linux-kernel - Re: [PATCH] sched: simplify sched_init

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Message-ID: <xhsmh8qjfith8.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 19 Aug 2025 10:52:19 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Srikar Dronamraju <srikar@...ux.ibm.com>, Yury Norov <yury.norov@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel
 Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: simplify sched_init_numa()

On 21/07/25 14:02, Srikar Dronamraju wrote:
> * Yury Norov <yury.norov@...il.com> [2025-07-19 17:07:51]:
>
>> From: Yury Norov (NVIDIA) <yury.norov@...il.com>
>>
>> The function opencodes for_each_set_bit() macro.
>>
>> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
>> ---
>>  kernel/sched/topology.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index b958fe48e020..7dc3c79aa480 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -1932,10 +1932,9 @@ void sched_init_numa(int offline_node)
>>              return;
>>      }
>>
>> -	for (i = 0, j = 0; i < nr_levels; i++, j++) {
>> -		j = find_next_bit(distance_map, NR_DISTANCE_VALUES, j);
>> -		distances[i] = j;
>> -	}
>> +	for_each_set_bit(j, distance_map, NR_DISTANCE_VALUES)
>> +		distances[i++] = j;
>> +
>
> Dont we need to reset the value of i;
>

That.

> Also now we may be iterating for NR_DISTANCE_VALUES instead of nr_levels.
> It should be okay, since NR_DISTANCE_VALUES is just 8.
>

DISTANCE_BITS is 8, NR_DISTANCE_VALUES is 1 << 8 aka 256.

But here the use of for_each_set_bit() means we'll get just one extra
iteration compared to using `nr_levels`. That said, since we *have* to
compute `nr_levels` to allocate sched_domains_numa_distance, IMO we're not
gaining much by using for_each_set_bit() here.

>>      rcu_assign_pointer(sched_domains_numa_distance, distances);
>>
>>      bitmap_free(distance_map);
>> --
>> 2.43.0
>>
>
> --
> Thanks and Regards
> Srikar Dronamraju