On 15 April 2014 14:43, Frederic Weisbecker fweisbec@gmail.com wrote:
Yeah. But not just that.
Using an inline saves a function call and reduce the offline case to a simple condition check. But there is also the jump label that reduce the condition check to an unconditional jump in the off case.
To summarize, here's how calling tick_nohz_task_switch() maps to final C code:
finish_task_switch() { //do things before calling tick_nohz_task_switch()... // call tick_nohz_task_switch goto offcase; if (tick_nohz_full_enabled()) __tick_nohz_task_switch(tsk); offcase: //end of call to tick_nohz_task_switch //do things before calling tick_nohz_task_switch()... }
In the offcase, the code is like above. We don't even do the check, thanks to the jump label code we unconditionally jump to what's next in finish_task_switch() (there is actually nothing afterward but that's for the picture).
Now if there is at least a CPU that is full dynticks on boot, it is enabled with context_tracking_cpu_set(). Then the jump label code patches the code in finish_task_switch() to turn the goto offcase into a nop. Then the condition is actually verified on every call to finish_task_switch().
So it goes beyond than just saving a function call.
Sorry, but my poor mind still couldn't understand what you are trying to tell me :(
So lets clarify things one by one :)
- What do you mean by offcase? CONFIG_NO_HZ_FULL not configured into the kernel or it is configured but none of the CPUs is running in that mode?
- Also what does it correspond to in code: goto offcase; ? There is no labels or goto statements in code that I can see.. This is how the code looks to me.
finish_task_switch() { //do things before calling tick_nohz_task_switch()... // call tick_nohz_task_switch if (tick_nohz_full_enabled()) __tick_nohz_task_switch(tsk); }
__tick_nohz_task_switch() may or maynot be available at all depending on CONFIG_NO_HZ_FULL is enabled into the kernel or not. But that was the case with tick_nohz_task_switch() as well in my patch. So shouldn't make a difference..
Again, sorry for not understanding what you are trying to explain here. I want to understand this once and for all and probably add a comment here as well :)
-- viresh