Messages in this thread | | | Date | Tue, 21 Mar 2023 14:24:28 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v5 1/2] function_graph: Support recording and printing the return value of function |
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On Mon, Mar 20, 2023 at 06:16:49AM -0700, Donglin Peng wrote: > When using the function_graph tracer to analyze system call failures, > it can be time-consuming to analyze the trace logs and locate the kernel > function that first returns an error. This change aims to simplify the > process by recording the function return value to the 'retval' member of > 'ftrace_graph_ent' and printing it when outputing the trace log. > > New trace options are introduced: funcgraph-retval and graph_retval_hex. > The former is used to control whether to display the return value, while > the latter is used to control the display format of the reutrn value. > > Note that even if a function's return type is void, a return value will > still be printed, so just ignore it. > > Currently, this modification supports the following commonly used > processor architectures: x86_64, x86, arm64, arm, riscv. > > Here is an example: > > I want to attach the demo process to a cpu cgroup, but it failed: > > echo `pidof demo` > /sys/fs/cgroup/cpu/test/tasks > -bash: echo: write error: Invalid argument > > The strace logs tells that the write system call returned -EINVAL(-22): > ... > write(1, "273\n", 4) = -1 EINVAL (Invalid argument) > ... > > Use the following commands to capture trace logs when calling the write > system call: > > cd /sys/kernel/debug/tracing/ > echo 0 > tracing_on > echo > trace > echo *sys_write > set_graph_function > echo *spin* > set_graph_notrace > echo *rcu* >> set_graph_notrace > echo *alloc* >> set_graph_notrace > echo preempt* >> set_graph_notrace > echo kfree* >> set_graph_notrace > echo $$ > set_ftrace_pid > echo function_graph > current_tracer > echo 1 > tracing_on > echo `pidof demo` > /sys/fs/cgroup/cpu/test/tasks > echo 0 > tracing_on > echo 1 > options/funcgraph-retval > cat trace > ~/trace.log > > Search the error code -22 directly in the file trace.log and find the > first function that return -22, then read the function code to get the > root cause. > > ... > > 1) | cgroup_migrate() { > 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ > 1) | cgroup_migrate_execute() { > 1) | cpu_cgroup_can_attach() { > 1) | cgroup_taskset_first() { > 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ > 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ > 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ > 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */ > 1) 4.369 us | } /* cgroup_migrate_execute = -22 */ > 1) 7.143 us | } /* cgroup_migrate = -22 */ > > ... > > Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > index 350ed81324ac..d1a5d76e6d72 100644 > --- a/arch/arm64/kernel/entry-ftrace.S > +++ b/arch/arm64/kernel/entry-ftrace.S > @@ -276,7 +276,15 @@ SYM_CODE_START(return_to_handler) > stp x4, x5, [sp, #32] > stp x6, x7, [sp, #48] > > +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL > + /* > + * Pass both the function return values in the register x0 and x1 > + * to ftrace_return_to_handler. > + */ > + mov x2, x29 // parent's fp > +#else > mov x0, x29 // parent's fp > +#endif > bl ftrace_return_to_handler// addr = ftrace_return_to_hander(fp); > mov x30, x0 // restore the original return address
Please don't make the calling convention of the asm change depending on a selectable config option.
We already store the regs here; I'd be happy to make that a struct ftrace_regs and pass a pointer to that to C code. Then it's be easy to acquire the value you want in the exact same way as upon entry, and it'll work even if we decide to return a structure by value somewhere (as that can use registers x2-x7 too).
Thanks, Mark.
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