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SubjectRe: [PATCH 1/2] rcu: Show the real fqs_state
On Wed, Sep 09, 2015 at 02:39:50PM +0200, Petr Mladek wrote:
> 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.

Thank you and here you go! Starting testing, and will let you
know how it goes.

Thanx, Paul

------------------------------------------------------------------------

commit bbe84e224959475fd5be9e9c18aede3a6abe4ab9
Author: Petr Mladek <pmladek@suse.com>
Date: Wed Sep 9 12:09:49 2015 -0700

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 was also used to decide when to collect dyntick-idle
snapshots. For this purpose, we add a boolean variable into the kthread,
which is set on the first call to rcu_gp_fqs() for a given grace period
and clear otherwise.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 655414e9a1f4..595148ab53aa 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -97,7 +97,7 @@ 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), \
@@ -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, 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;
@@ -1967,7 +1966,6 @@ 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;
@@ -1981,7 +1979,6 @@ 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;
}

/*
@@ -2045,7 +2042,7 @@ 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;
@@ -2063,7 +2060,7 @@ 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;
@@ -2095,7 +2092,7 @@ 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;
@@ -2123,7 +2120,8 @@ 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 d5f58e717c8b..9d92d161683e 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -417,13 +417,6 @@ 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
@@ -474,9 +467,8 @@ 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. */
@@ -545,7 +537,7 @@ 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 999c3672f990..ef7093cc9b5c 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",


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