Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754208AbaDOJxk (ORCPT ); Tue, 15 Apr 2014 05:53:40 -0400 Received: from mail-ob0-f172.google.com ([209.85.214.172]:47252 "EHLO mail-ob0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750898AbaDOJxh (ORCPT ); Tue, 15 Apr 2014 05:53:37 -0400 MIME-Version: 1.0 In-Reply-To: <20140415091323.GK1877@localhost.localdomain> References: <20140414232218.GE1877@localhost.localdomain> <20140415091323.GK1877@localhost.localdomain> Date: Tue, 15 Apr 2014 15:23:37 +0530 Message-ID: Subject: Re: [PATCH 29/38] tick-sched: remove wrapper around __tick_nohz_task_switch() From: Viresh Kumar To: Frederic Weisbecker Cc: Thomas Gleixner , Lists linaro-kernel , Linux Kernel Mailing List , Arvind Chauhan , Linaro Networking Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15 April 2014 14:43, Frederic Weisbecker 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/