linux-kernel - Re: [PATCH v3 15/38] x86/resctrl: Move monitor exit work to a restrl exit call

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Date: Mon, 1 Jul 2024 19:17:48 +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 15/38] x86/resctrl: Move monitor exit work to a restrl
 exit call

Hi Reinette,

On 28/06/2024 17:46, Reinette Chatre wrote:
> On 6/14/24 8:00 AM, James Morse wrote:
>> rdt_put_mon_l3_config() is called via the architecture's
>> resctrl_arch_exit() call, and appears to free the rmid_ptrs[]
>> and closid_num_dirty_rmid[] arrays. In reality this code is marked
>> __exit, and is removed by the linker as resctl can't be built
>> as a module.
>>
>> To separate the filesystem and architecture parts of resctrl,
>> this free()ing work needs to be triggered by the filesystem,
>> as these structures belong to the filesystem code.
>>
>> Rename rdt_put_mon_l3_config() resctrl_mon_resource_exit()
>> and call it from resctrl_exit(). The kfree() is currently
>> dependent on r->mon_capable. resctrl_mon_resource_init()

> resctrl_mon_resource_init() does not exist at this point making
> this motivation difficult to follow.

Right - I re-ordered the patches to make the diffs simpler. I'll drop this paragraph.


>> takes no arguments, so resctrl_mon_resource_exit() shouldn't
>> take any either. Add the check to dom_data_exit(), making it
>> take the resource as an argument. This makes it more symmetrical
>> with dom_data_init().

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index 3e5375c365e6..7d6aebce75c1 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>> @@ -965,10 +965,12 @@ static int dom_data_init(struct rdt_resource *r)
>>       return err;
>>   }
>>   -static void __exit dom_data_exit(void)
>> +static void dom_data_exit(struct rdt_resource *r)
>>   {
>> -    mutex_lock(&rdtgroup_mutex);
>> +    if (!r->mon_capable)
>> +        return;
>>   +    mutex_lock(&rdtgroup_mutex);
> 
> I know there has been a bit of back&forth on whether the mutex is needed
> here. With this change moving dom_data_exit() out from __exit I think
> the locking should aim to be consistent with existing runtime
> and thus the check of r->mon_capable should be with mutex held. Having
> this little snippet outside mutex will just cause confusion.

> Do you have motivation for needing this be done outside of mutex?

Just to avoid sleeping - then returning having done nothing.


> I think it
> ended up this way with this patch aiming to keep existing flow exactly,
> but that ended up as convenience in a flow where mutex was not really
> needed at all.

Sure - I don't think it matters either way. Its not a path where performance matters.
The property is read-only from when resctrl_init() is called. I'll make it look like
resctrl_offline_domain().


Thanks,

James