linux-kernel - Re: [PATCH v8 19/25] x86/resctrl: Auto assign/unassign counters when mbm_cntr

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Message-ID: <b4d9b572-4df3-4758-a40b-cb48fde0b595@amd.com>
Date: Tue, 15 Oct 2024 10:43:39 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 "Luck, Tony" <tony.luck@...el.com>
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Subject: Re: [PATCH v8 19/25] x86/resctrl: Auto assign/unassign counters when
 mbm_cntr_assign is enabled

Hi Reinette/Tony,

On 10/14/24 21:39,  wrote:
> Hi Babu,
> 
> On 10/14/24 9:35 AM, Moger, Babu wrote:
>> On 12/31/69 18:00, Luck, Tony wrote:
>  
>>>
>>> It is still the case that callers don't care about the return value.
>>
>> That is correct.
>>
> 
> Are you planning to change this? I think Tony has a good point that since
> assignment failures do not matter it unnecessarily complicates the code to
> have rdtgroup_assign_cntrs() return failure.
> 
> I also think the internals of rdtgroup_assign_cntrs() deserve a closer look.
> I assume that error handling within rdtgroup_assign_cntrs() was created with
> ABMC in mind. When only considering ABMC then the only reason why
> rdtgroup_assign_cntr_event() could fail is if the system ran out of counters
> and then indeed it makes no sense to attempt another call to rdtgroup_assign_cntr_event().
> 
> Now that the resctrl fs/arch split is clear the implementation does indeed expose
> another opportunity for failure ... if the arch callback, resctrl_arch_config_cntr()
> fails. It could thus be possible for the first rdtgroup_assign_cntr_event() to fail
> while the second succeeds. Earlier [1], Tony suggested to, within rdtgroup_assign_cntrs(),
> remove the local ret variable and have it return void. This sounds good to me.
> When doing so a function comment explaining the usage will be helpful.
> 
> I also think that rdtgroup_unassign_cntrs() deserves similar scrutiny. Even more
> so since I do not think that the second rdtgroup_unassign_cntr_event()
> should be prevented from running if the first rdtgroup_unassign_cntr_event() fails.


Sounds fine with me. Now it will look like this below.


static void rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
{
  struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;

 if (!resctrl_arch_mbm_cntr_assign_enabled(r))
      return;

 if (is_mbm_total_enabled())
   rdtgroup_assign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_TOTAL_EVENT_ID);

 if (is_mbm_local_enabled())
   rdtgroup_assign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_LOCAL_EVENT_ID);

}

/*
 * Called when a group is deleted. Counters are unassigned if it was in
 * assigned state.
 */
static void rdtgroup_unassign_cntrs(struct rdtgroup *rdtgrp)
{
  struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;

  if (!resctrl_arch_mbm_cntr_assign_enabled(r))
       return;

 if (is_mbm_total_enabled())
 rdtgroup_unassign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_TOTAL_EVENT_ID);

 if (is_mbm_local_enabled())
 rdtgroup_unassign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_LOCAL_EVENT_ID);

}


> 
> Reinette
> 
> [1] https://lore.kernel.org/all/ZwldvDBjEA3TSw2k@agluck-desk3.sc.intel.com/
> 

-- 
Thanks
Babu Moger