On Thu, Dec 13, 2018 at 03:28:46AM +0000, Zhang, Jun wrote: > Ok, we will test it, thanks!
But please also try the sysrq-y with the earlier patch after a hang! Thanx, Paul > -----Original Message----- > From: Paul E. McKenney [mailto:paul...@linux.ibm.com] > Sent: Thursday, December 13, 2018 10:43 > To: Zhang, Jun <jun.zh...@intel.com> > Cc: He, Bo <bo...@intel.com>; Steven Rostedt <rost...@goodmis.org>; > linux-kernel@vger.kernel.org; j...@joshtriplett.org; > mathieu.desnoy...@efficios.com; jiangshan...@gmail.com; Xiao, Jin > <jin.x...@intel.com>; Zhang, Yanmin <yanmin.zh...@intel.com>; Bai, Jie A > <jie.a....@intel.com>; Sun, Yi J <yi.j....@intel.com> > Subject: Re: rcu_preempt caused oom > > On Thu, Dec 13, 2018 at 02:11:35AM +0000, Zhang, Jun wrote: > > 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. > > Just to make sure that I understand, you are worried about something like the > following, correct? > > o __note_gp_changes() compares rnp->gp_seq_needed and rdp->gp_seq_needed > and finds them equal. > > o At just this time something like rcu_start_this_gp() assigns a new > (larger) value to rdp->gp_seq_needed. > > o Then __note_gp_changes() overwrites rdp->gp_seq_needed with the > old value. > > This cannot happen because __note_gp_changes() runs with interrupts disabled > on the CPU corresponding to the rcu_data structure referenced by the rdp > pointer. So there is no way for rcu_start_this_gp() to be invoked on the > same CPU during this "if" statement. > > Of course, there could be bugs. For example: > > o __note_gp_changes() might be called on a different CPU than that > corresponding to rdp. You can check this with something like: > > WARN_ON_ONCE(rdp->cpu != smp_processor_id()); > > o The same things could happen with rcu_start_this_gp(), and the > above WARN_ON_ONCE() would work there as well. > > o rcutree_prepare_cpu() is a special case, but is irrelevant unless > you are doing CPU-hotplug operations. (It can run on a CPU other > than rdp->cpu, but only at times when rdp->cpu is offline.) > > o Interrupts might not really be disabled. > > That said, your patch could reduce overhead slightly, given that the two > values will be equal much of the time. So it might be worth testing just for > that reason. > > So why not just test it anyway? If it makes the bug go away, I will be > surprised, but it would not be the first surprise for me. ;-) > > Thanx, Paul > > > 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:paul...@linux.ibm.com] > > Sent: Thursday, December 13, 2018 08:12 > > 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>; Sun, Yi J > > <yi.j....@intel.com> > > 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 <paul...@linux.ibm.com> > > > Sent: Thursday, December 13, 2018 5:03 AM > > > 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 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 <paul...@linux.ibm.com> > > 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 <paul...@linux.ibm.com> > > > > 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. > > */ > > >