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. -- Michal Hocko SUSE Labs