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SubjectRe: [PATCH 2/2] tracing - putting cond_resched into tace_pipe loop
On 03/12, Jiri Olsa wrote:
>
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3237,10 +3237,23 @@ waitagain:
> * One of the trace_seq_* functions is not used properly.
> */
> WARN_ON(iter->seq.full);
> +
> + /*
> + * There's a chance this loop might get quite tight,
> + * causing latency in non preemptive kernel.
> + */
> + cond_resched();
> + if (signal_pending(current)) {
> + sret = -EINTR;
> + break;

First of all: I do not pretend I understand this code ;) Still, a
couple of nits.

-EINTR doesn't look exactly right, I'd suggest -ERESTARTSYS. The same
for tracing_wait_pipe() btw, I think it should be fixed.



I wonder if it makes sense to simply "break" if signal_pending(), it
is possible we already have something to report via trace_seq_to_user().
Then we could do

- if (sret == -EBUSY)
- goto waitagain;
+ if (sret == -EBUSY) {
+ if (!signal_pending())
+ goto waitagain;
+ sret = -ERESTARTSYS;
+ }

Or we can change tracing_wait_pipe() to check signal_pending()
uncondditionally, I dunno.

Up to you, but note that otherwise the logic looks a bit strange.
Suppose that signal_pending() is already true when we call
tracing_wait_pipe(). In this case we are going to do the "unnecessary"
work and then return EINTR/ERESTART. This is correct, the next
invocation does trace_seq_to_user() before anything else, just
looks a bit strange.

Oleg.



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