linux-kernel - Re: [RFC PATCH 1/6] trace/stack: Move code to save the stack trace into a separate function

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Message-ID: <20210601112824.29c5f168@oasis.local.home>
Date:   Tue, 1 Jun 2021 11:28:24 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Michal Suchanek <msuchanek@...e.de>,
        Torsten Duwe <duwe@...e.de>, <linuxppc-dev@...ts.ozlabs.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/6] trace/stack: Move code to save the stack trace
 into a separate function

On Fri, 21 May 2021 12:18:36 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> In preparation to add support for stack tracer to powerpc, move code to
> save stack trace and to calculate the frame sizes into a separate weak
> function. Also provide access to some of the data structures used by the
> stack trace code so that architectures can update those.
> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
>  include/linux/ftrace.h     |  8 ++++
>  kernel/trace/trace_stack.c | 98 ++++++++++++++++++++------------------
>  2 files changed, 60 insertions(+), 46 deletions(-)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index a69f363b61bf73..8263427379f05c 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -368,10 +368,18 @@ static inline void arch_ftrace_set_direct_caller(struct pt_regs *regs,
>  
>  #ifdef CONFIG_STACK_TRACER
>  
> +#define STACK_TRACE_ENTRIES 500
> +
> +extern unsigned long stack_dump_trace[STACK_TRACE_ENTRIES];
> +extern unsigned stack_trace_index[STACK_TRACE_ENTRIES];
> +extern unsigned int stack_trace_nr_entries;
> +extern unsigned long stack_trace_max_size;
>  extern int stack_tracer_enabled;
>  
>  int stack_trace_sysctl(struct ctl_table *table, int write, void *buffer,
>  		       size_t *lenp, loff_t *ppos);
> +void stack_get_trace(unsigned long traced_ip, unsigned long *stack_ref,
> +					unsigned long stack_size, int *tracer_frame);
>  
>  /* DO NOT MODIFY THIS VARIABLE DIRECTLY! */
>  DECLARE_PER_CPU(int, disable_stack_tracer);
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index 63c28504205162..5b63dbd37c8c25 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -19,13 +19,11 @@
>  
>  #include "trace.h"
>  
> -#define STACK_TRACE_ENTRIES 500
> +unsigned long stack_dump_trace[STACK_TRACE_ENTRIES];
> +unsigned stack_trace_index[STACK_TRACE_ENTRIES];
>  
> -static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES];
> -static unsigned stack_trace_index[STACK_TRACE_ENTRIES];
> -
> -static unsigned int stack_trace_nr_entries;
> -static unsigned long stack_trace_max_size;
> +unsigned int stack_trace_nr_entries;
> +unsigned long stack_trace_max_size;
>  static arch_spinlock_t stack_trace_max_lock =
>  	(arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
>  
> @@ -152,49 +150,19 @@ static void print_max_stack(void)
>   * Although the entry function is not displayed, the first function (sys_foo)
>   * will still include the stack size of it.
>   */
> -static void check_stack(unsigned long ip, unsigned long *stack)

I just got back from PTO and have a ton of other obligations to attend
to before I can dig deeper into this. I'm not opposed to this change,
but the stack_tracer has not been getting the love that it deserves and
I think you hit one of the issues that needs to be addressed. I'm not
sure this is a PPC only issue, and would like to see if I can get more
time (or someone else can) to reevaluate the way stack tracer works,
and see if it can be made a bit more robust.

-- Steve