linux-kernel - Re: [PATCH v9 00/24] x86/resctrl: monitored closid+rmid together, separate arch/fs locking

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Date: Wed, 21 Feb 2024 12:06:34 +0000
From: James Morse <james.morse@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 Tony Luck <tony.luck@...el.com>, Thomas Gleixner <tglx@...utronix.de>
Cc: Borislav Petkov <bp@...en8.de>, x86@...nel.org,
 linux-kernel@...r.kernel.org, Fenghua Yu <fenghua.yu@...el.com>,
 Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
 Babu Moger <Babu.Moger@....com>, shameerali.kolothum.thodi@...wei.com,
 D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v9 00/24] x86/resctrl: monitored closid+rmid together,
 separate arch/fs locking

Hi Tony, Reinette,

On 20/02/2024 22:58, Reinette Chatre wrote:
> On 2/20/2024 12:59 PM, Tony Luck wrote:
>> On Mon, Feb 19, 2024 at 05:49:29PM +0100, Thomas Gleixner wrote:
>>> On Sat, Feb 17 2024 at 11:55, Borislav Petkov wrote:
>>>
>>>> On Tue, Feb 13, 2024 at 06:44:14PM +0000, James Morse wrote:
>>>>> Hello!
>>>>>
>>>>> It's been back and forth for whether this series should be rebased onto Tony's
>>>>> SNC series. This version isn't, its based on tip/x86/cache.
>>>>> (I have the rebased-and-tested versions if anyone needs them)
>>>>
>>>> The set applied ontop of tip:x86/cache gives:

>> Testing tip x86/cache that WARN fires while running
>> tools/tests/selftests/resctrl/resctrl_test.

I evidently need to build a newer version of that tool.


>> Everthing runs OK if I drop the top commit:
>>   fb700810d30b ("x86/resctrl: Separate arch and fs resctrl locks")
> 
> The new WARN_ON_ONCE() is why this encountered. The comment notes that
> lockdep_is_cpus_held() is used to determine if "someone is holding the
> CPUs lock" but it seems that lockdep_is_cpus_held() still only checks
> if "current" is holding cpu_hotplug_lock and that is not possible
> when running the code via IPI.

I was evidently mistaken that this was the difference between
lockdep_is_cpus_held() and lockdep_assert_cpus_held().

It's a false positive, ripping out the check is the simplest thing to do.


> So even though it is confirmed via lockdep_assert_cpus_held() that
> resctrl_arch_update_domains() holds cpu_hotplug_lock, it does not seem possible
> to have a similar lockdep check in the function called by it (resctrl_arch_update_domains())
> via IPI. It thus does not look like that lockdep checking within
> get_domain_from_cpu() can be accurate and I cannot see what it can be replaced with
> to make it accurate. Any guidance will be appreciated. Perhaps we should just drop (but
> with detailed context comments remaining) the lockdep check in get_domain_from_cpu()? 


Thanks,

James