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 theContinue reading “How to check page caches in the kernel memory.”

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 changeContinue reading “How intel_idle works”

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 beContinue reading “SystemTap to monitor SLAB usage”

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 youContinue reading “How to write mpykdump extension”

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: #define __PAGEFLAG(uname, lname) TESTPAGEFLAG(uname, lname) \ __SETPAGEFLAG(uname, lname) __CLEARPAGEFLAG(uname, lname) #define TESTPAGEFLAG(uname, lname) \ static inline int Page##uname(const struct page *page) \ { return test_bit(PG_##lname, &page->flags); } #define __SETPAGEFLAG(uname, lname) \ static inline void __SetPage##uname(structContinue reading “Where’s PageSlab() macro???”

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 callingContinue reading “What is ‘page_cache’, how it is managed and how ‘drop_caches’ dropping this pages?”

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)Continue reading “How to calculate available memory for mem_cgroup.”

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()Continue reading “What happens if you try two commands ‘ethtool -p ‘ and ‘ethtool ‘ in parallel.”

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} | |-Continue reading “Print callgraph of a function”

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 itContinue reading “Why error message not goes into pipe nor redirected path in ‘crash’?”

What happens if numa=off is provided in kernel parameter?

If “numa=off” is in kernel boot parameter, it will mark ‘numa_off’ global variable which will be checked during initialization function which is ‘x86_numa_init()’ in x86_64. This will make it not call ‘numa_init’ if numa_off is 1. static __init int numa_setup(char *opt) { if (!opt) return -EINVAL; if (!strncmp(opt, “off”, 3)) numa_off = 1; #ifdef CONFIG_NUMA_EMUContinue reading “What happens if numa=off is provided in kernel parameter?”

An example case with some of my commands

System got high load average and it wasn’t responding for long which is a typical hang situation. It shows total 56 tasks in D (Uninterruptible) state and 5 tasks were in D state longer than 120 seconds which is considered as a hung task. Let’s see what it was waiting for in this process. Alright,Continue reading “An example case with some of my commands”