linux-kernel - Re: [PATCH v9 09/24] x86/resctrl: Use __set_bit()/__clear

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Date: Tue, 20 Feb 2024 17:00:46 +0100
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
Subject: Re: [PATCH v9 09/24] x86/resctrl: Use __set_bit()/__clear_bit()
 instead of open coding

On 13.02.24 19:44, James Morse wrote:
> The resctrl CLOSID allocator uses a single 32bit word to track which
> CLOSID are free. The setting and clearing of bits is open coded.
> 
> Convert the existing open coded bit manipulations of closid_free_map
> to use __set_bit() and friends. These don't need to be atomic as this
> list is protected by the mutex.
> 
> Signed-off-by: James Morse <james.morse@....com>
> Tested-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Tested-by: Peter Newman <peternewman@...gle.com>
> Tested-by: Babu Moger <babu.moger@....com>
> Tested-by: Carl Worth <carl@...amperecomputing.com> # arm64
> Reviewed-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>
> Reviewed-by: Babu Moger <babu.moger@....com>
> ---
> Changes since v6:
>   * Use the __ inatomic helpers and add lockdep_assert_held() annotations to
>     document how this is safe.
>   * Fixed a resctrl_closid_is_free()/closid_allocated() rename in the commit
>     message.
>   * Use RESCTRL_RESERVED_CLOSID to improve readability.
> 
> Changes since v7:
>   * Removed paragraph explaining why this should be done now due to badword
>     'subsequent'.
>   * Changed a comment to refer to RESCTRL_RESERVED_CLOSID.
> ---
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 18 ++++++++++++------
>   1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index dcffd1c4a476..bc6e0f83c847 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -111,7 +111,7 @@ void rdt_staged_configs_clear(void)
>    * - Our choices on how to configure each resource become progressively more
>    *   limited as the number of resources grows.
>    */

That comment talks about "free CLOSIDs in a single integer". Once could 
think about rephrasing that to "free CLOSIDs in a simple bitmap."

> -static int closid_free_map;
> +static unsigned long closid_free_map;
>   static int closid_free_map_len;
>   
>   int closids_supported(void)
> @@ -130,8 +130,8 @@ static void closid_init(void)
>   
>   	closid_free_map = BIT_MASK(rdt_min_closid) - 1;

Now that we use "unsigned long", I was wondering for a second if we 
should use "proper" bitmap functions here like

	bitmap_fill(closid_free_map, rdt_min_closid);

and converting the alloc path (replace ffs() in closid_alloc()):

	closid = find_first_bit(closid_free_map, closid_free_map_len);
	if (closid == closid_free_map_len)
		return -ENOSPC;
	__clear_bit(closid, &closid_free_map);

(would get rid of the closid-- in closid_alloc())

Just a thought, so

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

-- 
Cheers,

David / dhildenb