On Thu, Dec 19, 2024 at 09:42:48AM -0800, Paul E. McKenney wrote:
> 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@xxxxxxxxx/)
> >
> > 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.
Another day, another 12 hours of 400*TREE03 passed. This gets us up
beyond 90% confidence that these patches help, and even more confidence
that they are not too severely hurting anything.
> 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! ;-)
Curiosity overcame me, and the #ifdef makes things at least appear to
work for CONFIG_SMP=n kernels. Experimental patch below, which I intend
to use for further testing, but I have no plans to push it upstream.
Thanx, Paul
------------------------------------------------------------------------
commit 61a0b92b3a3cfef69e3848806e51d1b99a9e9406
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date: Fri Dec 20 15:10:26 2024 -0800
EXP hrtimers: No-op enqueue_hrtimer_offline() if !HOTPLUG_CPU
In kernels built with CONFIG_HOTPLUG_CPU=n, this experimental commit
uses #ifdef to remove the body of the enqueue_hrtimer_offline() function.
This might or might not be the correct fix to the build complaint about
migration_base being undeclared.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Vlad Poenaru <vlad.wing@xxxxxxxxx>
Cc: Usama Arif <usamaarif642@xxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 48c0078d2c4f2..4235b7825b152 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1213,6 +1213,7 @@ static void enqueue_hrtimer_offline(struct hrtimer *timer,
struct hrtimer_clock_base *base,
const enum hrtimer_mode mode)
{
+#ifdef CONFIG_HOTPLUG_CPU
struct hrtimer_cpu_base *new_cpu_base, *old_cpu_base, *this_cpu_base;
struct hrtimer_clock_base *new_base;
int cpu;
@@ -1238,6 +1239,7 @@ static void enqueue_hrtimer_offline(struct hrtimer *timer,
if (enqueue_hrtimer(timer, new_base, mode))
smp_call_function_single_async(cpu, &new_cpu_base->csd);
+#endif
}
