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5-level paging in KVM
Latest kernels such as RHEL8 and above are supporting 5-level page table. You can check if this is available/enabled by looking at /boot/config-$(uname -r). If you don’t want to use 5-level page table, you can disable that by adding ‘no5lvl’ as a kernel parameter in grub file (/boot/grub2/grubenv) and reboot the system. This 5 level […]
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Install mpykdump on local machine
Here is the sequence of commands to build mpykdump on your system. This is based on crash version 8.0.0. You may want to check what version of crash your system has to match it. Below is the sequence on RHEL8. You can ignore the error from ‘crash’ compile as we are not going to use […]
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How to check page caches in the kernel memory.
If you would like to know what files were occupying the page caches which you can see with ‘free’ in command line or ‘kmem -I’ in kernel memory, you can start with ‘/proc/sys/vm/drop_caches’. I wrote a script that is extracting the page caches from the vmcore based on the above information. You can find the […]
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How intel_idle works
When the system is in IDLE state which means nothing to run and swapper is running, it calls cpuidle_idle_call() like shown in the below. This cpuidle_idle_call() is called from arch_cpu_idle(). cpuidle_idle_call() is the main idle loop which is checking idle driver and do further steps if the driver is installed and active. It can change […]
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SystemTap to monitor SLAB usage
Checking memory leak in SLAB is not an easy task. It is happening in kernel side and it can go through several different route. Here I want to show how we can use SystemTap to monitor those SLAB alloc/free activities. As a first step you need to find out the monitoring points. SLAB can be […]
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How to write mpykdump extension
If you are dealing with a vmcore (Linux memory dump), you must be familiar with ‘crash’. It is a powerful tool, but it doesn’t cover all the data you can find in Linux kernel. So, there comes ‘mpykdump’ which is a crash extension which understands python code. mpykdump comes with many prebuilt commands that you […]
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Where’s PageSlab() macro???
If you are having hard time to find the definition of PageSlab() in linux kernel, here’s the answer. In include/linux/page-flags.h: Above becomes like below during compile time.
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What is ‘page_cache’, how it is managed and how ‘drop_caches’ dropping this pages?
– The “buffers/cache” values reported by free include the page cache, but not the dentry cache which is saved in slab ‘dentry_cache’. – page cache is increased and decreased based on the disk access activities and managed by each super block (it means each disk). – ‘echo 1 > /proc/sys/vm/drop_caches’ frees page caches by calling […]
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How to calculate available memory for mem_cgroup.
mem_cgroup is checking available amount of memory the group can charge by calling the below function. /** * mem_cgroup_margin – calculate chargeable space of a memory cgroup * @memcg: the memory cgroup * * Returns the maximum amount of memory @mem can be charged with, in * pages. */ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg) […]
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What happens if you try two commands ‘ethtool -p ‘ and ‘ethtool ‘ in parallel.
If you start ‘ethtool -p ‘ and also start ‘ethtool ‘ after that, you may see the delays in ‘ethtool ‘ command. It is because any ethtool commands start by taking ‘rtnl_lock’ and ‘ethtool -p’ is keep running for LED on/off. In the below, bnx2x’s identity function just turns on or off the led. get_settings() […]
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Print callgraph of a function
Sometimes you may want to see what functions are called in a function in multiple level. Below command in my extension may help. crash> edis -c irq_exit {irq_exit} -+- {rcu_irq_exit} -+- {warn_slowpath_null} |- {idle_cpu} |- {tick_nohz_stop_sched_tick} -+- {ktime_get} | |- {update_ts_time_stats} | |- {sched_clock_idle_sleep_event} | |- {rcu_needs_cpu} | |- {select_nohz_load_balancer} | |- {rcu_enter_nohz} | |- […]
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Why error message not goes into pipe nor redirected path in ‘crash’?
In the below example, the error always shows in the console. crash> sym ffffffffa02ef86 > /dev/null sym: invalid address: ffffffffa02ef86 This ‘sym’ command is implemented in ‘void cmd_sym(void)’ function in crash. /* * This command may be used to: * * 1. Translate a symbol to its value. * 2. Translate a value to it […]
Sungju's Slow Life
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Sungju's Slow Life 