linux-kernel - Re: [PATCH v3 16/38] x86/resctrl: Move monitor init work to a resctrl init call

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Date: Mon, 1 Jul 2024 19:17:31 +0100
From: James Morse <james.morse@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Cc: Fenghua Yu <fenghua.yu@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 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>, Rex Nie <rex.nie@...uarmicro.com>,
 Dave Martin <dave.martin@....com>, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v3 16/38] x86/resctrl: Move monitor init work to a resctrl
 init call

Hi Reinette,

On 28/06/2024 17:47, Reinette Chatre wrote:
> On 6/14/24 8:00 AM, James Morse wrote:
>> rdt_get_mon_l3_config() is called from the architecture's
>> resctrl_arch_late_init(), and initialises both architecture specific
>> fields, such as hw_res->mon_scale and resctrl filesystem fields
>> by calling dom_data_init().
>>
>> To separate the filesystem and architecture parts of resctrl, this
>> function needs splitting up.
>>
>> Add resctrl_mon_resource_init() to do the filesystem specific work,
>> and call it from resctrl_init(). This runs later, but is still before
>> the filesystem is mounted and the rmid_ptrs[] array can be used.

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index 7d6aebce75c1..527c0e9d7b2e 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>> @@ -1016,12 +1016,28 @@ static void l3_mon_evt_init(struct rdt_resource *r)
>>           list_add_tail(&mbm_local_event.list, &r->evt_list);
>>   }
>>   +int resctrl_mon_resource_init(void)
> 
> (Lack of an __init is unexpected but I assume it was done since that will be removed
> in later patch anyway?)

Yup - I'll add and remove that if you find it surprising.


> This function needs a big warning to deter anybody from considering this to
> be the place where any and all monitor related allocations happen. It needs
> to warn developers that only resources that can only be touched after fs mount
> may be allocated here.

I'm afraid I don't follow. Can you give an example of the scenario you are worried about?

This is called from resctrl_init(), which is called once the architecture code has done
its setup, and reckons resctrl is something that can be supported on this platform. It
would be safe for the limbo/overflow callbacks to start ticking after this point - but
there is no point if the filesystem isn't mounted yet.
Filesystem mount is triggered through rdt_get_tree(). The filesystem can't be mounted
until resctrl_init() goes on to call register_filesystem().
These allocations could be made later (at mount time), but they're allocated once up-front
today.


I've added:
/**
 * resctrl_mon_resource_init() - Initialise monitoring structures.
 *
 * Allocate and initialise the rmid_ptrs[] used for the limbo and free lists.
 * Called once during boot after the struct rdt_resource's have been configured
 * but before the filesystem is mounted.
 */


Thanks,

James