proc/meminfo: avoid open coded reading of vm_committed_as

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 <feng.tang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Qian Cai <cai@lca.pw>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: kernel test robot <rong.a.chen@intel.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/1594389708-60781-1-git-send-email-feng.tang@intel.com
Link: http://lkml.kernel.org/r/1594389708-60781-2-git-send-email-feng.tang@intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>