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 <pmla...@suse.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 <pmla...@suse.com>
> Signed-off-by: Paul E. McKenney <paul...@linux.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" <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
into the state struct.
Reported-by: Petr Mladek <pmla...@suse.com>
Signed-off-by: Petr Mladek <pmla...@suse.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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