linux-kernel - Re: [PATCH v3 18/19] x86/resctrl: Add cpu offline callback for resctrl work

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Message-ID: <e965ab8e-5277-d2c2-5174-2159f050a19f@arm.com>
Date:   Thu, 27 Apr 2023 15:20:15 +0100
From:   James Morse <james.morse@....com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...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,
        xingxin.hx@...nanolis.org, baolin.wang@...ux.alibaba.com,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com
Subject: Re: [PATCH v3 18/19] x86/resctrl: Add cpu offline callback for
 resctrl work

Hi Ilpo,

On 21/03/2023 15:32, Ilpo Järvinen wrote:
> On Mon, 20 Mar 2023, James Morse wrote:
> 
>> The resctrl architecture specific code may need to free a domain when
>> a CPU goes offline, it also needs to reset the CPUs PQR_ASSOC register.
>> The resctrl filesystem code needs to move the overflow and limbo work
>> to run on a different CPU, and clear this CPU from the cpu_mask of
>> control and monitor groups.
>>
>> Currently this is all done in core.c and called from
>> resctrl_offline_cpu(), making the split between architecture and
>> filesystem code unclear.
>>
>> Move the filesystem work into a filesystem helper called
>> resctrl_offline_cpu(), and rename the one in core.c
>> resctrl_arch_offline_cpu().
>>
>> The rdtgroup_mutex is unlocked and locked again in the call in
>> preparation for changing the locking rules for the architecture
>> code.
>>
>> resctrl_offline_cpu() is called before any of the resource/domains
>> are updated, and makes use of the exclude_cpu feature that was
>> previously added.

>> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
>> index aafe4b74587c..4e5fc89dab6d 100644
>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>> @@ -578,22 +578,6 @@ static void domain_remove_cpu(int cpu, struct rdt_resource *r)
>>  
>>  		return;
>>  	}
>> -
>> -	if (r == &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl) {
>> -		if (is_mbm_enabled() && cpu == d->mbm_work_cpu) {
>> -			cancel_delayed_work(&d->mbm_over);
>> -			/*
>> -			 * exclude_cpu=-1 as this CPU has already been removed
>> -			 * by cpumask_clear_cpu()d
>> -			 */
> 
> This was added in 17/19 and now removed (not moved) in 18/19. Please avoid 
> such back-and-forth churn.

This is the cost of making small incremental changes that should be easier to review.
The intermediate step was a little odd, so came with a comment. (I normally mark those as
'temporary', but didn't bother this time as they are adjacent patches)

If you'd prefer, I can merge these patches together... but from Reinette's feedback its
likely I'll split them up even more.


Thanks,

James