ARC: [mm] optimize needless full mm TLB flush on munmap

munmap ends up calling tlb_flush() which for ARC was flushing the entire
TLB unconditionally (by moving the MMU to a new ASID)

do_munmap
  unmap_region
    unmap_vmas
      unmap_single_vma
         unmap_page_range
            tlb_start_vma
            zap_pud_range
            tlb_end_vma()
  tlb_finish_mmu
    tlb_flush()  ---> unconditional flush_tlb_mm()

So even a single page munmap, a frequent operation when uClibc dynamic
linker (ldso) is loading the dependent shared libraries, would move the
the ASID multiple times - needlessly invalidating the pre-faulted TLB
entries (and increasing the rate of ASID wraparound + full TLB flush).

This is now optimised to only be called if tlb->full_mm (which means
for exit/execve) cases only. And for those cases, flush_tlb_mm() is
already optimised to be a no-op for mm->mm_users == 0.

So essentially there are no mmore full mm flushes - except for fork which
anyhow needs it for properly COW'ing parent address space.

munmap now needs to do TLB range flush, which is implemented with
tlb_end_vma()

Results
-------
1. ASID now consistenly moves by 4 during a simple ls (as opposed to 5 or
   7 before).

2. LMBench microbenchmark also shows improvements

Basic system parameters
------------------------------------------------------------------------------
Host                 OS Description              Mhz  tlb  cache  mem scal
                                                     pages line   par load
                                                           bytes
--------- ------------- ----------------------- ---- ----- ----- ------ ----
3.9-rc5-0 Linux 3.9.0-r 3.9-rc5-0404-gcc-4.4-ba   80     8    64 1.1000 1
3.9-rc5-0 Linux 3.9.0-r 3.9-rc5-0405-avoid-full   80     8    64 1.1200 1

Processor, Processes - times in microseconds - smaller is better
------------------------------------------------------------------------------
Host                 OS  Mhz null null      open slct sig  sig  fork exec sh
                             call  I/O stat clos TCP  inst hndl proc proc proc
--------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
3.9-rc5-0 Linux 3.9.0-r   80 4.81 8.69 68.6 118. 239. 8.53 31.6 4839 13.K 34.K
3.9-rc5-0 Linux 3.9.0-r   80 4.46 8.36 53.8 91.3 223. 8.12 24.2 4725 13.K 33.K

File & VM system latencies in microseconds - smaller is better
-------------------------------------------------------------------------------
Host                 OS   0K File      10K File     Mmap    Prot   Page 100fd
                        Create Delete Create Delete Latency Fault  Fault selct
--------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
3.9-rc5-0 Linux 3.9.0-r  314.7  223.2 1054.9  390.2  3615.0 1.590 20.1 126.6
3.9-rc5-0 Linux 3.9.0-r  265.8  183.8 1014.2  314.1  3193.0 6.910 18.8 110.4
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/tlb.h

@@ -21,20 +21,28 @@
 
 #ifndef __ASSEMBLY__
 
-#define tlb_flush(tlb) local_flush_tlb_mm((tlb)->mm)
+#define tlb_flush(tlb)				\
+do {						\
+	if (tlb->fullmm)			\
+		flush_tlb_mm((tlb)->mm);	\
+} while (0)
 
 /*
  * This pair is called at time of munmap/exit to flush cache and TLB entries
  * for mappings being torn down.
  * 1) cache-flush part -implemented via tlb_start_vma( ) can be NOP (for now)
  *    as we don't support aliasing configs in our VIPT D$.
- * 2) tlb-flush part - implemted via tlb_end_vma( ) can be NOP as well-
- *    albiet for difft reasons - its better handled by moving to new ASID
+ * 2) tlb-flush part - implemted via tlb_end_vma( ) flushes the TLB range
  *
  * Note, read http://lkml.org/lkml/2004/1/15/6
  */
 #define tlb_start_vma(tlb, vma)
-#define tlb_end_vma(tlb, vma)
+
+#define tlb_end_vma(tlb, vma)						\
+do {									\
+	if (!tlb->fullmm)						\
+		flush_tlb_range(vma, vma->vm_start, vma->vm_end);	\
+} while (0)
 
 #define __tlb_remove_tlb_entry(tlb, ptep, address)