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Message-ID: <dee4dc13-b187-42df-93ce-f738cfab32ea@linux.ibm.com>
Date: Mon, 3 Nov 2025 14:02:11 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Nicolas Saenz Julienne <nsaenzju@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>, Arnd Bergmann <arnd@...db.de>,
Frederic Weisbecker <frederic@...nel.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Jason Baron <jbaron@...mai.com>, Steven Rostedt <rostedt@...dmis.org>,
Ard Biesheuvel <ardb@...nel.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
"David S. Miller" <davem@...emloft.net>,
Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
Joel Fernandes <joelagnelf@...dia.com>,
Josh Triplett
<josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>, Uladzislau Rezki <urezki@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Mel Gorman <mgorman@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Masahiro Yamada <masahiroy@...nel.org>, Han Shen <shenhan@...gle.com>,
Rik van Riel <riel@...riel.com>, Jann Horn <jannh@...gle.com>,
Dan Carpenter <dan.carpenter@...aro.org>,
Oleg Nesterov <oleg@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
Clark Williams <williams@...hat.com>,
Yair Podemsky <ypodemsk@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>, Daniel Wagner <dwagner@...e.de>,
Petr Tesarik <ptesarik@...e.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, rcu@...r.kernel.org, x86@...nel.org,
linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org,
linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v6 23/29] context-tracking: Introduce work deferral
infrastructure
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@...hat.com>
> Signed-off-by: Valentin Schneider <vschneid@...hat.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.
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