From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Mon, 17 Mar 2014 04:36:25 +0000 (-0700)
Subject: sched,rcu: Make cond_resched() report RCU quiescent states
X-Git-Tag: omap-for-v3.16/fixes-against-rc1~39^2~26^2^2~2^3~15
X-Git-Url: https://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ac1bea85781e9004da9b3e8a4b097c18492d857c;p=pandora-kernel.git

sched,rcu: Make cond_resched() report RCU quiescent states

Given a CPU running a loop containing cond_resched(), with no
other tasks runnable on that CPU, RCU will eventually report RCU
CPU stall warnings due to lack of quiescent states.  Fortunately,
every call to cond_resched() is a perfectly good quiescent state.
Unfortunately, invoking rcu_note_context_switch() is a bit heavyweight
for cond_resched(), especially given the need to disable preemption,
and, for RCU-preempt, interrupts as well.

This commit therefore maintains a per-CPU counter that causes
cond_resched(), cond_resched_lock(), and cond_resched_softirq() to call
rcu_note_context_switch(), but only about once per 256 invocations.
This ratio was chosen in keeping with the relative time constants of
RCU grace periods.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
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