sched/fair: Remove nohz.nr_cpus and use weight of cpumask instead

sched/fair: Remove nohz.nr_cpus and use weight of cpumask instead

nohz.nr_cpus was observed as contended cacheline when running
enterprise workload on large systems.

Fundamental scalability challenge with nohz.idle_cpus_mask
and nohz.nr_cpus is the following:

 (1) nohz_balancer_kick() observes (reads) nohz.nr_cpus
     (or nohz.idle_cpu_mask) and nohz.has_blocked to  see whether there's
     any nohz balancing work to do, in every scheduler tick.

 (2) nohz_balance_enter_idle() and nohz_balance_exit_idle()
     (through nohz_balancer_kick() via sched_tick()) modify (write)
     nohz.nr_cpus (and/or nohz.idle_cpu_mask) and nohz.has_blocked.

The characteristic frequencies are the following:

 (1) nohz_balancer_kick() happens at scheduler (busy)tick frequency
     on CPU(which has not gone idle). This is a relatively constant
     frequency  in the ~1 kHz range or lower.

 (2) happens at idle enter/exit frequency on every CPU that goes to idle.
     This is workload dependent, but can easily be hundreds of kHz for
     IO-bound loads and high CPU counts. Ie. can be orders of magnitude
     higher than (1), in which case a cachemiss at every invocation of (1)
     is almost inevitable. idle exit will trigger (1) on the CPU
     which is coming out of idle.

There's two types of costs from these functions:

 (A) scheduler tick cost via (1): this happens on busy CPUs too, and is
     thus a primary scalability cost. But the rate here is constant and
     typically much lower than (B), hence the absolute benefit to workload
     scalability will be lower as well.

 (B) idle cost via (2): going-to-idle and coming-from-idle costs are
     secondary concerns, because they impact power efficiency more than
     they impact scalability. But in terms of absolute cost this scales
     up with nr_cpus as well, and a much faster rate, and thus may also
     approach and negatively impact system limits like
     memory bus/fabric bandwidth.

Note that nohz.idle_cpus_mask and nohz.nr_cpus may appear to reside in the
same cacheline, however under CONFIG_CPUMASK_OFFSTACK=y the backing storage
for nohz.idle_cpus_mask will be elsewhere. With CPUMASK_OFFSTACK=n,
the nohz.idle_cpus_mask and rest of nohz fields are in different cachelines
under typical NR_CPUS=512/2048. This implies two separate cachelines
being dirtied upon idle entry / exit.

nohz.nr_cpus can be derived from the mask itself. Its usage doesn't warrant
a functionally correct value. This means one less cacheline being dirtied in
idle entry/exit path which helps to save some bus bandwidth w.r.t to those
nohz functions(approx 50%). This in turn helps to improve enterprise
workload throughput.

On system with 480 CPUs, running "hackbench 40 process 10000 loops"
(Avg of 3 runs)
baseline:
     0.81%  hackbench          [k] nohz_balance_exit_idle
     0.21%  hackbench          [k] nohz_balancer_kick
     0.09%  swapper            [k] nohz_run_idle_balance

With patch:
     0.35%  hackbench          [k] nohz_balance_exit_idle
     0.09%  hackbench          [k] nohz_balancer_kick
     0.07%  swapper            [k] nohz_run_idle_balance

[Ingo Molnar: scalability analysis changlog]

Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260115073524.376643-4-sshegde@linux.ibm.com