linux-kernel - Re: [PATCH rcu 0/3] RCU CPU stall-warning changes for v6.13

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Message-ID: <670f0257.d40a0220.9c8db.5c21@mx.google.com>
Date: Tue, 15 Oct 2024 20:01:23 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: frederic@...nel.org, rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, rostedt@...dmis.org
Subject: Re: [PATCH rcu 0/3] RCU CPU stall-warning changes for v6.13

On Tue, Oct 15, 2024 at 04:02:37PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 15, 2024 at 02:49:06PM -0400, Joel Fernandes wrote:
> > On Wed, Oct 09, 2024 at 11:05:02AM -0700, Paul E. McKenney wrote:
> > > Hello!
> > > 
> > > This series contains RCU CPU stall-warning changes for v6.13:
> > > 
> > > 1.	Delete unused rcu_gp_might_be_stalled() function.
> > > 
> > > 2.	Stop stall warning from dumping stacks if grace period ends.
> > > 
> > > 3.	Finer-grained grace-period-end checks in rcu_dump_cpu_stacks().
> > > 
> > 
> > Other than small nit in 2/3, 
> > 
> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> Applied, thank you, and I also added the data_race() in 2/3.
> 
> > I was curious if you're seeing perf or other improvements with the
> > finer-grained rnp locking.
> 
> This is about robustness rather that performance, though I suppose you
> could argue that lack of robustness is an extreme form of bad performance.
> Holding the leaf rcu_node locks for too long while dumping stacks can
> result in things like CSD-lock timeouts due to the dumping CPU having
> interrupts disabled for an extended period.
> 
> And earlier commit, 1ecd9d68eb44 ("rcu: Defer printing stall-warning
> backtrace when holding rcu_node lock"), took care of most of the issue
> by deferring the actual output.  But while in the area, it seemed wise
> to avoid up to 64 dumps being generated (but no longer printed) while
> holding a leaf rcu_node lock.
> 
> Hence this commit.

That's smart to do and makes sense, thanks!

 - Joel


> 
> 							Thanx, Paul
> 
> > thanks,
> > 
> >  - Joel
> > 
> > 
> > > 						Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > >  b/include/linux/rcutiny.h |    1 -
> > >  b/include/linux/rcutree.h |    1 -
> > >  b/kernel/rcu/tree_stall.h |   30 ------------------------------
> > >  kernel/rcu/tree_stall.h   |   35 ++++++++++++++++++++++-------------
> > >  4 files changed, 22 insertions(+), 45 deletions(-)
> > >