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

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Message-ID: <567bd687-a69f-46fb-ab19-3f8d95c5e798@intel.com>
Date: Mon, 14 Oct 2024 19:39:07 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....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 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.

Reinette

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