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Date: Tue, 15 Apr 2014 11:13:25 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: Viresh Kumar <viresh.kumar@...aro.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Lists linaro-kernel <linaro-kernel@...ts.linaro.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Arvind Chauhan <Arvind.Chauhan@....com>, Linaro Networking <linaro-networking@...aro.org> Subject: Re: [PATCH 29/38] tick-sched: remove wrapper around __tick_nohz_task_switch() On Tue, Apr 15, 2014 at 10:15:24AM +0530, Viresh Kumar wrote: > On 15 April 2014 04:52, Frederic Weisbecker <fweisbec@...il.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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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