Re: CPU Lockups in KVM with deferred hrtimer rearming

[Thomas Gleixner <tglx@kernel.org>]

On Thu, Apr 16 2026 at 20:50, Vishal L. Verma wrote:
> I tried out AI assisted and patch (below) which does happen to solve
> it, but I'm not familiar in this area, and not sure if this is the
> right fix.
>
> diff --git a/include/linux/entry-virt.h b/include/linux/entry-virt.h
> index bfa767702d9a..c4856c252412 100644
> --- a/include/linux/entry-virt.h
> +++ b/include/linux/entry-virt.h
> @@ -4,6 +4,7 @@
>  
>  #include <linux/static_call_types.h>
>  #include <linux/resume_user_mode.h>
> +#include <linux/hrtimer_rearm.h>
>  #include <linux/syscalls.h>
>  #include <linux/seccomp.h>
>  #include <linux/sched.h>
> @@ -58,6 +59,7 @@ int xfer_to_guest_mode_handle_work(void);
>  static inline void xfer_to_guest_mode_prepare(void)
>  {
>         lockdep_assert_irqs_disabled();
> +       hrtimer_rearm_deferred();
>         tick_nohz_user_enter_prepare();


This code should never be reached with a rearm pending. Something else
went wrong earlier. So while the patch "works" it papers over the
underlying problem.

Can you please do the following:

    1) Apply the patch below

    2) Enable function tracing and the hrtimer* trace events

    3) Enable tracing if it has been disabled already

       echo 1 >/sys/kernel/tracing/tracing_on

    4) Run the tests and wait for /sys/kernel/tracing/tracing_on to
       become 0, which means the problem triggered.

    5) Retrieve the trace from /sys/kernel/tracing/trace and provide it
       somewhere to download from or send it to me compressed offlist.

Thanks,

        tglx
---

diff --git a/include/linux/entry-virt.h b/include/linux/entry-virt.h
index bfa767702d9a..ab73963a7496 100644
--- a/include/linux/entry-virt.h
+++ b/include/linux/entry-virt.h
@@ -58,6 +58,10 @@ int xfer_to_guest_mode_handle_work(void);
 static inline void xfer_to_guest_mode_prepare(void)
 {
 	lockdep_assert_irqs_disabled();
+	if (test_thread_flag(TIF_HRTIMER_REARM)) {
+		tracing_off();
+		hrtimer_rearm_deferred();
+	}
 	tick_nohz_user_enter_prepare();
 }