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rcu: Update NOCB comments 

This commit updates a few obsolete comments in the RCU callback-offload
code.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-12-03 14:07:17 -08:00
parent b2c1955b88
commit a9fefdb257
1 changed files with 16 additions and 17 deletions
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33
kernel/rcu/tree_plugin.h
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@ -1857,22 +1857,24 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp)
/*
* Offload callback processing from the boot-time-specified set of CPUs
* specified by rcu_nocb_mask. For each CPU in the set, there is a
* kthread created that pulls the callbacks from the corresponding CPU,
* waits for a grace period to elapse, and invokes the callbacks.
* The no-CBs CPUs do a wake_up() on their kthread when they insert
* a callback into any empty list, unless the rcu_nocb_poll boot parameter
* has been specified, in which case each kthread actively polls its
* CPU. (Which isn't so great for energy efficiency, but which does
* reduce RCU's overhead on that CPU.)
* specified by rcu_nocb_mask. For the CPUs in the set, there are kthreads
* created that pull the callbacks from the corresponding CPU, wait for
* a grace period to elapse, and invoke the callbacks. These kthreads
* are organized into leaders, which manage incoming callbacks, wait for
* grace periods, and awaken followers, and the followers, which only
* invoke callbacks. Each leader is its own follower. The no-CBs CPUs
* do a wake_up() on their kthread when they insert a callback into any
* empty list, unless the rcu_nocb_poll boot parameter has been specified,
* in which case each kthread actively polls its CPU. (Which isn't so great
* for energy efficiency, but which does reduce RCU's overhead on that CPU.)
*
* This is intended to be used in conjunction with Frederic Weisbecker's
* adaptive-idle work, which would seriously reduce OS jitter on CPUs
* running CPU-bound user-mode computations.
*
* Offloading of callback processing could also in theory be used as
* an energy-efficiency measure because CPUs with no RCU callbacks
* queued are more aggressive about entering dyntick-idle mode.
* Offloading of callbacks can also be used as an energy-efficiency
* measure because CPUs with no RCU callbacks queued are more aggressive
* about entering dyntick-idle mode.
*/
@ -1976,10 +1978,7 @@ static void wake_nocb_leader_defer(struct rcu_data *rdp, int waketype,
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
}
/*
* Does the specified CPU need an RCU callback for this invocation
* of rcu_barrier()?
*/
/* Does rcu_barrier need to queue an RCU callback on the specified CPU? */
static bool rcu_nocb_cpu_needs_barrier(int cpu)
{
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
@ -1995,8 +1994,8 @@ static bool rcu_nocb_cpu_needs_barrier(int cpu)
* callbacks would be posted. In the worst case, the first
* barrier in rcu_barrier() suffices (but the caller cannot
* necessarily rely on this, not a substitute for the caller
* getting the concurrency design right!). There must also be
* a barrier between the following load an posting of a callback
* getting the concurrency design right!). There must also be a
* barrier between the following load and posting of a callback
* (if a callback is in fact needed). This is associated with an
* atomic_inc() in the caller.
*/