On RHEL servers, changing the semaphore value fails with a message “setting key “kernel.sem“: Numeri

浅浅的花香味﹌ 2022-09-14 00:15 8阅读 0赞

## 环境 ##

*  Red Hat Enterprise Linux 7.8
    
     *  starting with kernel version 3.10.0-1127.el7.x86\_64
 *  Red Hat Enterprise Linux 8.1
    
     *  starting with kernel version 4.18.0-147.el8\_1.x86\_64

## 问题 ##

*  On RHEL 7.8 and 8.1 servers, setting the value of semaphore parameter `SEMMNI` beyond `32768` fails with the error message `sysctl: setting key "kernel.sem": Numerical result out of range`.

[Raw][]

# sysctl -w kernel.sem="4096 2048000 32 32769"
    sysctl: setting key "kernel.sem": Numerical result out of range
    kernel.sem = 4096 2048000 32 32769

*  Anything up to and including 32768 succeeds :

[Raw][]

# sysctl -w kernel.sem="4096 2048000 32 32768"
    kernel.sem = 4096 2048000 32 32768

*  This also applies to `kernel.msgmni` and `kernel.shmmni`

## 决议 ##

*  The change in `SEMMNI` behaviour is a recent change and expected behaviour that has been backported to the RHEL kernel.
 *  Now, the maximum limit of `SEMMNI` has been set to `32768`.
 *  It is still possible to go beyond this limit by booting the server with the kernel command line parameter `ipcmni_extend`.

**For `RHEL 7`:**

*  Add this parameter `ipcmni_extend` in kernel commandline and reboot for changes to get reflected.
 *  Below are the steps:
 *  At the end of "GRUB\_CMDLINE\_LINUX" in "/etc/default/grub" and then save and exit the file.

[Raw][]

# cat /etc/default/grub 
    GRUB_TIMEOUT=5
    GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
    GRUB_DEFAULT=saved
    GRUB_DISABLE_SUBMENU=true
    GRUB_TERMINAL_OUTPUT="console"
    GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap console=tty0 console=ttyS0,115200 ipcmni_extend"<--
    GRUB_DISABLE_RECOVERY="true"

*  Rebuild the grub.

[Raw][]

for BIOS:
    # grub2-mkconfig -o /boot/grub2/grub.cfg 
    
    for UEFI:
    # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

*  Reboot the system for changes to get reflected.
 *  Once the changes are done, it will be possible to set the value to greater than 32768.

[Raw][]

# sysctl -w kernel.sem="4096 2048000 32 32769"
    kernel.sem = 4096 2048000 32 32769

**For `RHEL 8`:**

*  Use `grub2-editenv` to edit the grub :

[Raw][]

# grub2-editenv - set "kernelopts=root=UUID=XXXX [........] ipcmni_extend"

*  Reboot the server for changes to get reflected.
 *  Note: There is no need to rebuild the grub on `RHEL 8`.

## 根源 ##

*  The upstream commits that introduced the change are [6730e658017][] and [8c81ddd2acd][].
 *  These commits have been backported to RHEL in commits `5523711052c` and `e3ea54f485a`.
 *  The commit states :

[Raw][]

ipc: IPCMNI limit check for msgmni and shmmni
    
            Patch series "ipc: IPCMNI limit check for *mni & increase that limit", v9.
    
            The sysctl parameters msgmni, shmmni and semmni have an inherent limit of
            IPC_MNI (32k).  However, users may not be aware of that because they can
            write a value much higher than that without getting any error or
            notification.  Reading the parameters back will show the newly written
            values which are not real.
    
            The real IPCMNI limit is now enforced to make sure that users won't put in
            an unrealistic value.  The first 2 patches enforce the limits.
    
            There are also users out there requesting increase in the IPCMNI value.
            The last 2 patches attempt to do that by using a boot kernel parameter
            "ipcmni_extend" to increase the IPCMNI limit from 32k to 8M if the users
            really want the extended value.

## Additional Notes : ##

*  When a server with RHEL-8.0 / RHEL-7.7 or below kernel having the configured value of `SHMMNI` greater than 32768 will be updated  
    to RHEL-8.1 / RHEL-7.8 kernel, then the value of `SHMMNI` will fall back to default 4096.
 *  Similarly when a server with RHEL-8.0 / RHEL-7.7 or below kernel having the configured value of `MSGMNI` greater than 32768 will be updated  
    to RHEL-8.1 / RHEL-7.8 kernel, then the value of `MSGMNI` will fall back to default 32000.
 *  Likewise when a server with RHEL-8.0 / RHEL-7.7 or below kernel having the configured value of `SEMMNI` greater than 32768 will be updated  
    to RHEL-8.1 / RHEL-7.8 kernel, then the value of `SEMMNI` will fall back to default 128.

## 诊断步骤 ##

*  Update the **RHEL 7** kernel to the `3.10.0-1127.el7.x86_64` or **RHEL 8** kernel to `4.18.0-147.el8_1.x86_64`. Then set the parameter value as shown below which will fail :

[Raw][]

# sysctl -w kernel.sem="4096 2048000 32 32769"
    sysctl: setting key "kernel.sem": Numerical result out of range
    kernel.sem = 4096 2048000 32 32769

*  Anything up to and including 32768 will succeed :

[Raw][]

# sysctl -w kernel.sem="4096 2048000 32 32768"
    kernel.sem = 4096 2048000 32 32768

*  Boot the server with the kernel command line parameter `ipcmni_extend` and again set the value to 32769 which will succeed :

[Raw][]

# sysctl -w kernel.sem="4096 2048000 32 32769"
    kernel.sem = 4096 2048000 32 32769

转载至[https://access.redhat.com/solutions/4968021][https_access.redhat.com_solutions_4968021]

[Raw]: https://access.redhat.com/solutions/4968021#
[6730e658017]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6730e6580177d13f4612767873cb5a533ad63c61
[8c81ddd2acd]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8c81ddd2acd2c10979f5a64f6784ce7c6717495e
[https_access.redhat.com_solutions_4968021]: https://access.redhat.com/solutions/4968021