[RFC PATCH 2/6] powerpc/trace: Add support for stack tracer
From: Naveen N. Rao
Date: Fri May 21 2021 - 02:49:55 EST
With -mprofile-kernel and ppc32, we call into ftrace at function entry
before the function can establish its own stack frame. This breaks the
ABI since functions are expected to setup a stack frame before calling
into another function. As a consequence of this, when walking the stack,
the ftraced function does not show up in the stack trace.
Fix this by checking for ftrace functions (ftrace_[regs_]call+4) in the
stack trace and looking up the stored nip in pt_regs in its stackframe.
Use the back chain from the stack frame headers to accurately determine
the stack frame sizes, except for the ftraced function on
-mprofile-kernel and ppc32 where we set the frame size to 0.
The max stack tracer ftrace selftest (ftrace/func_stack_tracer.tc)
passes on -mprofile-kernel with this patch.
Before this patch, top of a stack trace with the stack tracer:
Depth Size Location (44 entries)
----- ---- --------
0) 7616 496 ftrace_call+0x4/0x44
1) 7120 64 __mod_lruvec_page_state+0x90/0x110
2) 7056 96 test_clear_page_writeback+0xe4/0x480
3) 6960 48 end_page_writeback+0xa0/0x1c0
4) 6912 256 ext4_finish_bio+0x2c0/0x350
5) 6656 176 ext4_end_bio+0x74/0x280
6) 6480 64 bio_endio+0x1cc/0x240
7) 6416 176 blk_update_request+0x2b8/0x640
8) 6240 64 blk_mq_end_request+0x3c/0x1e0
9) 6176 48 virtblk_request_done+0x48/0xd0
10) 6128 48 blk_complete_reqs+0x80/0xa0
11) 6080 240 __do_softirq+0x150/0x408
12) 5840 32 irq_exit+0x144/0x150
13) 5808 80 do_IRQ+0xc8/0x140
14) 5728 32 hardware_interrupt_common_virt+0x1a4/0x1b0
15) 5696 64 0x0
16) 5632 768 virtqueue_notify+0x40/0x80
17) 4864 240 virtio_queue_rq+0x568/0x610
18) 4624 256 blk_mq_dispatch_rq_list+0x190/0xbc0
19) 4368 160 __blk_mq_do_dispatch_sched+0x1f0/0x3d0
20) 4208 96 __blk_mq_sched_dispatch_requests+0x238/0x2c0
...
After this patch:
Depth Size Location (44 entries)
----- ---- --------
0) 7136 0 rcu_read_unlock_strict+0x8/0x10
1) 7136 64 __mod_lruvec_page_state+0x90/0x110
2) 7072 96 test_clear_page_writeback+0xe4/0x480
3) 6976 48 end_page_writeback+0xa0/0x1c0
4) 6928 256 ext4_finish_bio+0x2c0/0x350
5) 6672 176 ext4_end_bio+0x74/0x280
6) 6496 64 bio_endio+0x1cc/0x240
7) 6432 176 blk_update_request+0x2b8/0x640
8) 6256 64 blk_mq_end_request+0x3c/0x1e0
9) 6192 48 virtblk_request_done+0x48/0xd0
10) 6144 48 blk_complete_reqs+0x80/0xa0
11) 6096 240 __do_softirq+0x150/0x408
12) 5856 32 irq_exit+0x144/0x150
13) 5824 80 do_IRQ+0xc8/0x140
14) 5744 784 hardware_interrupt_common_virt+0x1a4/0x1b0
15) 4960 32 0x0
16) 4928 48 virtqueue_notify+0x40/0x80
17) 4880 240 virtio_queue_rq+0x568/0x610
18) 4640 256 blk_mq_dispatch_rq_list+0x190/0xbc0
19) 4384 160 __blk_mq_do_dispatch_sched+0x1f0/0x3d0
20) 4224 96 __blk_mq_sched_dispatch_requests+0x238/0x2c0
...
Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
---
arch/powerpc/include/asm/ftrace.h | 18 ++++++++
arch/powerpc/kernel/trace/ftrace.c | 70 ++++++++++++++++++++++++++++++
2 files changed, 88 insertions(+)
diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index debe8c4f706260..392296df70e96c 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h
@@ -126,6 +126,24 @@ static inline void this_cpu_enable_ftrace(void) { }
static inline void this_cpu_set_ftrace_enabled(u8 ftrace_enabled) { }
static inline u8 this_cpu_get_ftrace_enabled(void) { return 1; }
#endif /* CONFIG_PPC64 */
+
+#ifdef CONFIG_FUNCTION_TRACER
+/*
+ * With ppc64 -mprofile-kernel and ppc32, mcount call is made before a function
+ * establishes its own stack frame. While unwinding the stack, such functions
+ * do not appear in the trace. This helper returns the traced function if ip in
+ * the stack frame points to ftrace_[regs_]call.
+ *
+ * In ppc64 ELFv1, mcount call is after a function establishes its own
+ * stackframe. So, this always returns 0.
+ */
+unsigned long ftrace_get_traced_func_if_no_stackframe(unsigned long ip, unsigned long *stack);
+#else
+static inline unsigned long ftrace_get_traced_func_if_no_stackframe(unsigned long ip, unsigned long *stack)
+{
+ return 0;
+}
+#endif /* FUNCTION_TRACER */
#endif /* !__ASSEMBLY__ */
#endif /* _ASM_POWERPC_FTRACE */
diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
index ffe9537195aa33..ec1072d9a858d0 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -21,6 +21,7 @@
#include <linux/percpu.h>
#include <linux/init.h>
#include <linux/list.h>
+#include <linux/sched/task_stack.h>
#include <asm/asm-prototypes.h>
#include <asm/cacheflush.h>
@@ -987,3 +988,72 @@ char *arch_ftrace_match_adjust(char *str, const char *search)
return str;
}
#endif /* PPC64_ELF_ABI_v1 */
+
+static int is_ftrace_entry(unsigned long ip)
+{
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+ if (ip == (unsigned long)&ftrace_call + 4 || ip == (unsigned long)&ftrace_regs_call + 4)
+#else
+ if (ip == (unsigned long)&ftrace_call + 4)
+#endif
+ return 1;
+
+ return 0;
+}
+
+unsigned long ftrace_get_traced_func_if_no_stackframe(unsigned long ip, unsigned long *stack)
+{
+ if (!is_ftrace_entry(ip))
+ return 0;
+
+ if (IS_ENABLED(CONFIG_PPC32))
+ return stack[11]; /* see MCOUNT_SAVE_FRAME */
+
+ if (!IS_ENABLED(CONFIG_MPROFILE_KERNEL))
+ return 0;
+
+ return stack[(STACK_FRAME_OVERHEAD + offsetof(struct pt_regs, nip)) / sizeof(unsigned long)];
+}
+
+#ifdef CONFIG_STACK_TRACER
+void stack_get_trace(unsigned long traced_ip,
+ unsigned long *stack_ref __maybe_unused,
+ unsigned long stack_size __maybe_unused,
+ int *tracer_frame)
+{
+ unsigned long sp, newsp, top, ip;
+ int ftrace_call_found = 0;
+ unsigned long *stack;
+ int i = 0;
+
+ sp = current_stack_frame();
+ top = (unsigned long)task_stack_page(current) + THREAD_SIZE;
+
+ while (validate_sp(sp, current, STACK_FRAME_OVERHEAD) && i < STACK_TRACE_ENTRIES) {
+ stack = (unsigned long *) sp;
+ newsp = stack[0];
+ ip = stack[STACK_FRAME_LR_SAVE];
+
+ if (ftrace_call_found) {
+ stack_dump_trace[i] = ip;
+ stack_trace_index[i++] = top - sp;
+ }
+
+ if (is_ftrace_entry(ip)) {
+ if (IS_ENABLED(CONFIG_MPROFILE_KERNEL) || IS_ENABLED(CONFIG_PPC32)) {
+ stack_dump_trace[i] = ftrace_get_traced_func_if_no_stackframe(ip, stack);
+ stack_trace_index[i++] = top - newsp;
+ }
+ if (unlikely(!*tracer_frame)) {
+ *tracer_frame = newsp - (unsigned long)stack_ref;
+ stack_trace_max_size -= *tracer_frame;
+ }
+ ftrace_call_found = 1;
+ }
+
+ sp = newsp;
+ }
+
+ stack_trace_nr_entries = i;
+}
+#endif
--
2.30.2