RE: rcu_preempt caused oom

[Zhang, Jun]

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:paulmck@xxxxxxxxxxxxx] 
Sent: Thursday, December 13, 2018 08:12
To: He, Bo <bo.he@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; josh@xxxxxxxxxxxxxxxx; mathieu.desnoyers@xxxxxxxxxxxx; jiangshanlai@xxxxxxxxx; Zhang, Jun <jun.zhang@xxxxxxxxx>; Xiao, Jin <jin.xiao@xxxxxxxxx>; Zhang, Yanmin <yanmin.zhang@xxxxxxxxx>; Bai, Jie A <jie.a.bai@xxxxxxxxx>; Sun, Yi J <yi.j.sun@xxxxxxxxx>
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 <paulmck@xxxxxxxxxxxxx>
> Sent: Thursday, December 13, 2018 5:03 AM
> To: He, Bo <bo.he@xxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>; 
> linux-kernel@xxxxxxxxxxxxxxx; josh@xxxxxxxxxxxxxxxx; 
> mathieu.desnoyers@xxxxxxxxxxxx; jiangshanlai@xxxxxxxxx; Zhang, Jun 
> <jun.zhang@xxxxxxxxx>; Xiao, Jin <jin.xiao@xxxxxxxxx>; Zhang, Yanmin 
> <yanmin.zhang@xxxxxxxxx>; Bai, Jie A <jie.a.bai@xxxxxxxxx>
> 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 <paulmck@xxxxxxxxxxxxx>
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 <paulmck@xxxxxxxxxxxxx>

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.
  */