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

[Andi Kleen <andi.kleen@intel.com>]

On Tue, May 26, 2020 at 02:14:59PM -0400, Qian Cai wrote:
> On Thu, May 21, 2020 at 05:27:26PM -0400, Qian Cai wrote:
> > 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.
> 
> Andrew, Stephen, can you drop this series?

Hope you're happy now.

The warning is just not needed for the always case. The whole point
of the patch was to maintain these counters only approximately.
We could of course use _sum or _sum_positive in the warning, but that would
just be unnecessary overhead in this moderately hot path.

So this patch should fix it: (untested)

Possibly might need to move it even further down because the patch
also affected the _GUESS case.


diff --git a/mm/util.c b/mm/util.c
index 988d11e6c17c..fa78f90e29a1 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -798,10 +798,6 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 {
 	long allowed;
 
-	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
-			-(s64)vm_committed_as_batch * num_online_cpus(),
-			"memory commitment underflow");
-
 	vm_acct_memory(pages);
 
 	/*
@@ -810,6 +806,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 	if (sysctl_overcommit_memory == OVERCOMMIT_ALWAYS)
 		return 0;
 
+	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
+			-(s64)vm_committed_as_batch * num_online_cpus(),
+			"memory commitment underflow");
+
 	if (sysctl_overcommit_memory == OVERCOMMIT_GUESS) {
 		if (pages > totalram_pages() + total_swap_pages)
 			goto error;




> 
> > 
> > [ 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
> > > 
> > >