Re: [PATCH 29/30] Documentation: tracing: add TIF_NEED_RESCHED_LAZY

[Joel Fernandes]

On Wed, Feb 21, 2024 at 04:43:34PM -0500, Steven Rostedt wrote:
> On Mon, 12 Feb 2024 21:55:53 -0800
> Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
> 
> > Document various combinations of resched flags.
> > 
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> > ---
> >  Documentation/trace/ftrace.rst | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> > index 7e7b8ec17934..7f20c0bae009 100644
> > --- a/Documentation/trace/ftrace.rst
> > +++ b/Documentation/trace/ftrace.rst
> > @@ -1036,8 +1036,12 @@ explains which is which.
> >  		be printed here.
> >  
> >    need-resched:
> > -	- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
> > +	- 'B' all three, TIF_NEED_RESCHED, TIF_NEED_RESCHED_LAZY and PREEMPT_NEED_RESCHED are set,
> > +	- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED are set,
> > +	- 'L' both TIF_NEED_RESCHED_LAZY and PREEMPT_NEED_RESCHED are set,
> > +	- 'b' both TIF_NEED_RESCHED and TIF_NEED_RESCHED_LAZY are set,
> >  	- 'n' only TIF_NEED_RESCHED is set,
> > +	- 'l' only TIF_NEED_RESCHED_LAZY is set,
> >  	- 'p' only PREEMPT_NEED_RESCHED is set,

One thing I was curious about in current code, under which situation will
"only PREEMPT_NEED_RESCHED is set" case happen? All the code paths I see set
the PREEMPT_NEED_RESCHED when then TIF has already been set. That kind of
makes sense, why enter the scheduler on preempt_enable() unless there was a
reason to, and TIF should have recorded that reason.

So in other words, if 'p' above cannot happen, then it could be removed as a
potential need-resched stat. If it can happen, I'd love to learn in which
case?

Also considering all users of set_tsk_need_resched() also call the
set_preempt_need_resched() shortly after, should we add a helper to squash
the 2 calls and simplify callers?

There are some "special cases" where only TIF flag need be set (such as setting
rescheduling from an interrupt or when rescheduling a remote CPU). For those,
they can directly call the set_tsk_need_resched() like they do now.

thanks,

 - Joel