Re: [PATCH 05/12] perf top: Moving lost events warning to helpline

[Namhyung Kim]

On Mon, Nov 19, 2018 at 01:20:09PM +0100, Jiri Olsa wrote:
> We can't display the UI box saying that we are slow in reader
> thread. That will make perf top even slower and user even more
> angry ;-)
> 
> Moving the UI box message out of the reader thread into UI thread
> and changing it into helpline, so there's no 'press any key'
> necessary.
> 
> Link: http://lkml.kernel.org/n/tip-0rpmmt3omlait889ewp1cl61@xxxxxxxxxxxxxx
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  tools/perf/builtin-top.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index 047e99de09f3..1d77aa7650da 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -553,8 +553,6 @@ static void perf_top__sort_new_samples(void *arg)
>  	struct perf_evsel *evsel = t->sym_evsel;
>  	struct hists *hists;
>  
> -	perf_top__reset_sample_counters(t);
> -
>  	if (t->evlist->selected != NULL)
>  		t->sym_evsel = t->evlist->selected;
>  
> @@ -571,6 +569,11 @@ static void perf_top__sort_new_samples(void *arg)
>  
>  	hists__collapse_resort(hists, NULL);
>  	perf_evsel__output_resort(evsel, NULL);
> +
> +	if (t->lost)
> +		ui_helpline__fpush("Too slow to read ring buffer (change period (-c/-F) or limit CPUs (-C)\n");

In general, ui_helpline__[f]push() requires corresponding __pop()
which handles timeout logic (or something can dismiss the message).
The ui_helpline__show() is to show the message and overwrite the old
one.

But we don't use it strictly and I think just pr_err() or pr_warning()
is ok here (which call ui_helpline__show() internally).

Anyway, I found that tui_helpline__push() should use ui__lock.
Otherwise it could race with display thread and break rendering.
A bug from the beginning but no one reported... :)

Thanks,
Namhyung