When the CPU usage is 60%, the flame graphs(perf record) is used to capture the CPU usage. Why is 40% idle-related stack usage not displayed in the flame graphs? The usage of the idle stack is often less than 5%.

CodePudding user response：

For flame graphs, the point is normally to measure where a process spends CPU time while it's running, not which blocking functions it calls that make it sleep, or where it gets scheduled out and sleeps when it doesn't want to.

I capture performance for one cpu processor, not one process. According to the operating system design, if there is no active task on the CPU, the CPU calls an idle waiting function. For example, Linux often calls schedule_idle until it is interrupted by a new task. Therefore, it is expected that the schedule_idle can be found in flame gragh and it consumes 40% of the cpu usage.

Perf events like cycles don't increment when the clock is halted (e.g. cycles is cpu_clk_unhalted.thread_p or similar). If you really wanted to see time spend idle, you might be able to disable idle power saving to get Linux to just spin in a loop instead of using x86 monitor/mwait or even basic hlt to put the CPU into a C-state where the clock doesn't tick.

Or run your code pinned to one logical core, and on the other logical core, pin a task that runs the pause instruction in a loop. So the physical core's clock keeps ticking for the core you're counting events for.

You should still get counts for cpu_clk_unhalted.thread_any ([Core cycles when at least one thread on the physical core is not in halt state]) when recording that event on the logical core with your task, even when that logical core is asleep.

And you can also record counts for cpu_clk_unhalted.thread to count cycles when this (hardware) thread aka logical core isn't halted, to know how much CPU time you actually used. (Or use the software event task-clock for that.)

Use perf list to see events available on your CPU, and read their descriptions carefully.