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Message-ID: <88097441-534d-4b81-9801-680334092ab8@arm.com>
Date: Wed, 21 Feb 2024 12:06:46 +0000
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Subject: Re: [PATCH] x86/resctrl: Fix WARN in get_domain_from_cpu()
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To: Tony Luck <tony.luck@intel.com>,
 "Chatre, Reinette" <reinette.chatre@intel.com>,
 Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "Yu, Fenghua" <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
 H Peter Anvin <hpa@zytor.com>, Babu Moger <Babu.Moger@amd.com>,
 "shameerali.kolothum.thodi@huawei.com"
 <shameerali.kolothum.thodi@huawei.com>,
 D Scott Phillips OS <scott@os.amperecomputing.com>,
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 David Hildenbrand <david@redhat.com>
References: <20240213184438.16675-1-james.morse@arm.com>
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 <d409c32d-0f9a-4773-bc25-9d39de3c9e9b@intel.com>
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From: James Morse <james.morse@arm.com>
In-Reply-To: <ZdVFDIJmctsNaGd2@agluck-desk3>
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Hi Tony,

On 21/02/2024 00:34, Tony Luck wrote:
> reset_all_ctrls() and resctrl_arch_update_domains() use
> on_each_cpu_mask() to call rdt_ctrl_update() on potentially
> one CPU from each domain.
> 
> But this means rdt_ctrl_update() needs to figure out which domain
> to apply changes to. Doing so requires a search of all domains
> in a resource, which can only be done safely if cpus_lock is
> held. Both callers do hold this lock, but there isn't a way
> for a function called on another CPU via IPI to verify this.
> 
> Fix by adding the target domain to the msr_param structure and
> calling for each domain separately using smp_call_function_single()

Cunning - this trades the memory allocation for multiple IPI. I think this is much better
for the case where only the local domains configuration is modified.

With the double-IPI when both CDP configurations are changed fixed:
Reviewed-by: James Morse <james.morse@arm.com>

I think we should rip out the false positive check, I'll post a patch to do that.
I'll double check this was the only IPI path, if so its safe again after this patch and we
can add lockdep_assert_cpus_held(). If anyone ever hits this during a bisect its should be
clear(er).


Thanks,

James