linux-kernel - Re: [PATCH] x86/resctrl: Clear the staged configs when destroying schemata list

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Message-ID: <7728c846-0077-a34d-7fdc-bab9743edc4f@arm.com>
Date:   Wed, 28 Sep 2022 15:08:47 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        Shawn Wang <shawnwang@...ux.alibaba.com>, fenghua.yu@...el.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: Clear the staged configs when destroying
 schemata list

Hi Reinette,

On 27/09/2022 22:21, Reinette Chatre wrote:
> On 9/27/2022 6:06 AM, James Morse wrote:
>> On 27/09/2022 03:54, Shawn Wang wrote:

>>> The problem can be reproduced by the following commands:
>>>     # A system with 16 usable closids and mba disabled
>>>     mount -t resctrl resctrl -o cdp /sys/fs/resctrl
>>>     mkdir /sys/fs/resctrl/p{1..7}
>>>     umount /sys/fs/resctrl/
>>>     mount -t resctrl resctrl /sys/fs/resctrl
>>>     mkdir /sys/fs/resctrl/p{1..8}
>>
>> Thanks for the reproducer - but I don't see what could set have_new_ctrl in this sequence.
>> You can't call apply_config() to set CPUs in the mask without that being set.
>>
>> Creating a new control group, (your mkdir step) shouldn't touch the hardware at all, as it
>> should be left in its reset state from the last umount(), or setup.
> 
> There is an attempt to configure the hardware in the mkdir path:
> rdtgroup_mkdir()->rdtgroup_mkdir_ctrl_mon()->rdtgroup_init_alloc()->resctrl_arch_update_domains()

Aha! I'm not sure why my grepping around didn't find this.

This is a path that doesn't memset() the staged config first, so that explains it.

[..]

> What do you think about clearing the staged config within resctrl_arch_update_domains()
> after the configuration is complete and there is no more need for it? That may reduce
> complexity where each caller no longer need to remember to do so.
> I see "staged_config" as a temporary storage and it my help to understand the code better
> if it is treated as such. 

Yup, that would it with the idea of the value being consumed by
resctrl_arch_update_domains(), which is how I think of it.


Thanks,

James