Make the maxium TLB invalidate vectors depend on NR_CPUS linearly,
with a maximum of 32 vectors.

We currently only have 8 vectors for TLB invalidation and that is clearly
inadequate. If we have a lot of CPUs, the CPUs need share the 8 vectors and
tlbstate_lock is used to protect them. flush_tlb_page() is
heavily used in page reclaim, which will cause a lot of lock
contention for tlbstate_lock.

Andi Kleen suggested increasing the vectors number to 32, which should be
good for current typical systems to reduce the tlbstate_lock contention.

My test system has 4 sockets and 64G memory, and 64 CPUs. My
workload creates 64 processes. Each process mmap reads a big
empty sparse file. The total size of the files are 2*total_mem,
so this will cause a lot of page reclaim.

Below is the result I get from perf call-graph profiling:

 without the patch:
 ------------------

    24.25%           usemem  [kernel]                                   [k] _raw_spin_lock
                     |
                     --- _raw_spin_lock
                        |
                        |--42.15%-- native_flush_tlb_others

 with the patch:
 ------------------

    14.96%           usemem  [kernel]                                   [k] _raw_spin_lock
                     |
                     --- _raw_spin_lock
                        |--13.89%-- native_flush_tlb_others

So this heavily reduces the tlbstate_lock contention.
Suggested-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1295232727.1949.709.camel@sli10-conroe>
Signed-off-by: Ingo Molnar <mingo@elte.hu>