Re: [PATCH 0/3] make vm_committed_as_batch aware of vm overcommit policy

[Qian Cai]

On Fri, May 08, 2020 at 03:25:14PM +0800, Feng Tang wrote:
> 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 lift the batch number to 16X for OVERCOMMIT_ALWAYS and
> OVERCOMMIT_GUESS policies, and add a sysctl handler to adjust it
> when the policy is reconfigured.
> 
> 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 no change
> for some platforms, due to the test mmap size of the case is bigger
> than the batch number computed, though the patch will help mmap/munmap
> generally.
> 
> [1] https://lkml.org/lkml/2020/3/5/57
> 
> There are some style complain from checkpatch for patch 3, as it
> follows the similar format of sibling functions
> 
> patch1: a cleanup for /proc/meminfo
> patch2: a preparation patch which also improve the accuracy of
>         vm_memory_committed
> patch3: the main change
> 
> Please help to review, thanks!

Reverted this series fixed a warning under memory pressue.

[ 3319.257898] LTP: starting oom01
[ 3319.284417] ------------[ cut here ]------------
[ 3319.284439] memory commitment underflow
[ 3319.284456] WARNING: CPU: 13 PID: 130949 at mm/util.c:835 __vm_enough_memory+0x1dc/0x1f0
[ 3319.284485] Modules linked in: brd ext4 crc16 mbcache jbd2 loop kvm_hv kvm ip_tables x_tables xfs sd_mod ahci libahci bnx2x libata mdio tg3 libphy firmware_class dm_mirror dm_region_hash dm_log dm_mod
[ 3319.284538] CPU: 13 PID: 130949 Comm: oom01 Not tainted 5.7.0-rc6-next-20200521+ #115
[ 3319.284551] NIP:  c00000000043027c LR: c000000000430278 CTR: c0000000007bd120
[ 3319.284572] REGS: c00000064a42f710 TRAP: 0700   Not tainted  (5.7.0-rc6-next-20200521+)
[ 3319.284602] MSR:  900000000282b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 28222422  XER: 20040000
[ 3319.284638] CFAR: c000000000119098 IRQMASK: 0 
               GPR00: c000000000430278 c00000064a42f9a0 c000000001765500 000000000000001b 
               GPR04: c00000000179c3b0 000000000000000b 0000000066726564 c00000000429ac28 
               GPR08: 0000000000000001 0000000000000000 0000000000000000 c000001301facc80 
               GPR12: 0000000000002000 c000001fffff3d80 0000000000000000 0000000000000000 
               GPR16: 0000000000000000 0000000000000000 c0000014f358dd60 fffffffffff7dfff 
               GPR20: c0000003725a9128 c0000003725a3928 c000000b0a0c0670 c000000b0a0c0678 
               GPR24: c000000b0a0c0660 c00000064a42fa58 c000000b0a0c0650 c0000003725a9080 
               GPR28: 0000000000000001 0000000000000000 c000000001620178 0000000000000001 
[ 3319.284825] NIP [c00000000043027c] __vm_enough_memory+0x1dc/0x1f0
[ 3319.284853] LR [c000000000430278] __vm_enough_memory+0x1d8/0x1f0
[ 3319.284872] Call Trace:
[ 3319.284889] [c00000064a42f9a0] [c000000000430278] __vm_enough_memory+0x1d8/0x1f0 (unreliable)
[ 3319.284930] [c00000064a42fa30] [c000000000114258] dup_mm+0x2a8/0x700
[ 3319.284960] [c00000064a42fae0] [c000000000115c7c] copy_process+0xeac/0x1a00
[ 3319.284981] [c00000064a42fbf0] [c000000000116b3c] _do_fork+0xac/0xce0
[ 3319.285002] [c00000064a42fd00] [c000000000117928] __do_sys_clone+0x98/0xe0
[ 3319.285032] [c00000064a42fdc0] [c000000000039e78] system_call_exception+0x108/0x1d0
[ 3319.285064] [c00000064a42fe20] [c00000000000c9f0] system_call_common+0xf0/0x278
[ 3319.285093] Instruction dump:
[ 3319.285109] 60000000 73a90001 4182000c e8be0002 4bfffeb8 3c62ff51 39200001 3d42fff9 
[ 3319.285142] 38636690 992a6ad6 4bce8dbd 60000000 <0fe00000> e8be0002 4bfffe90 60000000 
[ 3319.285166] irq event stamp: 0
[ 3319.285183] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
[ 3319.285195] hardirqs last disabled at (0): [<c0000000001154c8>] copy_process+0x6f8/0x1a00
[ 3319.285216] softirqs last  enabled at (0): [<c0000000001154c8>] copy_process+0x6f8/0x1a00
[ 3319.285238] softirqs last disabled at (0): [<0000000000000000>] 0x0
[ 3319.285267] ---[ end trace 115e3f7fbb0653c2 ]---

> 
> - Feng
> 
> ----------------------------------------------------------------
> Changelog:
>   v2:
>      * 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 (3):
>   proc/meminfo: avoid open coded reading of vm_committed_as
>   mm/util.c: make vm_memory_committed() more accurate
>   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         | 19 +++++++++++++++----
>  mm/util.c            | 15 ++++++++++++++-
>  6 files changed, 37 insertions(+), 7 deletions(-)
> 
> -- 
> 2.7.4
> 
>