On Tue 2015-09-08 12:59:15, Paul E. McKenney wrote:
> On Mon, Sep 07, 2015 at 04:58:27PM +0200, Petr Mladek wrote:
> > 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:
[...]

> > > 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 ;-)
> 
> Good points, but perhaps an easier solution would be to have a
> "firsttime" argument to rcu_gp_fqs() that said whether or not this
> was the first call to rcu_gp_fqs() during the current grace period.
> If this is the first call, then take the "if" branch that passes
> dyntick_save_progress_counter() to force_qs_rnp(), otherwise take the
> other branch.

This seems to be the most elegant solution at the moment.

> But I am not generating the patch today, just flew across the Pacific
> yesterday.  ;-)

Please, find below the updated patch where I used the first_time
parameter.

Again, I am not sure about the commit person and Signed-off-by
tags. Many parts of the patch are yours. Feel free to update
them.


>From 7d7f2ee97a451f5cb055901a3bf22fec23a53bff Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paul...@linux.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
variable into the kthread.

Reported-by: Petr Mladek <pmla...@suse.com>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Signed-off-by: Petr Mladek <pmla...@suse.com>
---
 kernel/rcu/tree.c       | 18 ++++++++----------
 kernel/rcu/tree.h       | 14 +++-----------
 kernel/rcu/tree_trace.c |  2 +-
 3 files changed, 12 insertions(+), 22 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9f75f25cc5d9..5413d87a67c6 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, bool first_time)
 {
-       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 (first_time) {
                /* Collect dyntick-idle snapshots. */
                if (is_sysidle_rcu_state(rsp)) {
                        isidle = true;
@@ -1945,7 +1944,6 @@ 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;
        } else {
                /* Handle dyntick-idle and offline CPUs. */
                isidle = true;
@@ -1959,7 +1957,6 @@ 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 +2020,7 @@ 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 +2038,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
  */
 static int __noreturn rcu_gp_kthread(void *arg)
 {
-       int fqs_state;
+       bool first_gp_fqs;
        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;
+               first_gp_fqs = true;
                j = jiffies_till_first_fqs;
                if (j > HZ) {
                        j = HZ;
@@ -2101,7 +2098,8 @@ 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, first_gp_fqs);
+                               first_gp_fqs = false;
                                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..de370b611837 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
@@ -469,9 +462,8 @@ 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. */
@@ -539,7 +531,7 @@ 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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