+}
+
+void local_load_tlb_mm(struct mm_struct *mm)
+{
+	struct tlb_info *info = this_cpu_ptr(&tlbinfo);
+	struct tlb_context *contexts = info->contexts;
+	struct mm_struct *victim = NULL;
+	unsigned int i, pos = 0, min = UINT_MAX;
+
+	for (i = 0; i < MAX_LOADED_MM; i++) {
+		if (contexts[i].mm == mm) {
+			pos = i;
+			break;
+		}
+		if (min > contexts[i].gen) {
+			min = contexts[i].gen;
+			pos = i;
+		}
+	}
+
+	write_lock(&info->rwlock);
+
+	info->active_mm = mm;
+
+	if (contexts[pos].mm != mm) {
+		victim = contexts[pos].mm;
+		contexts[pos].mm = mm;
+	}
+	contexts[pos].gen = new_tlb_gen(info);
+
+	write_unlock(&info->rwlock);
+
+	if (victim) {
+		cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(victim));
+		local_flush_tlb_all_asid(get_mm_asid(victim));
+	}
+}
+
+#endif /* CONFIG_RISCV_LAZY_TLB_FLUSH */
-- 
2.20.1