From: Alexander Gordeev Date: Fri, 31 Oct 2014 14:55:05 +0000 (+0000) Subject: rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU X-Git-Tag: omap-for-v4.1/prcm-dts-mfd-syscon-fix~166^2^2^2~10 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ca9558a33f658155c3b69f92897c2e6a848684f5;p=pandora-kernel.git rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU Let's start assuming that something in the idle loop posts a callback, and scheduling-clock interrupt occurs: 1. The system is idle and stays that way, no runnable tasks. 2. Scheduling-clock interrupt occurs, rcu_check_callbacks() is called as result, which in turn calls rcu_is_cpu_rrupt_from_idle(). 3. rcu_is_cpu_rrupt_from_idle() reports the CPU was interrupted from idle, which results in rcu_sched_qs() call, which does a raise_softirq(RCU_SOFTIRQ). 4. Upon return from interrupt, rcu_irq_exit() is invoked, which calls rcu_idle_enter_common(), which in turn calls rcu_sched_qs() again, which does another raise_softirq(RCU_SOFTIRQ). 5. The softirq happens shortly and invokes rcu_process_callbacks(), which invokes __rcu_process_callbacks(). 6. So now callbacks can be invoked. At least they can be if ->donetail has been updated. Which it will have been because rcu_sched_qs() invokes rcu_qsctr_help(). In the described scenario rcu_sched_qs() and raise_softirq(RCU_SOFTIRQ) get called twice in steps 3 and 4. This redundancy could be eliminated by removing rcu_is_cpu_rrupt_from_idle() function. Signed-off-by: Alexander Gordeev Cc: Paul E. McKenney Signed-off-by: Paul E. McKenney --- Reading git-diff-tree failed