Re: [PATCH v2] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

["Luis Claudio R. Goncalves" <lgoncalv@redhat.com>]

On Thu, Apr 10, 2025 at 05:32:05PM +0200, Sebastian Andrzej Siewior wrote:
> On 2025-04-10 09:51:03 [+0200], Peter Zijlstra wrote:
> > > I complained about this special RT case in put_task_struct() when it was
> > > first got introduced. Couldn't we just just unconditionally do the RCU
> > > put?
> > 
> > Yeah, please make it simpler, not more complex.
> 
> Just so we clear: simpler as in everyone does call_rcu() or RT does
> always call_rcu() and everyone else __put_task_struct()? I mean we would
> end up with one call chain I am just not sure how expensive it gets for
> !RT.

Sebastian, I implemented the change where put_task_struct() unconditionally
resorted to:

	call_rcu(&t->rcu, __put_task_struct_rcu_cb);

I submitted the kernels I built with that change and a pristine upstream
kenrel to LTP and stress-ng and also ran 'perf bench all'. I built kernels
with and without lockdep and extra debug. All kernels survived the tests
without a scratch and I haven't observed differences in behaviors nor
timings (for the tests that had that information).

What would be a good benchmark to compare the kernels with and without the
put_task_struct() change? I would like to observe whether there is a
penalty or added overhead with the change in place.

Best,
Luis

> Sebastian
> 
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