sched/cache: Skip cache-aware scheduling for single-threaded processes

sched/cache: Skip cache-aware scheduling for single-threaded processes

For a single thread, the current wakeup path tends to place it
on the same LLC where it was previously running with cache-hot
data. There is no need to enable cache-aware scheduling for
single-threaded processes for the following reasons:

1. Cache-aware scheduling primarily benefits multi-threaded
   processes where threads share data. Single-threaded processes
   typically have no inter-thread data sharing and thus gain little.

2. Enabling it incurs the additional overhead of tracking the
   thread's residency in the LLCs.

3. Bypassing single-threaded processes avoids excessive
   concentration of such tasks on a single LLC.

Nevertheless, this check can be omitted if users explicitly
provide hints for such single-threaded workloads where different
processes have shared memory, e.g., via prctl() or other interfaces
to be added in the future.

Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Tingyin Duan <tingyin.duan@gmail.com>
Link: https://patch.msgid.link/8a59a13aa58fdb48e410ecb2aabd97fe3ea5d256.1778703694.git.tim.c.chen@linux.intel.com