Exadata image 18.1.19 pitfall

After Exadata image 18.1.19 have been installed we obtained some errors, concerned to new process creation:

The "tail  alert.log" bring the message:
$ tail -100f alert.log
-bash: fork: retry: Resource temporarily unavailable
-bash: fork: retry: Resource temporarily unavailable

After some time after reboot and DBs have started we were unable to switch to oracle account:
# su - oracle
su: /bin/bash: Resource temporarily unavailable

Because the problem is concerned to new process creation,  we checked some files and found they are ok.We found that these files were old enough, nobody modified it:
/etc/sysctl.conf
/etc/security/limits.conf
oracle soft nproc 400000
oracle hard nproc 400000
But "ulimit" show  the very different values:
[oracle@exa6dbadm01 ~]$ ulimit -u
2047
The number of oracle processes is close to 2048 value:
# ps -ef | grep oracle | wc -l
2326
The linux reboot didn't bring new ulimit values.

The "strace" is the way to found the root cause:

# strace su - oracle
...
open("/etc/security/limits.d/90-nproc.conf", O_RDONLY) = 3
...
setrlimit(RLIMIT_NPROC, {rlim_cur=2047, rlim_max=400000}) = 0
...

# cat /etc/security/limits.d/90-nproc.conf
# Default limit for number of user's processes to prevent
# accidental fork bombs.
# See rhbz #432903 for reasoning.
*          soft    nproc     1024
root       soft    nproc     unlimited
#Oracle recommended value for nproc is set to 2047 for user oracle
oracle  soft  nproc  2047

After we changed the "oracle  soft  nproc " to 400000 the system is work ok.
Bingo !