Re: [PATCH 2/2] s390/cpum_cf: Honor hwctr session cpu mask during hotplug

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] s390/cpum_cf: strict mask enforcement breaks legacy lshwc tools
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commit d3def3a5308e1d518c4fe69c3d9a0fc29e832d28
Author: Sumanth Korikkar <sumanthk@linux.ibm.com>

s390/cpum_cf: Honor hwctr session cpu mask during hotplug

This commit updates cfset_online_cpu() and cfset_offline_cpu() to start and
stop counter sets only for active hwctr sessions that include the CPU in
their requested mask. It prevents the kernel from dynamically modifying
the session mask on each hotplug event.

> diff --git a/arch/s390/kernel/perf_cpum_cf.c b/arch/s390/kernel/perf_cpum_cf.c
> index 1d63dc3b0ab99..a2334af515447 100644
> --- a/arch/s390/kernel/perf_cpum_cf.c
> +++ b/arch/s390/kernel/perf_cpum_cf.c
> @@ -1778,17 +1779,19 @@ static int cfset_online_cpu(unsigned int cpu)
>  
>  	if (!list_empty(&cfset_session.head)) {
>  		list_for_each_entry(rp, &cfset_session.head, node) {
> +			if (!cpumask_test_cpu(cpu, &rp->mask))
> +				continue;
> +

[Severity: Critical]
Could this strict mask enforcement cause a userspace regression with legacy
lshwc tools?

Legacy versions of lshwc initiate 'allcpu' sessions by providing the currently
online CPU mask instead of the possible CPU mask.

When a new CPU is hotplugged, this strict evaluation will silently exclude the
new CPU, preventing the legacy behavior of dynamically expanding the session
mask to newly onlined CPUs.

Since this change alters the semantic behavior of hwctr sessions, will systems
using legacy s390-tools on an updated kernel encounter incomplete data for newly
hotplugged CPUs?

Breaking existing userspace applications, even when fixing a bug such as the
incorrect inclusion of CPUs in subset masks, is generally considered an ABI
regression.

>  			p.sets = rp->ctrset;
>  			cfset_ioctl_on(&p);
> -			cpumask_set_cpu(cpu, &rp->mask);
>  		}
>  	}
>  	return 0;
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/20261006111304.4023017-1-sumanthk@linux.ibm.com?part=2