Date:	Mon, 7 Sep 2015 16:58:27 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Jiri Kosina <jkosina@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rcu: Show the real fqs_state

On Fri 2015-09-04 16:24:22, Paul E. McKenney wrote:
> On Fri, Sep 04, 2015 at 02:11:29PM +0200, Petr Mladek wrote:
> > The value of "fqs_state" in struct rcu_state is always RCU_GP_IDLE.
> > 
> > The real state is stored in a local variable in rcu_gp_kthread().
> > It is modified by rcu_gp_fqs() via parameter and return value.
> > But the actual value is never stored to rsp->fqs_state.
> > 
> > The result is that print_one_rcu_state() does not show the real
> > state.
> > 
> > This code has been added 3 years ago by the commit 4cdfc175c25c89ee
> > ("rcu: Move quiescent-state forcing into kthread"). I guess that it
> > was an overlook or optimization.
> > 
> > Anyway, the value seems to be manipulated only by the thread, except
> > for shoving the status. I do not see any risk in updating it directly
> > in the struct.
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> 
> Good catch, but how about the following fix instead?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
>     rcu: Finish folding ->fqs_state into ->gp_state
>     
>     Commit commit 4cdfc175c25c89ee ("rcu: Move quiescent-state forcing
>     into kthread") started the process of folding the old ->fqs_state
>     into ->gp_state, but did not complete it.  This situation does not
>     cause any malfunction, but can result in extremely confusing trace
>     output.  This commit completes this task of eliminating ->fqs_state
>     in favor of ->gp_state.

It makes sense but it breaks dynticks handling in rcu_gp_fqs(), see
below.

>     
>     Reported-by: Petr Mladek <pmladek@...e.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 69ab7ce2cf7b..04234936d897 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1949,16 +1949,15 @@ static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
>  /*
>   * Do one round of quiescent-state forcing.
>   */
> -static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
> +static void rcu_gp_fqs(struct rcu_state *rsp)
>  {
> -	int fqs_state = fqs_state_in;
>  	bool isidle = false;
>  	unsigned long maxj;
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  
>  	WRITE_ONCE(rsp->gp_activity, jiffies);
>  	rsp->n_force_qs++;
> -	if (fqs_state == RCU_SAVE_DYNTICK) {
> +	if (rsp->gp_state == RCU_SAVE_DYNTICK) {

This will never happen because rcu_gp_kthread() modifies rsp->gp_state
many times. The last value before calling rcu_gp_fqs() is
RCU_GP_DOING_FQS.

I think about passing this information via a separate bool.

[...]

> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index d5f58e717c8b..9faad70a8246 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -417,12 +417,11 @@ struct rcu_data {
>  	struct rcu_state *rsp;
>  };
>  
> -/* Values for fqs_state field in struct rcu_state. */
> +/* Values for gp_state field in struct rcu_state. */
>  #define RCU_GP_IDLE		0	/* No grace period in progress. */

This value seems to be used instead of the new RCU_GP_WAIT_INIT.

>  #define RCU_GP_INIT		1	/* Grace period being
>  #initialized. */

This value is unused.

>  #define RCU_SAVE_DYNTICK	2	/* Need to scan dyntick
>  #state. */

This one is not longer preserved when merged with the other state.

>  #define RCU_FORCE_QS		3	/* Need to force quiescent
>  #state. */

The meaning of this one is strange. If I get it correctly,
it is set after the state was forced. But the comment suggests
that it is before.

By other words, these states seems to get obsoleted by

/* Values for rcu_state structure's gp_flags field. */
#define RCU_GP_WAIT_INIT 0	/* Initial state. */
#define RCU_GP_WAIT_GPS  1	/* Wait for grace-period start. */
#define RCU_GP_DONE_GPS  2	/* Wait done for grace-period start. */
#define RCU_GP_WAIT_FQS  3	/* Wait for force-quiescent-state time. */
#define RCU_GP_DOING_FQS 4	/* Wait done for force-quiescent-state time. */
#define RCU_GP_CLEANUP   5	/* Grace-period cleanup started. */
#define RCU_GP_CLEANED   6	/* Grace-period cleanup complete. */


Please, find below your commit updated with my ideas:

	+ used bool save_dyntick instead of RCU_SAVE_DYNTICK
	  and RCU_FORCE_QS states
	+ rename RCU_GP_WAIT_INIT -> RCU_GP_IDLE
	+ remove all the obsolete states

I am sorry if I handled "Signed-off-by" flags a wrong way. It is
basically your patch with few small updates from me. I am not sure
what is the right process in this case. Feel free to use Reviewed-by
instead of Signed-off-by with my name.

Well, I guess that this is not the final state ;-)


>From 61a1bf6659f4f4c0c4021f185bc156f8c83f9ea5 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Date: Fri, 4 Sep 2015 16:24:22 -0700
Subject: [PATCH] rcu: Finish folding ->fqs_state into ->gp_state

Commit commit 4cdfc175c25c89ee ("rcu: Move quiescent-state forcing
into kthread") started the process of folding the old ->fqs_state
into ->gp_state, but did not complete it.  This situation does not
cause any malfunction, but can result in extremely confusing trace
output.  This commit completes this task of eliminating ->fqs_state
in favor of ->gp_state.

The old fqs_state had one side effect.  It was used to decide whether
to collect dyntick-idle snapshots.  For this purpose, we add a boolean
into the state struct.

Reported-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 kernel/rcu/tree.c       | 17 +++++++----------
 kernel/rcu/tree.h       | 16 +++++-----------
 kernel/rcu/tree_trace.c |  2 +-
 3 files changed, 13 insertions(+), 22 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9f75f25cc5d9..f47067fdc783 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -98,7 +98,7 @@ struct rcu_state sname##_state = { \
 	.level = { &sname##_state.node[0] }, \
 	.rda = &sname##_data, \
 	.call = cr, \
-	.fqs_state = RCU_GP_IDLE, \
+	.gp_state = RCU_GP_IDLE, \
 	.gpnum = 0UL - 300UL, \
 	.completed = 0UL - 300UL, \
 	.orphan_lock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.orphan_lock), \
@@ -1927,16 +1927,15 @@ static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
 /*
  * Do one round of quiescent-state forcing.
  */
-static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
+static void rcu_gp_fqs(struct rcu_state *rsp)
 {
-	int fqs_state = fqs_state_in;
 	bool isidle = false;
 	unsigned long maxj;
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	WRITE_ONCE(rsp->gp_activity, jiffies);
 	rsp->n_force_qs++;
-	if (fqs_state == RCU_SAVE_DYNTICK) {
+	if (rsp->save_dyntick) {
 		/* Collect dyntick-idle snapshots. */
 		if (is_sysidle_rcu_state(rsp)) {
 			isidle = true;
@@ -1945,7 +1944,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 		force_qs_rnp(rsp, dyntick_save_progress_counter,
 			     &isidle, &maxj);
 		rcu_sysidle_report_gp(rsp, isidle, maxj);
-		fqs_state = RCU_FORCE_QS;
+		rsp->save_dyntick = false;
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
 		isidle = true;
@@ -1959,7 +1958,6 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 			   READ_ONCE(rsp->gp_flags) & ~RCU_GP_FLAG_FQS);
 		raw_spin_unlock_irq(&rnp->lock);
 	}
-	return fqs_state;
 }
 
 /*
@@ -2023,7 +2021,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	/* Declare grace period done. */
 	WRITE_ONCE(rsp->completed, rsp->gpnum);
 	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
-	rsp->fqs_state = RCU_GP_IDLE;
+	rsp->gp_state = RCU_GP_IDLE;
 	rdp = this_cpu_ptr(rsp->rda);
 	/* Advance CBs to reduce false positives below. */
 	needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
@@ -2041,7 +2039,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
  */
 static int __noreturn rcu_gp_kthread(void *arg)
 {
-	int fqs_state;
 	int gf;
 	unsigned long j;
 	int ret;
@@ -2073,7 +2070,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 		}
 
 		/* Handle quiescent-state forcing. */
-		fqs_state = RCU_SAVE_DYNTICK;
+		rsp->save_dyntick = true;
 		j = jiffies_till_first_fqs;
 		if (j > HZ) {
 			j = HZ;
@@ -2101,7 +2098,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 				trace_rcu_grace_period(rsp->name,
 						       READ_ONCE(rsp->gpnum),
 						       TPS("fqsstart"));
-				fqs_state = rcu_gp_fqs(rsp, fqs_state);
+				rcu_gp_fqs(rsp);
 				trace_rcu_grace_period(rsp->name,
 						       READ_ONCE(rsp->gpnum),
 						       TPS("fqsend"));
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 2e991f8361e4..12303ff25077 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -412,13 +412,6 @@ struct rcu_data {
 	struct rcu_state *rsp;
 };
 
-/* Values for fqs_state field in struct rcu_state. */
-#define RCU_GP_IDLE		0	/* No grace period in progress. */
-#define RCU_GP_INIT		1	/* Grace period being initialized. */
-#define RCU_SAVE_DYNTICK	2	/* Need to scan dyntick state. */
-#define RCU_FORCE_QS		3	/* Need to force quiescent state. */
-#define RCU_SIGNAL_INIT		RCU_SAVE_DYNTICK
-
 /* Values for nocb_defer_wakeup field in struct rcu_data. */
 #define RCU_NOGP_WAKE_NOT	0
 #define RCU_NOGP_WAKE		1
@@ -469,15 +462,16 @@ struct rcu_state {
 
 	/* The following fields are guarded by the root rcu_node's lock. */
 
-	u8	fqs_state ____cacheline_internodealigned_in_smp;
-						/* Force QS state. */
-	u8	boost;				/* Subject to priority boost. */
+	u8	boost ____cacheline_internodealigned_in_smp;
+						/* Subject to priority boost. */
 	unsigned long gpnum;			/* Current gp number. */
 	unsigned long completed;		/* # of last completed gp. */
 	struct task_struct *gp_kthread;		/* Task for grace periods. */
 	wait_queue_head_t gp_wq;		/* Where GP task waits. */
 	short gp_flags;				/* Commands for GP task. */
 	short gp_state;				/* GP kthread sleep state. */
+	bool save_dyntick;			/* Collect dyntick-idle */
+						/* snapshots when forcing QS. */
 
 	/* End of fields guarded by root rcu_node's lock. */
 
@@ -539,7 +533,7 @@ struct rcu_state {
 #define RCU_GP_FLAG_FQS  0x2	/* Need grace-period quiescent-state forcing. */
 
 /* Values for rcu_state structure's gp_flags field. */
-#define RCU_GP_WAIT_INIT 0	/* Initial state. */
+#define RCU_GP_IDLE	 0	/* Initial state and no GP in progress. */
 #define RCU_GP_WAIT_GPS  1	/* Wait for grace-period start. */
 #define RCU_GP_DONE_GPS  2	/* Wait done for grace-period start. */
 #define RCU_GP_WAIT_FQS  3	/* Wait for force-quiescent-state time. */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index 6fc4c5ff3bb5..1d61f5ba4641 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -268,7 +268,7 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
 	gpnum = rsp->gpnum;
 	seq_printf(m, "c=%ld g=%ld s=%d jfq=%ld j=%x ",
 		   ulong2long(rsp->completed), ulong2long(gpnum),
-		   rsp->fqs_state,
+		   rsp->gp_state,
 		   (long)(rsp->jiffies_force_qs - jiffies),
 		   (int)(jiffies & 0xffff));
 	seq_printf(m, "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",
-- 
1.8.5.6


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