Re: [PATCH v6 23/29] context-tracking: Introduce work deferral infrastructure

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Valentin.

On 10/10/25 9:08 PM, Valentin Schneider wrote:
> smp_call_function() & friends have the unfortunate habit of sending IPIs to
> isolated, NOHZ_FULL, in-userspace CPUs, as they blindly target all online
> CPUs.
> 
> Some callsites can be bent into doing the right, such as done by commit:
> 
>    cc9e303c91f5 ("x86/cpu: Disable frequency requests via aperfmperf IPI for nohz_full CPUs")
> 
> Unfortunately, not all SMP callbacks can be omitted in this
> fashion. However, some of them only affect execution in kernelspace, which
> means they don't have to be executed *immediately* if the target CPU is in
> userspace: stashing the callback and executing it upon the next kernel entry
> would suffice. x86 kernel instruction patching or kernel TLB invalidation
> are prime examples of it.
> 
> Reduce the RCU dynticks counter width to free up some bits to be used as a
> deferred callback bitmask. Add some build-time checks to validate that
> setup.
> 
> Presence of CT_RCU_WATCHING in the ct_state prevents queuing deferred work.
> 
> Later commits introduce the bit:callback mappings.
> 
> Link: https://lore.kernel.org/all/20210929151723.162004989@infradead.org/
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>   arch/Kconfig                                 |  9 +++
>   arch/x86/Kconfig                             |  1 +
>   arch/x86/include/asm/context_tracking_work.h | 16 +++++
>   include/linux/context_tracking.h             | 21 ++++++
>   include/linux/context_tracking_state.h       | 30 ++++++---
>   include/linux/context_tracking_work.h        | 26 ++++++++
>   kernel/context_tracking.c                    | 69 +++++++++++++++++++-
>   kernel/time/Kconfig                          |  5 ++
>   8 files changed, 165 insertions(+), 12 deletions(-)
>   create mode 100644 arch/x86/include/asm/context_tracking_work.h
>   create mode 100644 include/linux/context_tracking_work.h
> 
> diff --git a/include/linux/context_tracking_work.h b/include/linux/context_tracking_work.h
> new file mode 100644
> index 0000000000000..c68245f8d77c5
> --- /dev/null
> +++ b/include/linux/context_tracking_work.h
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_CONTEXT_TRACKING_WORK_H
> +#define _LINUX_CONTEXT_TRACKING_WORK_H
> +
> +#include <linux/bitops.h>
> +
> +enum {
> +	CT_WORK_n_OFFSET,
> +	CT_WORK_MAX_OFFSET
> +};
> +
> +enum ct_work {
> +	CT_WORK_n        = BIT(CT_WORK_n_OFFSET),
> +	CT_WORK_MAX      = BIT(CT_WORK_MAX_OFFSET)
> +};
> +
> +#include <asm/context_tracking_work.h>
> +

It fails to compile on powerpc (likey any arch other than x86)

In file included from ./include/linux/context_tracking_state.h:8,
                  from ./include/linux/hardirq.h:5,
                  from ./include/linux/interrupt.h:11,
                  from ./include/linux/kernel_stat.h:8,
                  from ./include/linux/cgroup.h:27,
                  from ./include/linux/memcontrol.h:13,
                  from ./include/linux/swap.h:9,
                  from ./include/linux/suspend.h:5,
                  from arch/powerpc/kernel/asm-offsets.c:21:
./include/linux/context_tracking_work.h:17:10: fatal error: 
asm/context_tracking_work.h: No such file or directory
    17 | #include <asm/context_tracking_work.h>
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Gating works for compile, but no benefit of the series.

+#ifdef HAVE_CONTEXT_TRACKING_WORK
  #include <asm/context_tracking_work.h>
+#endif


I have been trying to debug/understand the issue seen with isolcpus= and
nohz_full=. system is idle, even then it occasionally woken up to do 
some work. So I was interesting if this series can help.