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Questions tagged [linux-kernel]

The Linux kernel is the operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software.

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How to dump the kernel stack trace of a failed system call?

This is a canonical question. I want to understand why a specific system call is failing. Is it possible to display the kernel stack trace of the syscall: when the return is non-zero or is negative (...
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1answer
14 views

OOM kills a process while there is still a lot of available memory

I am aware that there are many threads like this one, but none of them actually give a methodology on how to read an OOM message. I have a db that is being monitored through NRPE for available ...
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2answers
35 views

AMD Epyc, occassional crappy performance

I'm working on a 2x AMD EPYC 7451 server, with scaling governor set to performance, yet there are issues when the server is under low load. When the load is low all the cores are basically downscaled ...
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2answers
29 views

how to decently configure net.ipv4.neigh.default.gc_thresh_n

We recently had a storm of kernel: neighbour: arp_cache: neighbor table overflow! errors on some nodes of our kubernetes cluster. After research we've figured out that we needed to bump our servers ...
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2answers
1k views

How to avoid downtime with linux?

Frequently software updates to Ubuntu require reboots (which can have side effects such as downtime). I see Ubuntu has https://www.ubuntu.com/livepatch which allows for kernel updates without reboots,...
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5answers
147 views

Do Containers Have An OS?

I know that containers use the host's kernel and from what I understand , that's the reason we do not need an OS. My questions are (and I couldn't find any good explanation online) : 1) If that's ...
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1answer
30 views

What is tmon and how can I use it?

I found a tmon utility in my CentOS 7.5 and I've googled that: TMON is conceived as a tool to help visualize, tune, and test the complex thermal subsystem. ... For monitoring only: ...
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0answers
21 views

Linux Kernel Mode Vs User Mode [migrated]

I'm trying to better understand the process of switching from user mode to kernel mode on linux systems. When a process requires access to the low level HW/restricted memory spaces, does it issue a ...
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2answers
613 views

nic: Something Wicked happened! ffff

I'm configuring a GNU/Linux Router with Debian Stretch (9.7) and before rebooting the server, all traffic from one interface to another was going ok, but after that, I received a message like this (...
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1answer
77 views

FATAL: modpost: GPL-incompatible Module. Uses GPL-only symbol 'sme_active'

Full Error Message FATAL: modpost: GPL-incompatible module celerity16fcmp.ko uses GPL-only symbol 'sme_active' CentOS 7.6 machine unable to install ATTO Fibre Channel drivers due to GPL error in ...
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0answers
81 views

rcu_sched self-detected stall on CPU + watchdog: BUG: soft lockup - CPU#3 stuck for 22s

Being unable to ssh into a machine I connected it to a monitor and found the following: The machine is running Ubuntu Server 18.04 LTS and is a first generation 8 core Ryzen 1700. I've restarted the ...
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0answers
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what is the best practice when rescanning scsi disks reliably in linux?

I'm constantly having to make SAN changes which means that I am also constantly having to rescan the scsi bus so that my various changes are reflected in the machines I've exported disks to. I've ...
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1answer
53 views

Debugging and fine-tuning the linux process scheduler

When debugging sporadic web application slowdowns, I've found that all the processes have quite high maximum scheduling delay. # perf sched record -- sleep 10 && perf sched latency --sort max ...
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1answer
31 views

AMD Geode LX linux : how to instruct driver to enable flat panel? [closed]

Platform : PC/104 with AMD Geode LX CPU with integrated graphics. Buildroot framework, kernel 4.18.10. I am building a custom Linux system with Buildroot. Since I compiled the Geode LX frame buffer ...
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0answers
25 views

QEMU crashed while trying to boot up i.MX6q device kernel

When tried to boot up Kernel uImage of an i.MX6q based embedded device, it seems that QEMU crashes with "Unable to handle kernel NULL pointer dereference at virtual address 00000044". Before crashing ...
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1answer
49 views

How to keep custom kernel modules on kernel updates installed?

I've got a NIC (Asus XG C100C) that is not properly supported in the current Linux kernel. Hence, I installed driver which I compiled from source. Every time when the kernel is updated, I have to ...
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1answer
94 views

fsck -n /dev/nandb giving errors . how to solve fsck errors [closed]

I want to know how to solve this error? help me out!! INPUT "fsck -n /dev/nandb" OUTPUT fsck from util-linux 2.25.2 e2fsck 1.42.12 (29-Aug-2014) Warning! /dev/nandb is mounted. /dev/nandb ...
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1answer
90 views

Are these errors in dmesg (related to PCI/NVME drive) anything to be concerned about?

Anyone know what this error below (dmesg output) indicates? I'm getting it when periodically writing to an Intel NVME drive (attached to a PCI card) under Linux. Not sure if "no further action" means ...
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1answer
113 views

How to enable IBRS when it is disabled?

I have been updating some servers to the latest patchlevels and wanted to check the security state regarding Meltdown, Spectre and Spectre-NG. Using the current V 0.39+ of the spectre-meltdown-...
1
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1answer
69 views

iptables: Rate too fast Error

I want to set hard limit for incoming UDP packets on my Linux machine which is inside 10 Gigabit LAN. Following command works fine /sbin/iptables -A UDP-LIMIT -m limit --limit 10000/s --limit-burst ...
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0answers
36 views

amd_iommu=force_isolation.what does it mean?

When I use iommu to passthrough sriov device,I find all VFs and PF are in a same iommu group. Can I use “amd_iommu=force_isolation”? Does it mean every device will have a single IOMMU Group? Does it ...
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1answer
90 views

cannot boot to the newly compiled kernel version

I have a server with RHEL 7.5 OS. The inbox kernel version on the system is 4.14.0-49. I have a requirement to use the latest available kernel, so I downloaded the Kernel 4.17.18 source package and ...
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1answer
87 views

IPv4 identification field manipulation in linux

When Don't Fragment flag is set, IPv4 RFC allows set Identification filed to 0x0000. I got a peer which is behind firewall that brakes RFC and block any IPv4 packets with such 0x0000 ID field. My "...
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1answer
339 views

How can iperf reporting packet loss in udp

Iperf is the well known tool to calculate throughput. When i tried udp throughput using iperf on my linuxpc, It reported that 10% of packet loss. In UDP protocol, data gram did not receice any ...
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0answers
133 views

Centos 6.8 kernel 2.6.32-642.el6.x86_64 crash at linux-2.6.32-642.el6.x86_64/mm/vmalloc.c: 630

I have intel servers running CentOS 6.8 as KVM host,and there was an OOPS or BUG occurs. kernel crash at kernel-2.6.32-642.el6/linux-2.6.32-642.el6.x86_64/mm/vmalloc.c: 630 Did anyone else have this ...
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1answer
61 views

Linux - memory size

How to check exact amount of memory installed in server? MemTotal from /proc/meminfo show something a bit less. I.e. I have 2097152K and meminfo says 2054348K. I can sum all banks reported by ...
3
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1answer
53 views

CPU utilization impact due to granularity kernel parameter RHEL6 vs RHEL7

Following kernel parameters are showing very different behaviour from R6 to R7 and we are not able to figure out why. Any help appreciated. kernel.sched_min_granularity_ns kernel....
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0answers
36 views

Why in CoreOS Kernel parameter sotflockup_panic is activated?

I looked through different Linux distros such as Ubuntu, CentOS, openSUSE, RHEL, PhotonOS, and Alpine for kernel parameter softlockup_panic. In all of them this kernel parameter is deactivated. # ...
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1answer
70 views

When someone gains remote access to a server are there other ways to execute system commands other than the bash shell?

I am hardening a server and attempting to build a restrictive layer to a potential hacker even with root access to the server to do harm. If a user gains root or user access to the shell via say ...
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0answers
15 views

How to tune the performance of a Nginx webserver in ubuntu16.04 [duplicate]

My server is ubuntu 16.04 and 8 core machine with 16gb RAM. I need to get the best performance with this configuration. How can I tune the server with Nginx level and system level? Running a PHP LMS ...
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0answers
51 views

Can /boot/efi partition causes issues for kernel boot?

I have a fresh installation of CentOS 7 on a server. I have manually partitioned system and, per installer suggestion, I have created /boot/efi (never asked for that before). After the installation ...
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0answers
48 views

localhost kernel: possible SYN flooding on port 8222. Sending cookies

I get this error message(centos 6): Jun 12 07:42:40 localhost kernel: possible SYN flooding on port 8222. Sending cookies. Jun 12 08:07:05 localhost kernel: possible SYN flooding on port 8222. ...
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1answer
278 views

Are Kerberos service tickets cached by mount.cifs or kernel?

When mounting a CIFS share using krbr5i and destroying my Kerberos tickets with kdestroy afterwards its still possible to mount other shares on the same server. How is this possible? Doesn't mount....
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1answer
1k views

ZFS datasets no longer automatically mount on reboot after system upgrade

I have an Arch linux ZFS backup system which wasn't updated for a long time because it was working perfectly and there was no reason to bother. After a recent upgrade, though, my ZFS datasets are no ...
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1answer
215 views

What are the parameters in iostat

I used iostat and trying to understand the iostat output using this link : https://linoxide.com/linux-command/linux-iostat-command/ but It is not clearly explained. PF the output at different times ...
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1answer
258 views

linux (ubuntu 17.10) fan stopped working

At some point recently, my fan stopped working. I've confirmed it's not the hardware, because in the bios I can start and stop the fan. This is an ubuntu 17.10 desktop host. I noticed that lm-...
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0answers
71 views

How to setup nvmetcli for FC loopback mode?

I have a NVMe SSD, KC1000, from Kingston and a HBA, QLE2672 2-port 16Gb Fibre Channel Adapter, from QLogic and are installed on Linux kernel 4.16.6. Then I would like to set up a scenario which has a ...
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0answers
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How to count the packets inside Linux network namespace using NETFILTER framework?

I am newbie to Netfilter. Trying to implement netfilter framework (INPUT-HOOK, OUTPUT-HOOK, FORWARD-HOOK) in Linux to count the number of packets went inside the network namespace. But not able to ...
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1answer
159 views

Improve single HTTP transfer bandwith over high latency connection (Ubuntu Linux)

I'm trying to improve the file transfer speed for my web server in Roubaix, France (hosted by OVH) over high distance/latency connections. I know that TCP connections are affected by higher latency, ...
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0answers
105 views

Redhat vs Oracle Linux benchmarks

I have two servers, both have Intel 8160 processor, 768 GB memory, same hardware specs, same software/package versions. The only difference being that one is running RedHat Enterprise Linux 7 and the ...
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0answers
74 views

iptables rejected message log references another message in '[…]' - is that other one real?

I've got a set of rules in my iptables to detect, log, and drop "conntrack-invalid" packets: -A local-before-input -m conntrack --ctstate INVALID -j local-conntrack-invalid -A local-conntrack-invalid ...
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1answer
211 views

I can't connect to my AWS server after yum update on centOS 7

I created a new server (EC2) with centOS 7, and after the first "yum update" and a reboot I can't connect anymore. =====================================================================================...
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0answers
460 views

Kernel Panic on Guest-VM (Debian 9)

I have two servers running with Debian Jessie Kernel 3.16.0-5-amd64. Both servers have the exact same configuration, only hardware differs. I use KVM as hypervisor together with QEMU on both servers. ...
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0answers
88 views

Nice not working though disabled kernel autogrouping via sys.kernel.sched_autogroup_enabled

Context I have a Ubuntu 16.04 Kernel 4.4 desktop installation and two randomly CPU-intensive tasks (docker, numbercruncher). I want to prioritize the docker process over the numberbruncher task, ...
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1answer
56 views

Diagnose Sudden Slowness on Linux

I have two identical Linux laptops (Core™ i7-4810MQ with 32GB RAM) both running Ubuntu 16.04. Running seemingly any command on machine 2 takes about 4-12 times as long as on machine 1, depending on ...
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3answers
2k views

Where to get newer RHEL 7 kernel mentioned on bugzilla.redhat.com (for CentOS)

I am using Centos 7.4 and have encountered a bug, which I found a solution for: https://bugzilla.redhat.com/show_bug.cgi?id=1502601 The problem is, solution mentions that bug is fixed in kernel-3.10....
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0answers
127 views

Ubuntu kernel source code for AWS instances

I'm trying to find the source code for a specific AWS kernel - ubuntu instance (e.g. linux-image-4.4.0-1022-aws). The ubuntu kernel git repository doesn't have tags for these AWS instances. Please don'...
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1answer
91 views

zram filesystem reports different usage at device level to that reported by the filesystem

We have an 80GB zram device defined on our host, and within this a 170GB ext4 filesystem: echo 170G > /sys/block/zram0/disksize echo 80G > /sys/block/zram0/mem_limit /usr/sbin/mkfs.ext4 -...
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1answer
65 views

VGA monitor blinks with KVM

Tried using several computers with a KVM switch that uses the 25th port on the VGA connection for USB (for the keyboard and mouse - StarTech.com SV1631DUSBUK). Manufacture says the cable they use to ...
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1answer
73 views

Cannot explain memory occupancy in linux

I'm working on a debian 9.3, the free command return total used free shared buff/cache available Mem: 131940516 100433176 29189576 33940 ...