Re: rcu_preempt caused oom

From: Paul E. McKenney
Date: Wed Dec 12 2018 - 10:42:32 EST

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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.

Thanx, Paul

> -----Original Message-----
> From: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> Sent: Wednesday, December 12, 2018 10:25 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 01:37:40AM +0000, He, Bo wrote:
> > We reproduced the issue panic in hung_task with the patch "Improve diagnostics for failed RCU grace-period start", but unfortunately maybe it's due to the loglevel, the show_rcu_gp_kthreads doesn't print any logs, we will improve the build and run the test to double check.
>
> Well, at least the diagnostics didn't prevent the problem from happening. ;-)
>
> Thanx, Paul
>
> > -----Original Message-----
> > From: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> > Sent: Tuesday, December 11, 2018 12:47 PM
> > 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 Mon, Dec 10, 2018 at 04:38:38PM -0800, Paul E. McKenney wrote:
> > > On Mon, Dec 10, 2018 at 06:56:18AM +0000, He, Bo wrote:
> > > > Hi,
> > > > We have start the test with the CONFIG_PROVE_RCU=y, and also add one 2s to detect the preempt rcu hang, hope we can get more useful logs tomorrow.
> > > > I also enclosed the config and the debug patches for you review.
> > >
> > > I instead suggest the (lightly tested) debug patch shown below,
> > > which tracks wakeups of RCU's grace-period kthreads and dumps them
> > > out if a given requested grace period fails to start. Again, it is
> > > necessary to build with CONFIG_PROVE_RCU=y, that is, with CONFIG_PROVE_LOCKING=y.
> >
> > Right. This time without commenting out the wakeup as a test of the
> > diagnostic. :-/
> >
> > Please use the patch below instead of the one that I sent in my previous email.
> >
> > Thanx, Paul
> >
> > ----------------------------------------------------------------------
> > --
> >
> > commit adfc7dff659495a3433d5084256be59eee0ac6df
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> > Date: Mon Dec 10 16:33:59 2018 -0800
> >
> > rcu: Improve diagnostics for failed RCU grace-period start
> >
> > Backported from v4.21/v5.0
> >
> > If a grace period fails to start (for example, because you commented
> > out the last two lines of rcu_accelerate_cbs_unlocked()), rcu_core()
> > will invoke rcu_check_gp_start_stall(), which will notice and complain.
> > However, this complaint is lacking crucial debugging information such
> > as when the last wakeup executed and what the value of ->gp_seq was at
> > that time. This commit therefore removes the current pr_alert() from
> > rcu_check_gp_start_stall(), instead invoking show_rcu_gp_kthreads(),
> > which has been updated to print the needed information, which is collected
> > by rcu_gp_kthread_wake().
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index
> > 0b760c1369f7..4bcd8753e293 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -626,25 +626,57 @@ void rcu_sched_force_quiescent_state(void)
> > }
> > EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);
> >
> > +/*
> > + * Convert a ->gp_state value to a character string.
> > + */
> > +static const char *gp_state_getname(short gs) {
> > + if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
> > + return "???";
> > + return gp_state_names[gs];
> > +}
> > +
> > +/*
> > + * Return the root node of the specified rcu_state structure.
> > + */
> > +static struct rcu_node *rcu_get_root(struct rcu_state *rsp) {
> > + return &rsp->node[0];
> > +}
> > +
> > /*
> > * Show the state of the grace-period kthreads.
> > */
> > void show_rcu_gp_kthreads(void)
> > {
> > int cpu;
> > + unsigned long j;
> > + unsigned long ja;
> > + unsigned long jr;
> > + unsigned long jw;
> > struct rcu_data *rdp;
> > struct rcu_node *rnp;
> > struct rcu_state *rsp;
> >
> > + j = jiffies;
> > for_each_rcu_flavor(rsp) {
> > - pr_info("%s: wait state: %d ->state: %#lx\n",
> > - rsp->name, rsp->gp_state, rsp->gp_kthread->state);
> > + ja = j - READ_ONCE(rsp->gp_activity);
> > + jr = j - READ_ONCE(rsp->gp_req_activity);
> > + jw = j - READ_ONCE(rsp->gp_wake_time);
> > + pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
> > + rsp->name, gp_state_getname(rsp->gp_state),
> > + rsp->gp_state,
> > + rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL,
> > + ja, jr, jw, (long)READ_ONCE(rsp->gp_wake_seq),
> > + (long)READ_ONCE(rsp->gp_seq),
> > + (long)READ_ONCE(rcu_get_root(rsp)->gp_seq_needed),
> > + READ_ONCE(rsp->gp_flags));
> > rcu_for_each_node_breadth_first(rsp, rnp) {
> > if (ULONG_CMP_GE(rsp->gp_seq, rnp->gp_seq_needed))
> > continue;
> > - pr_info("\trcu_node %d:%d ->gp_seq %lu ->gp_seq_needed %lu\n",
> > - rnp->grplo, rnp->grphi, rnp->gp_seq,
> > - rnp->gp_seq_needed);
> > + pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
> > + rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
> > + (long)rnp->gp_seq_needed);
> > if (!rcu_is_leaf_node(rnp))
> > continue;
> > for_each_leaf_node_possible_cpu(rnp, cpu) { @@ -653,8 +685,8 @@ void show_rcu_gp_kthreads(void)
> > ULONG_CMP_GE(rsp->gp_seq,
> > rdp->gp_seq_needed))
> > continue;
> > - pr_info("\tcpu %d ->gp_seq_needed %lu\n",
> > - cpu, rdp->gp_seq_needed);
> > + pr_info("\tcpu %d ->gp_seq_needed %ld\n",
> > + cpu, (long)rdp->gp_seq_needed);
> > }
> > }
> > /* sched_show_task(rsp->gp_kthread); */ @@ -690,14 +722,6 @@ void
> > rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags, }
> > EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
> >
> > -/*
> > - * Return the root node of the specified rcu_state structure.
> > - */
> > -static struct rcu_node *rcu_get_root(struct rcu_state *rsp) -{
> > - return &rsp->node[0];
> > -}
> > -
> > /*
> > * Enter an RCU extended quiescent state, which can be either the
> > * idle loop or adaptive-tickless usermode execution.
> > @@ -1285,16 +1309,6 @@ static void record_gp_stall_check_time(struct rcu_state *rsp)
> > rsp->n_force_qs_gpstart = READ_ONCE(rsp->n_force_qs); }
> >
> > -/*
> > - * Convert a ->gp_state value to a character string.
> > - */
> > -static const char *gp_state_getname(short gs) -{
> > - if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
> > - return "???";
> > - return gp_state_names[gs];
> > -}
> > -
> > /*
> > * Complain about starvation of grace-period kthread.
> > */
> > @@ -1693,7 +1707,8 @@ static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
> > * Don't do a self-awaken, and don't bother awakening when there is
> > * nothing for the grace-period kthread to do (as in several CPUs
> > * raced to awaken, and we lost), and finally don't try to awaken
> > - * a kthread that has not yet been created.
> > + * a kthread that has not yet been created. If all those checks are
> > + * passed, track some debug information and awaken.
> > */
> > static void rcu_gp_kthread_wake(struct rcu_state *rsp) { @@ -1701,6 +1716,8 @@ static void rcu_gp_kthread_wake(struct rcu_state *rsp)
> > !READ_ONCE(rsp->gp_flags) ||
> > !rsp->gp_kthread)
> > return;
> > + WRITE_ONCE(rsp->gp_wake_time, jiffies);
> > + WRITE_ONCE(rsp->gp_wake_seq, READ_ONCE(rsp->gp_seq));
> > swake_up_one(&rsp->gp_wq);
> > }
> >
> > @@ -2802,16 +2819,11 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
> > raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > return;
> > }
> > - pr_alert("%s: g%ld->%ld gar:%lu ga:%lu f%#x gs:%d %s->state:%#lx\n",
> > - __func__, (long)READ_ONCE(rsp->gp_seq),
> > - (long)READ_ONCE(rnp_root->gp_seq_needed),
> > - j - rsp->gp_req_activity, j - rsp->gp_activity,
> > - rsp->gp_flags, rsp->gp_state, rsp->name,
> > - rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL);
> > WARN_ON(1);
> > if (rnp_root != rnp)
> > raw_spin_unlock_rcu_node(rnp_root);
> > raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > + show_rcu_gp_kthreads();
> > }
> >
> > /*
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index
> > 4e74df768c57..0e051d9b5f1a 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -327,6 +327,8 @@ struct rcu_state {
> > struct swait_queue_head gp_wq; /* Where GP task waits. */
> > short gp_flags; /* Commands for GP task. */
> > short gp_state; /* GP kthread sleep state. */
> > + unsigned long gp_wake_time; /* Last GP kthread wake. */
> > + unsigned long gp_wake_seq; /* ->gp_seq at ^^^. */
> >
> > /* End of fields guarded by root rcu_node's lock. */
> >
> >
>
>

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