proc/meminfo: avoid open coded reading of vm_committed_as

Patch series "make vm_committed_as_batch aware of vm overcommit policy", v6.

When checking a performance change for will-it-scale scalability mmap test
[1], we found very high lock contention for spinlock of percpu counter
'vm_committed_as':

    94.14%     0.35%  [kernel.kallsyms]         [k] _raw_spin_lock_irqsave
    48.21% _raw_spin_lock_irqsave;percpu_counter_add_batch;__vm_enough_memory;mmap_region;do_mmap;
    45.91% _raw_spin_lock_irqsave;percpu_counter_add_batch;__do_munmap;

Actually this heavy lock contention is not always necessary.  The
'vm_committed_as' needs to be very precise when the strict
OVERCOMMIT_NEVER policy is set, which requires a rather small batch number
for the percpu counter.

So keep 'batch' number unchanged for strict OVERCOMMIT_NEVER policy, and
enlarge it for not-so-strict OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS
policies.

Benchmark with the same testcase in [1] shows 53% improvement on a 8C/16T
desktop, and 2097%(20X) on a 4S/72C/144T server.  And for that case,
whether it shows improvements depends on if the test mmap size is bigger
than the batch number computed.

We tested 10+ platforms in 0day (server, desktop and laptop).  If we lift
it to 64X, 80%+ platforms show improvements, and for 16X lift, 1/3 of the
platforms will show improvements.

And generally it should help the mmap/unmap usage,as Michal Hocko
mentioned:

: I believe that there are non-synthetic worklaods which would benefit
: from a larger batch. E.g. large in memory databases which do large
: mmaps during startups from multiple threads.

Note: There are some style complain from checkpatch for patch 4, as sysctl
handler declaration follows the similar format of sibling functions

[1] https://lore.kernel.org/lkml/20200305062138.GI5972@shao2-debian/

This patch (of 4):

Use the existing vm_memory_committed() instead, which is also convenient
for future change.

Signed-off-by: Feng Tang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Qian Cai <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Tejun Heo <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

fs/proc/meminfo.c

@@ -41,7 +41,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 
 	si_meminfo(&i);
 	si_swapinfo(&i);
-	committed = percpu_counter_read_positive(&vm_committed_as);
+	committed = vm_memory_committed();
 
 	cached = global_node_page_state(NR_FILE_PAGES) -
 			total_swapcache_pages() - i.bufferram;