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 <paul...@linux.ibm.com>
> Sent: Tuesday, December 11, 2018 12:47 PM
> To: He, Bo <bo...@intel.com>
> Cc: Steven Rostedt <rost...@goodmis.org>; linux-kernel@vger.kernel.org;
> j...@joshtriplett.org; mathieu.desnoy...@efficios.com;
> jiangshan...@gmail.com; Zhang, Jun <jun.zh...@intel.com>; Xiao, Jin
> <jin.x...@intel.com>; Zhang, Yanmin <yanmin.zh...@intel.com>; Bai, Jie A
> <jie.a....@intel.com>
> 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 <paul...@linux.ibm.com>
> 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 <paul...@linux.ibm.com>
>
> 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. */
>
>
