linux-kernel - Re: [PATCH v1 01/31] x86/resctrl: Fix allocation of cleanest CLOSID on platforms with no monitors

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Date: Tue, 9 Apr 2024 10:05:33 +0200
From: David Hildenbrand <david@...hat.com>
To: James Morse <james.morse@....com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Cc: 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,
 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,
 Rex Nie <rex.nie@...uarmicro.com>, Dave Martin <dave.martin@....com>
Subject: Re: [PATCH v1 01/31] x86/resctrl: Fix allocation of cleanest CLOSID
 on platforms with no monitors

On 21.03.24 17:50, James Morse wrote:
> commit 6eac36bb9eb0 ("x86/resctrl: Allocate the cleanest CLOSID by
> searching closid_num_dirty_rmid") added a Kconfig option that causes
> resctrl to search for the CLOSID with the fewest dirty cache lines when
> creating a new control group. This depends on the values read from the
> llc_occupancy counters.
> 
> This support missed that some platforms may not have these counters.
> This causes a NULL pointer dereference when creating a new control
> group as the array was not allocated by dom_data_init().
> 
> As this feature isn't necessary on platforms that don't have cache
> occupancy monitors, add this to the check that occurs when a new
> control group is allocated.
> 
> The existing code is not selected by any upstream platform, it makes
> no sense to backport this patch to stable.
> 

It's weird to not see RESCTRL_RMID_DEPENDS_ON_CLOSID appear in any Kconfig file.
I guess it will all make sense once the refactoring is done :)

As Reinette comments, likely we want here:

Fixes: 6eac36bb9eb0 ("x86/resctrl: Allocate the cleanest CLOSID by searching closid_num_dirty_rmid")

> Signed-off-by: James Morse <james.morse@....com>
> ---
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 011e17efb1a6..1767c1affa60 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -149,7 +149,8 @@ static int closid_alloc(void)
>   
>   	lockdep_assert_held(&rdtgroup_mutex);
>   
> -	if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) {
> +	if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID) &&
> +	    is_llc_occupancy_enabled()) {
>   		cleanest_closid = resctrl_find_cleanest_closid();
>   		if (cleanest_closid < 0)
>   			return cleanest_closid;

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb