linux-kernel - Re: [PATCH 0/3] hrtimer: Fix timers queued locally from offline CPUs

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Message-ID: <7d19b58a-2cd2-4c29-8596-f0cda605112f@paulmck-laptop>
Date: Thu, 19 Dec 2024 09:42:48 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, vlad.wing@...il.com,
	rcu@...r.kernel.org, boqun.feng@...il.com, joel@...lfernandes.org,
	neeraj.upadhyay@....com, urezki@...il.com,
	qiang.zhang1211@...il.com, Cheng-Jui.Wang@...iatek.com,
	leitao@...ian.org, kernel-team@...a.com,
	Usama Arif <usamaarif642@...il.com>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [PATCH 0/3] hrtimer: Fix timers queued locally from offline CPUs

On Wed, Dec 18, 2024 at 05:50:05PM +0100, Frederic Weisbecker wrote:
> 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")
> was introduced to fix stalls with scheduler bandwidth timers getting
> migrated while some kthreads handling CPU hotplug rely on bandwidth.
> 
> However this has introduced several other issues which used to be
> confined to RCU. But not anymore as it is spreading to hotplug code
> itself (https://lore.kernel.org/all/20241213203739.1519801-1-usamaarif642@gmail.com/)
> 
> Instead of introducing yet another new hackery, fix the problem in
> hrtimers for everyone.

The good news is that this passes 12 hours of 400*TREE03.  (Yay!!!)

The so-so news is that this gives only about 70% confidence that these
patches help, but on the other hand, it also gives much higher confidence
that these patches are not hurting anything.

At least for TREE03.

The not-so-good news is that this series causes build failures for
rcutorture scenarios (such as SRCU-T) that build with CONFIG_SMP=n:

------------------------------------------------------------------------
kernel/time/hrtimer.c: In function ‘enqueue_hrtimer_offline’:
kernel/time/hrtimer.c:1229:42: error: ‘migration_base’ undeclared (first use in this function); did you mean ‘is_migration_base’?
------------------------------------------------------------------------

When built with KCSAN enabled (--kcsan to kvm.sh), there is this
additional build failure on that same line of code:

------------------------------------------------------------------------
kernel/time/hrtimer.c:1229:3: error: incompatible pointer types assigning to 'volatile typeof (timer->base)' (aka 'struct hrtimer_clock_base *volatile') from 'bool (*)(struct hrtimer_clock_base *)' (aka '_Bool (*)(struct hrtimer_clock_base *)') [-Werror,-Wincompatible-pointer-types]

 1229 |                 WRITE_ONCE(timer->base, &migration_base);
------------------------------------------------------------------------

Me, I am a bit surprised that enqueue_hrtimer_offline() is even built
in a CONFIG_SMP=n kernel.

But there might be some reason why #ifdef-ing out that function's body
would be a bad idea, so over to you!  ;-)

							Thanx, Paul

> Frederic Weisbecker (3):
>   hrtimers: Force migrate away hrtimers queued after
>     CPUHP_AP_HRTIMERS_DYING
>   rcu: Remove swake_up_one_online() bandaid
>   Revert "rcu/nocb: Fix rcuog wake-up from offline softirq"
> 
>  include/linux/hrtimer_defs.h |  1 +
>  kernel/rcu/tree.c            | 34 +-------------------
>  kernel/rcu/tree_exp.h        |  2 +-
>  kernel/rcu/tree_nocb.h       | 10 ++----
>  kernel/time/hrtimer.c        | 60 +++++++++++++++++++++++++++++++-----
>  5 files changed, 58 insertions(+), 49 deletions(-)
> 
> -- 
> 2.46.0
>