Re: [PATCH v4 09/10] user_events: Optimize writing events by only copying data once

[Steven Rostedt <rostedt@goodmis.org>]

On Thu,  4 Nov 2021 10:04:32 -0700
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> Pass iterator through to probes to allow copying data directly to the
> probe buffers instead of taking multiple copies. Enables eBPF user and
> raw iterator types out to programs for no-copy scenarios.
> 
> Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> ---
>  kernel/trace/trace_events_user.c | 97 +++++++++++++++++++++++---------
>  1 file changed, 69 insertions(+), 28 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index b5fe0550b489..d50118b9630a 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -39,6 +39,10 @@
>  #define MAX_EVENT_DESC 512
>  #define EVENT_NAME(user_event) ((user_event)->tracepoint.name)
>  
> +#define MAX_BPF_COPY_SIZE PAGE_SIZE
> +#define MAX_STACK_BPF_DATA 512
> +#define copy_nofault copy_from_iter_nocache
> +
>  static char *register_page_data;
>  
>  static DEFINE_MUTEX(reg_mutex);
> @@ -63,8 +67,7 @@ struct user_event_refs {
>  	struct user_event *events[];
>  };
>  
> -typedef void (*user_event_func_t) (struct user_event *user,
> -				   void *data, u32 datalen,
> +typedef void (*user_event_func_t) (struct user_event *user, struct iov_iter *i,
>  				   void *tpdata);
>  
>  static int user_event_parse(char *name, char *args, char *flags,
> @@ -491,7 +494,7 @@ static struct user_event *find_user_event(char *name, u32 *outkey)
>  /*
>   * Writes the user supplied payload out to a trace file.
>   */
> -static void user_event_ftrace(struct user_event *user, void *data, u32 datalen,
> +static void user_event_ftrace(struct user_event *user, struct iov_iter *i,
>  			      void *tpdata)
>  {
>  	struct trace_event_file *file;
> @@ -506,41 +509,82 @@ static void user_event_ftrace(struct user_event *user, void *data, u32 datalen,
>  		return;
>  
>  	entry = trace_event_buffer_reserve(&event_buffer, file,
> -					   sizeof(*entry) + datalen);
> +					   sizeof(*entry) + i->count);
>  
>  	if (unlikely(!entry))
>  		return;
>  
> -	memcpy(entry + 1, data, datalen);
> +	if (unlikely(!copy_nofault(entry + 1, i->count, i)))

Need:
		__trace_event_discard_commit(event_buffer.buffer, event_buffer.event);

Because the trace_event_buffer_reserve() will not only allocate space on
the ring buffer, but may also disable preemption.

-- Steve


> +		return;
>  
>  	trace_event_buffer_commit(&event_buffer);
>  }
>  
>  #ifdef CONFIG_PERF_EVENTS