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Date: Tue, 30 Mar 2021 13:50:15 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zhouyi Zhou <zhouzhouyi@...il.com>
Cc: 340442286@...com, josh@...htriplett.org, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
joel@...lfernandes.org, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RCU: some improvements to comments of tree.c
On Sun, Mar 21, 2021 at 11:17:25AM +0800, Zhouyi Zhou wrote:
> During my study of RCU, I go through tree.c many times
> and try to make some improvements to the comments.
>
> Thanks a lot.
Thank you for looking this over and for the patch!
The commit log should read something like this:
This commit provides some cleanups to comments and code in the
kernel/rcu/tree.c file.
I have provided responses below, followed by a modified commit.
Please let me know if you have objections to having your Signed-off-by
on this modified commit.
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> ---
> kernel/rcu/tree.c | 28 ++++++++++++++--------------
> 1 file changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index da6f5213fb74..50c6b8fd8d08 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -201,7 +201,7 @@ EXPORT_SYMBOL_GPL(rcu_get_gp_kthreads_prio);
> * the need for long delays to increase some race probabilities with the
> * need for fast grace periods to increase other race probabilities.
> */
> -#define PER_RCU_NODE_PERIOD 3 /* Number of grace periods between delays. */
> +#define PER_RCU_NODE_PERIOD 3 /* Number of grace periods between delays, for debug purpose. */
This should be something like "delays for debugging".
> /*
> * Compute the mask of online CPUs for the specified rcu_node structure.
> @@ -835,9 +835,9 @@ void noinstr rcu_irq_exit(void)
>
> /**
> * rcu_irq_exit_preempt - Inform RCU that current CPU is exiting irq
> - * towards in kernel preemption
> + * towards kernel preemption
> *
> - * Same as rcu_irq_exit() but has a sanity check that scheduling is safe
> + * Same as rcu_irq_exit() but has some sanity checks that scheduling is safe
> * from RCU point of view. Invoked from return from interrupt before kernel
> * preemption.
> */
Huh. It does not look like this function is used. And Thomas Gleixner
confirmed that it can be removed, which I did as a separate commit.
> @@ -959,7 +959,7 @@ EXPORT_SYMBOL_GPL(rcu_idle_exit);
> */
> void noinstr rcu_user_exit(void)
> {
> - rcu_eqs_exit(1);
> + rcu_eqs_exit(true);
Fair enough, might as well be consistent with the other call to
rcu_eqs_exit().
> }
>
> /**
> @@ -1226,7 +1226,7 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
> #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
>
> /*
> - * We are reporting a quiescent state on behalf of some other CPU, so
> + * We may reporting a quiescent state on behalf of some other CPU, so
Fair enough, although we are trying to report or preparing to report
a quiescent state when invoking this function, we might end up not doing
so. I reworded as follows:
* When trying to report a quiescent state on behalf of some other CPU,
> * it is our responsibility to check for and handle potential overflow
> * of the rcu_node ->gp_seq counter with respect to the rcu_data counters.
> * After all, the CPU might be in deep idle state, and thus executing no
> @@ -1808,9 +1808,9 @@ static bool rcu_gp_init(void)
> return false;
> }
>
> - /* Advance to a new grace period and initialize state. */
> - record_gp_stall_check_time();
> /* Record GP times before starting GP, hence rcu_seq_start(). */
> + record_gp_stall_check_time();
> + /* Advance to a new grace period and initialize state. */
Recording the grace-period stall machinery is part of advancing to a new
grace period, so I left this one as is.
> rcu_seq_start(&rcu_state.gp_seq);
> ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
> @@ -2630,7 +2630,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
> * state, for example, user mode or idle loop. It also schedules RCU
> * core processing. If the current grace period has gone on too long,
> * it will ask the scheduler to manufacture a context switch for the sole
> - * purpose of providing a providing the needed quiescent state.
> + * purpose of providing the needed quiescent state.
Good catch, applied as you suggested.
> */
> void rcu_sched_clock_irq(int user)
> {
> @@ -3260,7 +3260,7 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
>
> /*
> * This function is invoked in workqueue context after a grace period.
> - * It frees all the objects queued on ->bhead_free or ->head_free.
> + * It frees all the objects queued on ->bkvhead_free or ->head_free.
And for this one as well.
> */
> static void kfree_rcu_work(struct work_struct *work)
> {
> @@ -3287,7 +3287,7 @@ static void kfree_rcu_work(struct work_struct *work)
> krwp->head_free = NULL;
> raw_spin_unlock_irqrestore(&krcp->lock, flags);
>
> - // Handle two first channels.
> + // Handle first two channels.
This one I translated the rest of the way to English as follows:
// Handle the first two channels.
> for (i = 0; i < FREE_N_CHANNELS; i++) {
> for (; bkvhead[i]; bkvhead[i] = bnext) {
> bnext = bkvhead[i]->next;
> @@ -3530,7 +3530,7 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
>
> /*
> * Queue a request for lazy invocation of appropriate free routine after a
> - * grace period. Please note there are three paths are maintained, two are the
> + * grace period. Please note there are three paths maintained, two are the
I made this "Please note that three paths are maintained" and fixed up a few
other things, resulting in this:
* Queue a request for lazy invocation of the appropriate free routine
* after a grace period. Please note that three paths are maintained,
* two for the common case using arrays of pointers and a third one that
* is used only when the main paths cannot be used, for example, due to
* memory pressure.
> * main ones that use array of pointers interface and third one is emergency
> * one, that is used only when the main path can not be maintained temporary,
> * due to memory pressure.
> @@ -3813,7 +3813,7 @@ EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
>
> /*
> * Check to see if there is any immediate RCU-related work to be done by
> - * the current CPU, returning 1 if so and zero otherwise. The checks are
> + * the current CPU, returning 1 if so and 0 otherwise. The checks are
I am not feeling it for this one...
> * in order of increasing expense: checks that can be carried out against
> * CPU-local state are performed first. However, we must check for CPU
> * stalls first, else we might not get a chance.
> @@ -4153,7 +4153,7 @@ int rcutree_online_cpu(unsigned int cpu)
> }
>
> /*
> - * Near the beginning of the process. The CPU is still very much alive
> + * Near the beginning of the offline process. The CPU is still very much alive
> * with pretty much all services enabled.
> */
> int rcutree_offline_cpu(unsigned int cpu)
The name of the function suffices here.
I will agree that finding all the pieces of RCU's CPU-hotplug processing
can be a bit annoying. I have been thinking in terms of gathering these
functions into a tree_hotplug.c or similar.
Thoughts?
> @@ -4651,7 +4651,7 @@ void __init rcu_init(void)
> rcutree_online_cpu(cpu);
> }
>
> - /* Create workqueue for expedited GPs and for Tree SRCU. */
> + /* Create workqueue for Tree SRCU and for expedited GPs. */
Fair enough...
> rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
> WARN_ON(!rcu_gp_wq);
> rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
------------------------------------------------------------------------
commit 3f0d0d00b8f3fe58754071eb7726c5128b5816da
Author: Zhouyi Zhou <zhouzhouyi@...il.com>
Date: Tue Mar 30 13:47:42 2021 -0700
rcu: Improve tree.c comments and add code cleanups
This commit cleans up some comments and code in kernel/rcu/tree.c.
Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 418c743..ce5b4cd 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -202,7 +202,7 @@ EXPORT_SYMBOL_GPL(rcu_get_gp_kthreads_prio);
* the need for long delays to increase some race probabilities with the
* need for fast grace periods to increase other race probabilities.
*/
-#define PER_RCU_NODE_PERIOD 3 /* Number of grace periods between delays. */
+#define PER_RCU_NODE_PERIOD 3 /* Number of grace periods between delays for debugging. */
/*
* Compute the mask of online CPUs for the specified rcu_node structure.
@@ -938,7 +938,7 @@ EXPORT_SYMBOL_GPL(rcu_idle_exit);
*/
void noinstr rcu_user_exit(void)
{
- rcu_eqs_exit(1);
+ rcu_eqs_exit(true);
}
/**
@@ -1204,7 +1204,7 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
#endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
/*
- * We are reporting a quiescent state on behalf of some other CPU, so
+ * When trying to report a quiescent state on behalf of some other CPU,
* it is our responsibility to check for and handle potential overflow
* of the rcu_node ->gp_seq counter with respect to the rcu_data counters.
* After all, the CPU might be in deep idle state, and thus executing no
@@ -2608,7 +2608,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
* state, for example, user mode or idle loop. It also schedules RCU
* core processing. If the current grace period has gone on too long,
* it will ask the scheduler to manufacture a context switch for the sole
- * purpose of providing a providing the needed quiescent state.
+ * purpose of providing the needed quiescent state.
*/
void rcu_sched_clock_irq(int user)
{
@@ -3237,7 +3237,7 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
/*
* This function is invoked in workqueue context after a grace period.
- * It frees all the objects queued on ->bhead_free or ->head_free.
+ * It frees all the objects queued on ->bkvhead_free or ->head_free.
*/
static void kfree_rcu_work(struct work_struct *work)
{
@@ -3264,7 +3264,7 @@ static void kfree_rcu_work(struct work_struct *work)
krwp->head_free = NULL;
raw_spin_unlock_irqrestore(&krcp->lock, flags);
- // Handle two first channels.
+ // Handle the first two channels.
for (i = 0; i < FREE_N_CHANNELS; i++) {
for (; bkvhead[i]; bkvhead[i] = bnext) {
bnext = bkvhead[i]->next;
@@ -3531,11 +3531,11 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
}
/*
- * Queue a request for lazy invocation of appropriate free routine after a
- * grace period. Please note there are three paths are maintained, two are the
- * main ones that use array of pointers interface and third one is emergency
- * one, that is used only when the main path can not be maintained temporary,
- * due to memory pressure.
+ * Queue a request for lazy invocation of the appropriate free routine
+ * after a grace period. Please note that three paths are maintained,
+ * two for the common case using arrays of pointers and a third one that
+ * is used only when the main paths cannot be used, for example, due to
+ * memory pressure.
*
* Each kvfree_call_rcu() request is added to a batch. The batch will be drained
* every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch will
@@ -4709,7 +4709,7 @@ void __init rcu_init(void)
rcutree_online_cpu(cpu);
}
- /* Create workqueue for expedited GPs and for Tree SRCU. */
+ /* Create workqueue for Tree SRCU and for expedited GPs. */
rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
WARN_ON(!rcu_gp_wq);
rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
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