[PATCH bpf v3 3/3] bpf: Cache build IDs in sleepable stackmap path

[Ihor Solodrai <ihor.solodrai@linux.dev>]

Stack traces often contain adjacent IPs from the same VMA or from
different VMAs backed by the same ELF file. Cache the last successfully
parsed build ID together with the resolved VMA range and backing file
so the sleepable build-ID path can avoid repeated VMA locking and file
parsing in common cases.

Suggested-by: Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 kernel/bpf/stackmap.c | 51 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index c1e96df360c3..318ce9ed0dd5 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -226,13 +226,34 @@ static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *i
 		.vma = NULL,
 		.mm = mm,
 	};
-	unsigned long vm_pgoff, vm_start;
+	struct {
+		struct file *file;
+		const char *build_id;
+		unsigned long vm_start;
+		unsigned long vm_end;
+		unsigned long vm_pgoff;
+	} cache = {};
+	unsigned long vm_pgoff, vm_start, vm_end;
 	struct vm_area_struct *vma;
 	struct file *file;
 	u64 ip;
 
 	for (u32 i = 0; i < trace_nr; i++) {
 		ip = READ_ONCE(id_offs[i].ip);
+
+		/*
+		 * Range cache fast path: if ip falls within the previously
+		 * resolved VMA range, reuse the cache build_id without
+		 * re-acquiring the VMA lock.
+		 */
+		if (cache.build_id && ip >= cache.vm_start && ip < cache.vm_end) {
+			memcpy(id_offs[i].build_id, cache.build_id, BUILD_ID_SIZE_MAX);
+			vm_start = cache.vm_start;
+			vm_end = cache.vm_end;
+			vm_pgoff = cache.vm_pgoff;
+			goto build_id_valid;
+		}
+
 		vma = stack_map_lock_vma(&lock, ip);
 		if (!vma || !vma->vm_file) {
 			stack_map_build_id_set_ip(&id_offs[i]);
@@ -240,9 +261,22 @@ static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *i
 			continue;
 		}
 
-		file = get_file(vma->vm_file);
+		file = vma->vm_file;
 		vm_pgoff = vma->vm_pgoff;
 		vm_start = vma->vm_start;
+		vm_end = vma->vm_end;
+
+		if (file == cache.file) {
+			/*
+			 * Same backing file as previous (e.g. different VMAs
+			 * of the same ELF binary). Reuse the cache build_id.
+			 */
+			memcpy(id_offs[i].build_id, cache.build_id, BUILD_ID_SIZE_MAX);
+			stack_map_unlock_vma(&lock);
+			goto build_id_valid;
+		}
+
+		file = get_file(file);
 		stack_map_unlock_vma(&lock);
 
 		/* build_id_parse_file() may block on filesystem reads */
@@ -251,11 +285,22 @@ static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *i
 			fput(file);
 			continue;
 		}
-		fput(file);
 
+		if (cache.file)
+			fput(cache.file);
+		cache.file = file;
+		cache.build_id = id_offs[i].build_id;
+
+build_id_valid:
+		cache.vm_start = vm_start;
+		cache.vm_end = vm_end;
+		cache.vm_pgoff = vm_pgoff;
 		id_offs[i].offset = (vm_pgoff << PAGE_SHIFT) + ip - vm_start;
 		id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
 	}
+
+	if (cache.file)
+		fput(cache.file);
 }
 
 /*
-- 
2.54.0