Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap
From: Kirill A. Shutemov
Date: Tue Jul 25 2017 - 10:17:35 EST
On Mon, Jul 24, 2017 at 06:11:47PM +0200, Michal Hocko wrote:
> On Mon 24-07-17 17:51:42, Kirill A. Shutemov wrote:
> > On Mon, Jul 24, 2017 at 04:15:26PM +0200, Michal Hocko wrote:
> [...]
> > > What kind of scalability implication you have in mind? There is
> > > basically a zero contention on the mmap_sem that late in the exit path
> > > so this should be pretty much a fast path of the down_write. I agree it
> > > is not 0 cost but the cost of the address space freeing should basically
> > > make it a noise.
> >
> > Even in fast path case, it adds two atomic operation per-process. If the
> > cache line is not exclusive to the core by the time of exit(2) it can be
> > noticible.
> >
> > ... but I guess it's not very hot scenario.
> >
> > I guess I'm just too cautious here. :)
>
> I definitely did not want to handwave your concern. I just think we can
> rule out the slow path and didn't think about the fast path overhead.
>
> > > > Should we do performance/scalability evaluation of the patch before
> > > > getting it applied?
> > >
> > > What kind of test(s) would you be interested in?
> >
> > Can we at lest check that number of /bin/true we can spawn per second
> > wouldn't be harmed by the patch? ;)
>
> OK, so measuring a single /bin/true doesn't tell anything so I've done
> root@test1:~# cat a.sh
> #!/bin/sh
>
> NR=$1
> for i in $(seq $NR)
> do
> /bin/true
> done
>
> in my virtual machine (on a otherwise idle host) with 4 cpus and 2GB of
> RAM
>
> Unpatched kernel
> root@test1:~# /usr/bin/time -v ./a.sh 100000
> Command being timed: "./a.sh 100000"
> User time (seconds): 53.57
> System time (seconds): 26.12
> Percent of CPU this job got: 100%
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:19.46
> root@test1:~# /usr/bin/time -v ./a.sh 100000
> Command being timed: "./a.sh 100000"
> User time (seconds): 53.90
> System time (seconds): 26.23
> Percent of CPU this job got: 100%
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:19.77
> root@test1:~# /usr/bin/time -v ./a.sh 100000
> Command being timed: "./a.sh 100000"
> User time (seconds): 54.02
> System time (seconds): 26.18
> Percent of CPU this job got: 100%
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:19.92
>
> patched kernel
> root@test1:~# /usr/bin/time -v ./a.sh 100000
> Command being timed: "./a.sh 100000"
> User time (seconds): 53.81
> System time (seconds): 26.55
> Percent of CPU this job got: 100%
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:19.99
> root@test1:~# /usr/bin/time -v ./a.sh 100000
> Command being timed: "./a.sh 100000"
> User time (seconds): 53.78
> System time (seconds): 26.15
> Percent of CPU this job got: 100%
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:19.67
> root@test1:~# /usr/bin/time -v ./a.sh 100000
> Command being timed: "./a.sh 100000"
> User time (seconds): 54.08
> System time (seconds): 26.87
> Percent of CPU this job got: 100%
> Elapsed (wall clock) time (h:mm:ss or m:ss): 1:20.52
>
> the results very quite a lot (have a look at the user time which
> shouldn't have no reason to vary at all - maybe the virtual machine
> aspect?). I would say that we are still reasonably close to a noise
> here. Considering that /bin/true would close to the worst case I think
> this looks reasonably. What do you think?
>
> If you absolutely insist, I can make the lock conditional only for oom
> victims. That would still mean current->signal->oom_mm pointers fetches
> and a 2 branches.
Below are numbers for the same test case, but from bigger machine (48
threads, 64GiB of RAM).
v4.13-rc2:
Performance counter stats for './a.sh 100000' (5 runs):
159857.233790 task-clock:u (msec) # 1.000 CPUs utilized ( +- 3.21% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
8,761,843 page-faults:u # 0.055 M/sec ( +- 0.64% )
38,725,763,026 cycles:u # 0.242 GHz ( +- 0.18% )
272,691,643,016 stalled-cycles-frontend:u # 704.16% frontend cycles idle ( +- 3.16% )
22,221,416,575 instructions:u # 0.57 insn per cycle
# 12.27 stalled cycles per insn ( +- 0.00% )
5,306,829,649 branches:u # 33.197 M/sec ( +- 0.00% )
240,783,599 branch-misses:u # 4.54% of all branches ( +- 0.15% )
159.808721098 seconds time elapsed ( +- 3.15% )
v4.13-rc2 + the patch:
Performance counter stats for './a.sh 100000' (5 runs):
167628.094556 task-clock:u (msec) # 1.007 CPUs utilized ( +- 1.63% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
8,838,314 page-faults:u # 0.053 M/sec ( +- 0.26% )
38,862,240,137 cycles:u # 0.232 GHz ( +- 0.10% )
282,105,057,553 stalled-cycles-frontend:u # 725.91% frontend cycles idle ( +- 1.64% )
22,219,273,623 instructions:u # 0.57 insn per cycle
# 12.70 stalled cycles per insn ( +- 0.00% )
5,306,165,194 branches:u # 31.654 M/sec ( +- 0.00% )
240,473,075 branch-misses:u # 4.53% of all branches ( +- 0.07% )
166.497005412 seconds time elapsed ( +- 1.61% )
IMO, there is something to think about. ~4% slowdown is not insignificant.
I expect effect to be bigger for larger machines.
--
Kirill A. Shutemov