On Thu, 3 Oct 2024 11:16:33 -0400
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> In preparation for allowing system call enter/exit instrumentation to
> handle page faults, make sure that perf can handle this change by
> explicitly disabling preemption within the perf system call tracepoint
> probes to respect the current expectations within perf ring buffer code.
>
> This change does not yet allow perf to take page faults per se within
> its probe, but allows its existing probes to adapt to the upcoming
> change.
Frederic,
Does the perf ring buffer expect preemption to be disabled when used?
In other words, is this patch needed?
-- Steve
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Yonghong Song <yhs@xxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Cc: bpf@xxxxxxxxxxxxxxx
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
> include/trace/perf.h | 41 +++++++++++++++++++++++++++++++----
> kernel/trace/trace_syscalls.c | 12 ++++++++++
> 2 files changed, 49 insertions(+), 4 deletions(-)
>
> diff --git a/include/trace/perf.h b/include/trace/perf.h
> index ded997af481e..5650c1bad088 100644
> --- a/include/trace/perf.h
> +++ b/include/trace/perf.h
> @@ -12,10 +12,10 @@
> #undef __perf_task
> #define __perf_task(t) (__task = (t))
>
> -#undef DECLARE_EVENT_CLASS
> -#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
> +#undef __DECLARE_EVENT_CLASS
> +#define __DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
> static notrace void \
> -perf_trace_##call(void *__data, proto) \
> +do_perf_trace_##call(void *__data, proto) \
> { \
> struct trace_event_call *event_call = __data; \
> struct trace_event_data_offsets_##call __maybe_unused __data_offsets;\
> @@ -55,8 +55,38 @@ perf_trace_##call(void *__data, proto) \
> head, __task); \
> }
>
> +/*
> + * Define unused __count and __task variables to use @args to pass
> + * arguments to do_perf_trace_##call. This is needed because the
> + * macros __perf_count and __perf_task introduce the side-effect to
> + * store copies into those local variables.
> + */
> +#undef DECLARE_EVENT_CLASS
> +#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
> +__DECLARE_EVENT_CLASS(call, PARAMS(proto), PARAMS(args), PARAMS(tstruct), \
> + PARAMS(assign), PARAMS(print)) \
> +static notrace void \
> +perf_trace_##call(void *__data, proto) \
> +{ \
> + u64 __count __attribute__((unused)); \
> + struct task_struct *__task __attribute__((unused)); \
> + \
> + do_perf_trace_##call(__data, args); \
> +}
> +
> #undef DECLARE_EVENT_SYSCALL_CLASS
> -#define DECLARE_EVENT_SYSCALL_CLASS DECLARE_EVENT_CLASS
> +#define DECLARE_EVENT_SYSCALL_CLASS(call, proto, args, tstruct, assign, print) \
> +__DECLARE_EVENT_CLASS(call, PARAMS(proto), PARAMS(args), PARAMS(tstruct), \
> + PARAMS(assign), PARAMS(print)) \
> +static notrace void \
> +perf_trace_##call(void *__data, proto) \
> +{ \
> + u64 __count __attribute__((unused)); \
> + struct task_struct *__task __attribute__((unused)); \
> + \
> + guard(preempt_notrace)(); \
> + do_perf_trace_##call(__data, args); \
> +}
>
> /*
> * This part is compiled out, it is only here as a build time check
> @@ -76,4 +106,7 @@ static inline void perf_test_probe_##call(void) \
> DEFINE_EVENT(template, name, PARAMS(proto), PARAMS(args))
>
> #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
> +
> +#undef __DECLARE_EVENT_CLASS
> +
> #endif /* CONFIG_PERF_EVENTS */
> diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> index ab4db8c23f36..edcfa47446c7 100644
> --- a/kernel/trace/trace_syscalls.c
> +++ b/kernel/trace/trace_syscalls.c
> @@ -596,6 +596,12 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
> int rctx;
> int size;
>
> + /*
> + * Syscall probe called with preemption enabled, but the ring
> + * buffer and per-cpu data require preemption to be disabled.
> + */
> + guard(preempt_notrace)();
> +
> syscall_nr = trace_get_syscall_nr(current, regs);
> if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
> return;
> @@ -698,6 +704,12 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
> int rctx;
> int size;
>
> + /*
> + * Syscall probe called with preemption enabled, but the ring
> + * buffer and per-cpu data require preemption to be disabled.
> + */
> + guard(preempt_notrace)();
> +
> syscall_nr = trace_get_syscall_nr(current, regs);
> if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
> return;
