After calling switch_to(), the kernel stack is switched to that of the task named in next. Changing the address space, etc, is handled in eg context_switch(). schedule() cannot be called in atomic context, including from an interrupt (see the check in schedule_debug()). If a reschedule is needed, the TIF_NEED_RESCHED task flag is set, which is … Read more
As wikipedia knows in its Context switch article, “context switch is the process of storing and restoring the state (context) of a process so that execution can be resumed from the same point at a later time.“. I’ll assume context switch between two processes of the same OS, not the user/kernel mode transition (syscall) which … Read more
It’s much easier to explain those in reverse order because a process-switch always involves a thread-switch. A typical thread context switch on a single-core CPU happens like this: All context switches are initiated by an ‘interrupt’. This could be an actual hardware interrupt that runs a driver, (eg. from a network card, keyboard, memory-management or … Read more
You need to understand that a thread/process context has multiple parts, one, directly associated with execution and is held in the CPU and certain system tables in memory that the CPU uses (e.g. page tables), and the other, which is needed for the OS, for bookkeeping (think of the various IDs, handles, special OS-specific permissions, … Read more
Use perf: perf stat ./yourapp See the kernel wiki perf tutorial for details. This uses the hardware performance counters of your CPU, so the overhead is very small. Example from the wiki: perf stat -B dd if=/dev/zero of=/dev/null count=1000000 Performance counter stats for ‘dd if=/dev/zero of=/dev/null count=1000000’: 5,099 cache-misses # 0.005 M/sec (scaled from 66.58%) … Read more
At a high level, there are two separate mechanisms to understand. The first is the kernel entry/exit mechanism: this switches a single running thread from running usermode code to running kernel code in the context of that thread, and back again. The second is the context switch mechanism itself, which switches in kernel mode from … Read more
The main distinction between a thread switch and a process switch is that during a thread switch, the virtual memory space remains the same, while it does not during a process switch. Both types involve handing control over to the operating system kernel to perform the context switch. The process of switching in and out … Read more