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

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Message-ID: <xhsmhjz05ud0r.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 04 Nov 2025 14:45:56 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Nicolas Saenz Julienne <nsaenzju@...hat.com>, Thomas Gleixner
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Subject: Re: [PATCH v6 23/29] context-tracking: Introduce work deferral
 infrastructure

On 03/11/25 14:02, Shrikanth Hegde wrote:
> Hi Valentin.
>
> On 10/10/25 9:08 PM, Valentin Schneider wrote:
>> +++ 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)
>

Woops, thanks for testing!

> 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
>

Huh, looking at this I wonder why I shoved that here. That include belongs
in context_tracking.c itself; I'll fix that.

>
> 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.

Is there some ongoing thread for this particular issue or is this just
something you're experimenting with?

If you suspect stray IPIs hitting isolated CPUs I'd recommend tracing with
these events enabled:
  ipi_send_cpu
  ipi_send_cpumask