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Message-ID: <b504ff21-4941-49e5-b1cb-3e5ed5177836@redhat.com>
Date: Tue, 20 Feb 2024 17:44:50 +0100
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Subject: Re: [PATCH v9 09/24] x86/resctrl: Use __set_bit()/__clear_bit()
 instead of open coding
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To: James Morse <james.morse@arm.com>, x86@kernel.org,
 linux-kernel@vger.kernel.org
Cc: Fenghua Yu <fenghua.yu@intel.com>,
 Reinette Chatre <reinette.chatre@intel.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
 Babu Moger <Babu.Moger@amd.com>, shameerali.kolothum.thodi@huawei.com,
 D Scott Phillips OS <scott@os.amperecomputing.com>,
 carl@os.amperecomputing.com, lcherian@marvell.com,
 bobo.shaobowang@huawei.com, tan.shaopeng@fujitsu.com,
 baolin.wang@linux.alibaba.com, Jamie Iles <quic_jiles@quicinc.com>,
 Xin Hao <xhao@linux.alibaba.com>, peternewman@google.com,
 dfustini@baylibre.com, amitsinght@marvell.com
References: <20240213184438.16675-1-james.morse@arm.com>
 <20240213184438.16675-10-james.morse@arm.com>
 <b5bc883a-8a8a-4f07-a6ff-b1bd02ffc99d@redhat.com>
 <91b58253-f682-49f7-914d-8faf1ce181ef@arm.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <91b58253-f682-49f7-914d-8faf1ce181ef@arm.com>
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On 20.02.24 17:27, James Morse wrote:
> Hi David,
> 
> On 20/02/2024 16:00, David Hildenbrand wrote:
>> 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.
> 
>>> 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())
> 
> Yup. I have this as something to post after all the MPAM changes as it's not necessary to
> get MPAM going. The patch[0] uses the bitmap APIs you suggest to remove the fixed limit on
> the number of CLOSID/PARTID.
> MPAM systems are being built with more than 32, but will work without that patch.

Make sense, thanks!

-- 
Cheers,

David / dhildenb