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Subject[PATCH v4 0/4] make vm_committed_as_batch aware of vm overcommit policy
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

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
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 3,
as sysctl handler declaration follows the similar format of sibling

patch1: a cleanup for /proc/meminfo
patch2: a preparation patch which also improve the accuracy of
patch3: remove the VM_WARN_ONCE for vm_committed_as underflow check
patch4: main change

This is against today's linux-mm git tree on github.

Please help to review, thanks!

- Feng


* Remove the VM_WARN_ONCE check for vm_committed_as underflow,
thanks to Qian Cai for finding and testing the warning

* refine commit log and cleanup code, according to comments
from Michal Hocko and Matthew Wilcox
* change the lift from 16X and 64X after test

* add the sysctl handler to cover runtime overcommit policy
change, as suggested by Andres Morton
* address the accuracy concern of vm_memory_committed()
from Andi Kleen

Feng Tang (4):
proc/meminfo: avoid open coded reading of vm_committed_as
mm/util.c: make vm_memory_committed() more accurate
mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check
mm: adjust vm_committed_as_batch according to vm overcommit policy

fs/proc/meminfo.c | 2 +-
include/linux/mm.h | 2 ++
include/linux/mman.h | 4 ++++
kernel/sysctl.c | 2 +-
mm/mm_init.c | 18 ++++++++++++++----
mm/util.c | 22 +++++++++++++---------
6 files changed, 35 insertions(+), 15 deletions(-)


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