Re: [PATCH v1 2/8] tracing/ftrace: guard syscall probe with preempt_notrace

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 3 Oct 2024 20:26:29 -0400
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:


> static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
> {
>          struct trace_array *tr = data;
>          struct trace_event_file *trace_file;
>          struct syscall_trace_enter *entry;
>          struct syscall_metadata *sys_data;
>          struct trace_event_buffer fbuffer;
>          unsigned long args[6];
>          int syscall_nr;
>          int size;
> 
>          syscall_nr = trace_get_syscall_nr(current, regs);
>          if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
>                  return;
> 
>          /* Here we're inside tp handler's rcu_read_lock_sched (__DO_TRACE) */
>          trace_file = rcu_dereference_sched(tr->enter_syscall_files[syscall_nr]);
> 
> ^^^^ this function explicitly states that preempt needs to be disabled by
> tracepoints.

Ah, I should have known it was the syscall portion. I don't care for this
hidden dependency. I rather add a preempt disable here and not expect it to
be disabled when called.

> 
>          if (!trace_file)
>                  return;
> 
>          if (trace_trigger_soft_disabled(trace_file))
>                  return;
> 
>          sys_data = syscall_nr_to_meta(syscall_nr);
>          if (!sys_data)
>                  return;
> 
>          size = sizeof(*entry) + sizeof(unsigned long) * sys_data->nb_args;
> 
>          entry = trace_event_buffer_reserve(&fbuffer, trace_file, size);
> ^^^^ it reserves space in the ring buffer without disabling preemption explicitly.
> 
> And also:
> 
> void *trace_event_buffer_reserve(struct trace_event_buffer *fbuffer,
>                                   struct trace_event_file *trace_file,
>                                   unsigned long len)
> {
>          struct trace_event_call *event_call = trace_file->event_call;
> 
>          if ((trace_file->flags & EVENT_FILE_FL_PID_FILTER) &&
>              trace_event_ignore_this_pid(trace_file))
>                  return NULL;
> 
>          /*
>           * If CONFIG_PREEMPTION is enabled, then the tracepoint itself disables
>           * preemption (adding one to the preempt_count). Since we are
>           * interested in the preempt_count at the time the tracepoint was
>           * hit, we need to subtract one to offset the increment.
>           */
> ^^^ This function also explicitly expects preemption to be disabled.
> 
> So I rest my case. The change I'm introducing for tracepoints
> don't make any assumptions about whether or not each tracer require
> preempt off or not: it keeps the behavior the _same_ as it was before.
> 
> Then it's up to each tracer's developer to change the behavior of their
> own callbacks as they see fit. But I'm not introducing regressions in
> tracers with the "big switch" change of making syscall tracepoints
> faultable. This will belong to changes that are specific to each tracer.


I rather remove these dependencies at the source. So, IMHO, these places
should be "fixed" first.

At least for the ftrace users. But I think the same can be done for the
other users as well. BPF already stated it just needs "migrate_disable()".
Let's see what perf has.

We can then audit all the tracepoint users to make sure they do not need
preemption disabled.

-- Steve