Hello, Paul
I think the next patch is better.
Because ULONG_CMP_GE could cause double write, which has risk that write back
old value.
Please help review.
I don't test it. If you agree, we will test it.
Thanks!
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0b760c1..c00f34e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1849,7 +1849,7 @@ static bool __note_gp_changes(struct rcu_state *rsp,
struct rcu_node *rnp,
zero_cpu_stall_ticks(rdp);
}
rdp->gp_seq = rnp->gp_seq; /* Remember new grace-period state. */
- if (ULONG_CMP_GE(rnp->gp_seq_needed, rdp->gp_seq_needed) || rdp->gpwrap)
+ if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
rdp->gp_seq_needed = rnp->gp_seq_needed;
WRITE_ONCE(rdp->gpwrap, false);
rcu_gpnum_ovf(rnp, rdp);
-----Original Message-----
From: Paul E. McKenney [mailto:[email protected]]
Sent: Thursday, December 13, 2018 08:12
To: He, Bo <[email protected]>
Cc: Steven Rostedt <[email protected]>; [email protected];
[email protected]; [email protected]; [email protected];
Zhang, Jun <[email protected]>; Xiao, Jin <[email protected]>; Zhang, Yanmin
<[email protected]>; Bai, Jie A <[email protected]>; Sun, Yi J
<[email protected]>
Subject: Re: rcu_preempt caused oom
On Wed, Dec 12, 2018 at 11:13:22PM +0000, He, Bo wrote:
> I don't see the rcutree.sysrq_rcu parameter in v4.19 kernel, I also checked
> the latest kernel and the latest tag v4.20-rc6, not see the sysrq_rcu.
> Please correct me if I have something wrong.
That would be because I sent you the wrong patch, apologies! :-/
Please instead see the one below, which does add sysrq_rcu.
Thanx, Paul
> -----Original Message-----
> From: Paul E. McKenney <[email protected]>
> Sent: Thursday, December 13, 2018 5:03 AM
> To: He, Bo <[email protected]>
> Cc: Steven Rostedt <[email protected]>;
> [email protected]; [email protected];
> [email protected]; [email protected]; Zhang, Jun
> <[email protected]>; Xiao, Jin <[email protected]>; Zhang, Yanmin
> <[email protected]>; Bai, Jie A <[email protected]>
> Subject: Re: rcu_preempt caused oom
>
> On Wed, Dec 12, 2018 at 07:42:24AM -0800, Paul E. McKenney wrote:
> > On Wed, Dec 12, 2018 at 01:21:33PM +0000, He, Bo wrote:
> > > we reproduce on two boards, but I still not see the
> > > show_rcu_gp_kthreads() dump logs, it seems the patch can't catch the
> > > scenario.
> > > I double confirmed the CONFIG_PROVE_RCU=y is enabled in the config as
> > > it's extracted from the /proc/config.gz.
> >
> > Strange.
> >
> > Are the systems responsive to sysrq keys once failure occurs? If
> > so, I will provide you a sysrq-R or some such to dump out the RCU state.
>
> Or, as it turns out, sysrq-y if booting with rcutree.sysrq_rcu=1 using the
> patch below. Only lightly tested.
------------------------------------------------------------------------
commit 04b6245c8458e8725f4169e62912c1fadfdf8141
Author: Paul E. McKenney <[email protected]>
Date: Wed Dec 12 16:10:09 2018 -0800
rcu: Add sysrq rcu_node-dump capability
Backported from v4.21/v5.0
Life is hard if RCU manages to get stuck without triggering RCU CPU
stall warnings or triggering the rcu_check_gp_start_stall() checks
for failing to start a grace period. This commit therefore adds a
boot-time-selectable sysrq key (commandeering "y") that allows manually
dumping Tree RCU state. The new rcutree.sysrq_rcu kernel boot parameter
must be set for this sysrq to be available.
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index
0b760c1369f7..e9392a9d6291 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -61,6 +61,7 @@
#include <linux/trace_events.h>
#include <linux/suspend.h>
#include <linux/ftrace.h>
+#include <linux/sysrq.h>
#include "tree.h"
#include "rcu.h"
@@ -128,6 +129,9 @@ int num_rcu_lvl[] = NUM_RCU_LVL_INIT; int rcu_num_nodes
__read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
/* panic() on RCU Stall sysctl. */
int sysctl_panic_on_rcu_stall __read_mostly;
+/* Commandeer a sysrq key to dump RCU's tree. */ static bool sysrq_rcu;
+module_param(sysrq_rcu, bool, 0444);
/*
* The rcu_scheduler_active variable is initialized to the value @@ -662,6
+666,27 @@ void show_rcu_gp_kthreads(void) }
EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
+/* Dump grace-period-request information due to commandeered sysrq. */
+static void sysrq_show_rcu(int key) {
+ show_rcu_gp_kthreads();
+}
+
+static struct sysrq_key_op sysrq_rcudump_op = {
+ .handler = sysrq_show_rcu,
+ .help_msg = "show-rcu(y)",
+ .action_msg = "Show RCU tree",
+ .enable_mask = SYSRQ_ENABLE_DUMP,
+};
+
+static int __init rcu_sysrq_init(void)
+{
+ if (sysrq_rcu)
+ return register_sysrq_key('y', &sysrq_rcudump_op);
+ return 0;
+}
+early_initcall(rcu_sysrq_init);
+
/*
* Send along grace-period-related data for rcutorture diagnostics.
*/
