On Tue, Apr 15, 2014 at 10:15:24AM +0530, Viresh Kumar wrote:
On 15 April 2014 04:52, Frederic Weisbecker fweisbec@gmail.com wrote:
On Mon, Apr 14, 2014 at 09:53:51PM +0530, Viresh Kumar wrote:
__tick_nohz_task_switch() was called only from tick_nohz_task_switch() and there is nothing much in tick_nohz_task_switch() as well. IOW, we don't need unnecessary wrapper over __tick_nohz_task_switch() to be there. Merge all code from __tick_nohz_task_switch() into tick_nohz_task_switch() and move it to tick-sched.c.
This also moves check for tick_nohz_tick_stopped() outside of irq_save() context.
No, the wrapper is there on purpose in order to optimize the full dynticks off case in the context switch path with the jump label'ed check on tick_nohz_full_enabled().
Just to clarify, you are saying that:
Wrapper was there to save an extra function call when tick_nohz_full_enabled() returns false, as tick_nohz_task_switch() will be inlined ?
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.
In this case probably we can move !can_stop_full_tick() as well to the wrapper ?
Do you mean moving all the code of __tick_nohz_task_switch() to tick_nohz_task_switch()? I much prefer we don't do that. This is going to make can_stop_full_tick() a publicly visible nohz internal. And it may uglify tick.h as well.