When SQL Server uses a portion of the server processors , the is_online column on the sys.dm_os_schedulers will return 0.

 Basically , the scheduler will not be used for query processing. The situation is caused by either affinity masking or licensing restrictions

 Be careful if you attempt to change these number – either be confident you know how to use affinity masking or if Virtualization is used , speak to the VM administrator about apportioning more cores per virtual socket. In either case analyse the impact and test thoroughly

 Read more on Amount of sql server processors with sys.dm_os_schedulers

--how many CPUs is SQL Server actually using
select cpu_id,scheduler_id,is_online,status from sys.dm_os_schedulers where status = 'VISIBLE ONLINE'

--how many CPUs are not being used
select cpu_id,scheduler_id,is_online,status from sys.dm_os_schedulers where status = 'VISIBLE OFFLINE'

--how many CPUs an instance can see
select cpu_count from sys.dm_os_sys_info

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SQL Server – SOS_SCHEDULER_YIELD and how to reduce it