[RFC PATCH v8 1/4] arm64: Make all stack walking functions use arch_stack_walk()
Madhavan T. Venkataraman
madvenka at linux.microsoft.com
Sat Oct 9 16:41:46 PDT 2021
On 8/24/21 8:13 AM, Mark Rutland wrote:
> On Thu, Aug 12, 2021 at 02:06:00PM -0500, madvenka at linux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka at linux.microsoft.com>
>>
>> Currently, there are multiple functions in ARM64 code that walk the
>> stack using start_backtrace() and unwind_frame(). Convert all of
>> them to use arch_stack_walk(). This makes maintenance easier.
>
> It would be good to split this into a series of patches as Mark Brown
> suggested in v7.
>
>> Here is the list of functions:
>>
>> perf_callchain_kernel()
>> get_wchan()
>> return_address()
>> dump_backtrace()
>> profile_pc()
>
> Note that arch_stack_walk() depends on CONFIG_STACKTRACE (which is not in
> defconfig), so we'll need to reorganise things such that it's always defined,
> or factor out the core of that function and add a wrapper such that we
> can always use it.
>
>> Signed-off-by: Madhavan T. Venkataraman <madvenka at linux.microsoft.com>
>> ---
>> arch/arm64/include/asm/stacktrace.h | 3 ---
>> arch/arm64/kernel/perf_callchain.c | 5 +---
>> arch/arm64/kernel/process.c | 39 ++++++++++++++++++-----------
>> arch/arm64/kernel/return_address.c | 6 +----
>> arch/arm64/kernel/stacktrace.c | 38 +++-------------------------
>> arch/arm64/kernel/time.c | 22 +++++++++-------
>> 6 files changed, 43 insertions(+), 70 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
>> index 8aebc00c1718..e43dea1c6b41 100644
>> --- a/arch/arm64/include/asm/stacktrace.h
>> +++ b/arch/arm64/include/asm/stacktrace.h
>> @@ -61,9 +61,6 @@ struct stackframe {
>> #endif
>> };
>>
>> -extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
>> -extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
>> - bool (*fn)(void *, unsigned long), void *data);
>> extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>> const char *loglvl);
>>
>> diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
>> index 4a72c2727309..2f289013c9c9 100644
>> --- a/arch/arm64/kernel/perf_callchain.c
>> +++ b/arch/arm64/kernel/perf_callchain.c
>> @@ -147,15 +147,12 @@ static bool callchain_trace(void *data, unsigned long pc)
>> void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
>> struct pt_regs *regs)
>> {
>> - struct stackframe frame;
>> -
>> if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
>> /* We don't support guest os callchain now */
>> return;
>> }
>>
>> - start_backtrace(&frame, regs->regs[29], regs->pc);
>> - walk_stackframe(current, &frame, callchain_trace, entry);
>> + arch_stack_walk(callchain_trace, entry, current, regs);
>> }
>
> We can also update callchain_trace take the return value of
> perf_callchain_store into acount, e.g.
>
> | static bool callchain_trace(void *data, unsigned long pc)
> | {
> | struct perf_callchain_entry_ctx *entry = data;
> | return perf_callchain_store(entry, pc) == 0;
> | }
>
>>
>> unsigned long perf_instruction_pointer(struct pt_regs *regs)
>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>> index c8989b999250..52c12fd26407 100644
>> --- a/arch/arm64/kernel/process.c
>> +++ b/arch/arm64/kernel/process.c
>> @@ -544,11 +544,28 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
>> return last;
>> }
>>
>> +struct wchan_info {
>> + unsigned long pc;
>> + int count;
>> +};
>> +
>> +static bool get_wchan_cb(void *arg, unsigned long pc)
>> +{
>> + struct wchan_info *wchan_info = arg;
>> +
>> + if (!in_sched_functions(pc)) {
>> + wchan_info->pc = pc;
>> + return false;
>> + }
>> + wchan_info->count--;
>> + return !!wchan_info->count;
>> +}
>
> This will terminate one entry earlier than the old logic since we used
> to use a post-increment (testing the prior value), and now we're
> effectively using a pre-decrement (testing the new value).
>
> I don't think that matters all that much in practice, but it might be
> best to keep the original logic, e.g. initialize `count` to 0 and here
> do:
>
> return wchan_info->count++ < 16;
>
>> +
>> unsigned long get_wchan(struct task_struct *p)
>> {
>> - struct stackframe frame;
>> - unsigned long stack_page, ret = 0;
>> - int count = 0;
>> + unsigned long stack_page;
>> + struct wchan_info wchan_info;
>> +
>> if (!p || p == current || task_is_running(p))
>> return 0;
>>
>> @@ -556,20 +573,12 @@ unsigned long get_wchan(struct task_struct *p)
>> if (!stack_page)
>> return 0;
>>
>> - start_backtrace(&frame, thread_saved_fp(p), thread_saved_pc(p));
>> + wchan_info.pc = 0;
>> + wchan_info.count = 16;
>> + arch_stack_walk(get_wchan_cb, &wchan_info, p, NULL);
>>
>> - do {
>> - if (unwind_frame(p, &frame))
>> - goto out;
>> - if (!in_sched_functions(frame.pc)) {
>> - ret = frame.pc;
>> - goto out;
>> - }
>> - } while (count++ < 16);
>> -
>> -out:
>> put_task_stack(p);
>> - return ret;
>> + return wchan_info.pc;
>> }
>
> Other than the comment above, this looks good to me.
>
>> unsigned long arch_align_stack(unsigned long sp)
>> diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
>> index a6d18755652f..92a0f4d434e4 100644
>> --- a/arch/arm64/kernel/return_address.c
>> +++ b/arch/arm64/kernel/return_address.c
>> @@ -35,15 +35,11 @@ NOKPROBE_SYMBOL(save_return_addr);
>> void *return_address(unsigned int level)
>> {> struct return_address_data data;
>> - struct stackframe frame;
>>
>> data.level = level + 2;
>> data.addr = NULL;
>>
>> - start_backtrace(&frame,
>> - (unsigned long)__builtin_frame_address(0),
>> - (unsigned long)return_address);
>> - walk_stackframe(current, &frame, save_return_addr, &data);
>> + arch_stack_walk(save_return_addr, &data, current, NULL);
>>
>> if (!data.level)
>> return data.addr;
>
> Nor that arch_stack_walk() will start with it's caller, so
> return_address() will be included in the trace where it wasn't
> previously, which implies we need to skip an additional level.
>
> That said, I'm not entirely sure why we need to skip 2 levels today; it
> might be worth checking that's correct.
>
> We should also mark return_address() as noinline to avoid surprises with
> LTO.
>
>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>> index 8982a2b78acf..1800310f92be 100644
>> --- a/arch/arm64/kernel/stacktrace.c
>> +++ b/arch/arm64/kernel/stacktrace.c
>> @@ -151,23 +151,21 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
>> }
>> NOKPROBE_SYMBOL(walk_stackframe);
>>
>> -static void dump_backtrace_entry(unsigned long where, const char *loglvl)
>> +static bool dump_backtrace_entry(void *arg, unsigned long where)
>> {
>> + char *loglvl = arg;
>> printk("%s %pSb\n", loglvl, (void *)where);
>> + return true;
>> }
>>
>> void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>> const char *loglvl)
>> {
>> - struct stackframe frame;
>> - int skip = 0;
>> -
>> pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
>>
>> if (regs) {
>> if (user_mode(regs))
>> return;
>> - skip = 1;
>> }
>
> We can simplifiy this to:
>
> if (regs && user_mode(regs))
> return;
>
>>
>> if (!tsk)
>> @@ -176,36 +174,8 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>> if (!try_get_task_stack(tsk))
>> return;
>>
>> - if (tsk == current) {
>> - start_backtrace(&frame,
>> - (unsigned long)__builtin_frame_address(0),
>> - (unsigned long)dump_backtrace);
>> - } else {
>> - /*
>> - * task blocked in __switch_to
>> - */
>> - start_backtrace(&frame,
>> - thread_saved_fp(tsk),
>> - thread_saved_pc(tsk));
>> - }
>> -
>> printk("%sCall trace:\n", loglvl);
>> - do {
>> - /* skip until specified stack frame */
>> - if (!skip) {
>> - dump_backtrace_entry(frame.pc, loglvl);
>> - } else if (frame.fp == regs->regs[29]) {
>> - skip = 0;
>> - /*
>> - * Mostly, this is the case where this function is
>> - * called in panic/abort. As exception handler's
>> - * stack frame does not contain the corresponding pc
>> - * at which an exception has taken place, use regs->pc
>> - * instead.
>> - */
>> - dump_backtrace_entry(regs->pc, loglvl);
>> - }
>> - } while (!unwind_frame(tsk, &frame));
>> + arch_stack_walk(dump_backtrace_entry, (void *)loglvl, tsk, regs);
>
> It turns out we currently need this skipping to get the balance the
> ftrace call stack, and arch_stack_walk() doesn't currently do the right
> thing when starting from regs. That balancing isn't quite right, and
> will be wrong in some case when unwinding across exception boundaries;
> we could implement HAVE_FUNCTION_GRAPH_RET_ADDR_PTR using the FP to
> solve that.
>
Hi Mark,
It seems that the behavior is the same in the old and new code. Do you
agree?
In the old code when regs is used, the stack trace starts from regs->regs[29]
and the first PC displayed is regs->pc. arch_stack_walk() does the same thing.
Can you elaborate what change you want me to make here?
Madhavan
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