| Message ID | 20160629164415.GG4650@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
Hi Paul, On Wed, Jun 29, 2016 at 6:44 PM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Wed, Jun 29, 2016 at 04:54:44PM +0200, Geert Uytterhoeven wrote: >> On Thu, Jun 23, 2016 at 4:53 AM, Paul E. McKenney >> <paulmck@linux.vnet.ibm.com> wrote: >> > On Wed, Jun 22, 2016 at 07:47:42PM -0700, Paul E. McKenney wrote: > > [ . . . ] > >> > @@ -4720,11 +4720,18 @@ static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp) >> > pr_info(" "); >> > level = rnp->level; >> > } >> > - pr_cont("%d:%d ^%d ", rnp->grplo, rnp->grphi, rnp->grpnum); >> > + pr_cont("%d:%d/%#lx/%#lx ^%d ", rnp->grplo, rnp->grphi, >> > + rnp->qsmask, >> > + rnp->qsmaskinit | rnp->qsmaskinitnext, rnp->grpnum); >> > } >> > pr_cont("\n"); >> > } >> >> For me it always crashes during the 37th call of synchronize_sched() in >> setup_kmem_cache_node(), which is the first call after secondary CPU bring up. >> With your and my debug code, I get: >> >> CPU: Testing write buffer coherency: ok >> CPU0: thread -1, cpu 0, socket 0, mpidr 80000000 >> Setting up static identity map for 0x40100000 - 0x40100058 >> cnt = 36, sync >> CPU1: thread -1, cpu 1, socket 0, mpidr 80000001 >> Brought up 2 CPUs >> SMP: Total of 2 processors activated (2132.00 BogoMIPS). >> CPU: All CPU(s) started in SVC mode. >> rcu_node tree layout dump >> 0:1/0x0/0x3 ^0 > > Thank you for running this! > > OK, so RCU knows about both CPUs (the "0x3"), and the previous > grace period has seen quiescent states from both of them (the "0x0"). > That would indicate that your synchronize_sched() showed up when RCU was > idle, so it had to start a new grace period. It also rules out failure > modes where RCU thinks that there are more CPUs than really exist. > (Don't laugh, such things have really happened.) > >> devtmpfs: initialized >> VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1 >> clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, >> max_idle_ns: 19112604462750000 ns >> >> I hope it helps. Thanks! > > I am going to guess that this was the first grace period since the second > CPU came online. When there only on CPU online, synchronize_sched() > is a no-op. > > OK, this showed some things that aren't a problem. What might the > problem be? > > o The grace-period kthread has not yet started. It -should- start > at early_initcall() time, but who knows? Adding code to print > out that kthread's task_struct address. > > o The grace-period kthread might not be responding to wakeups. > Checking this requires that a grace period be in progress, > so please put a call_rcu_sched() just before the call to > rcu_dump_rcu_node_tree(). (Sample code below.) Adding code > to my patch to print out more GP-kthread state as well. > > o One of the CPUs might not be responding to RCU. That -should- > result in an RCU CPU stall warning, so I will ignore this > possibility for the moment. > > That said, do you have some way to determine whether scheduling > clock interrupts are really happening? Without these interrupts, > no RCU CPU stall warnings. I believe there are no clocksources yet. The jiffies clocksource is the first clocksource found, and that happens after the first call to synchronize_sched(), cfr. my dmesg snippet above. In a working boot: # cat /sys/bus/clocksource/devices/clocksource0/available_clocksource e0180000.timer jiffies # cat /sys/bus/clocksource/devices/clocksource0/current_clocksource e0180000.timer > OK, that should be enough for the next phase, please see the end for the > patch. This patch applies on top of my previous one. > > Could you please set this up as follows? > > struct rcu_head rh; > > rcu_dump_rcu_node_tree(&rcu_sched_state); /* Initial state. */ > call_rcu(&rh, do_nothing_cb); I added an empty do_nothing_cb() for this: static void do_nothing_cb(struct rcu_head *rcu_head) { } According to the debugging technique "comment everything out until it boots", it now hangs in call_rcu(). > schedule_timeout_uninterruptible(5 * HZ); /* Or whatever delay. */ > rcu_dump_rcu_node_tree(&rcu_sched_state); /* GP state. */ > synchronize_sched(); /* Probably hangs. */ > rcu_barrier(); /* Drop RCU's references to rh before return. */ Thanks! Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
On Wed, Jun 29, 2016 at 07:52:06PM +0200, Geert Uytterhoeven wrote: > Hi Paul, > > On Wed, Jun 29, 2016 at 6:44 PM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > On Wed, Jun 29, 2016 at 04:54:44PM +0200, Geert Uytterhoeven wrote: > >> On Thu, Jun 23, 2016 at 4:53 AM, Paul E. McKenney > >> <paulmck@linux.vnet.ibm.com> wrote: > >> > On Wed, Jun 22, 2016 at 07:47:42PM -0700, Paul E. McKenney wrote: > > > > [ . . . ] > > > >> > @@ -4720,11 +4720,18 @@ static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp) > >> > pr_info(" "); > >> > level = rnp->level; > >> > } > >> > - pr_cont("%d:%d ^%d ", rnp->grplo, rnp->grphi, rnp->grpnum); > >> > + pr_cont("%d:%d/%#lx/%#lx ^%d ", rnp->grplo, rnp->grphi, > >> > + rnp->qsmask, > >> > + rnp->qsmaskinit | rnp->qsmaskinitnext, rnp->grpnum); > >> > } > >> > pr_cont("\n"); > >> > } > >> > >> For me it always crashes during the 37th call of synchronize_sched() in > >> setup_kmem_cache_node(), which is the first call after secondary CPU bring up. > >> With your and my debug code, I get: > >> > >> CPU: Testing write buffer coherency: ok > >> CPU0: thread -1, cpu 0, socket 0, mpidr 80000000 > >> Setting up static identity map for 0x40100000 - 0x40100058 > >> cnt = 36, sync > >> CPU1: thread -1, cpu 1, socket 0, mpidr 80000001 > >> Brought up 2 CPUs > >> SMP: Total of 2 processors activated (2132.00 BogoMIPS). > >> CPU: All CPU(s) started in SVC mode. > >> rcu_node tree layout dump > >> 0:1/0x0/0x3 ^0 > > > > Thank you for running this! > > > > OK, so RCU knows about both CPUs (the "0x3"), and the previous > > grace period has seen quiescent states from both of them (the "0x0"). > > That would indicate that your synchronize_sched() showed up when RCU was > > idle, so it had to start a new grace period. It also rules out failure > > modes where RCU thinks that there are more CPUs than really exist. > > (Don't laugh, such things have really happened.) > > > >> devtmpfs: initialized > >> VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1 > >> clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, > >> max_idle_ns: 19112604462750000 ns > >> > >> I hope it helps. Thanks! > > > > I am going to guess that this was the first grace period since the second > > CPU came online. When there only on CPU online, synchronize_sched() > > is a no-op. > > > > OK, this showed some things that aren't a problem. What might the > > problem be? > > > > o The grace-period kthread has not yet started. It -should- start > > at early_initcall() time, but who knows? Adding code to print > > out that kthread's task_struct address. > > > > o The grace-period kthread might not be responding to wakeups. > > Checking this requires that a grace period be in progress, > > so please put a call_rcu_sched() just before the call to > > rcu_dump_rcu_node_tree(). (Sample code below.) Adding code > > to my patch to print out more GP-kthread state as well. > > > > o One of the CPUs might not be responding to RCU. That -should- > > result in an RCU CPU stall warning, so I will ignore this > > possibility for the moment. > > > > That said, do you have some way to determine whether scheduling > > clock interrupts are really happening? Without these interrupts, > > no RCU CPU stall warnings. > > I believe there are no clocksources yet. The jiffies clocksource is the first > clocksource found, and that happens after the first call to > synchronize_sched(), cfr. my dmesg snippet above. > > In a working boot: > # cat /sys/bus/clocksource/devices/clocksource0/available_clocksource > e0180000.timer jiffies > # cat /sys/bus/clocksource/devices/clocksource0/current_clocksource > e0180000.timer Ah! But if there is no jiffies clocksource, then schedule_timeout() and friends will never return, correct? If so, I guarantee you that synchronize_sched() will unconditionally hang. So if I understand correctly, the fix is to get the jiffies clocksource running before the first call to synchronize_sched(). Thanx, Paul > > OK, that should be enough for the next phase, please see the end for the > > patch. This patch applies on top of my previous one. > > > > Could you please set this up as follows? > > > > struct rcu_head rh; > > > > rcu_dump_rcu_node_tree(&rcu_sched_state); /* Initial state. */ > > call_rcu(&rh, do_nothing_cb); > > I added an empty do_nothing_cb() for this: > > static void do_nothing_cb(struct rcu_head *rcu_head) > { > } > > According to the debugging technique "comment everything out until it boots", > it now hangs in call_rcu(). > > > schedule_timeout_uninterruptible(5 * HZ); /* Or whatever delay. */ > > rcu_dump_rcu_node_tree(&rcu_sched_state); /* GP state. */ > > synchronize_sched(); /* Probably hangs. */ > > rcu_barrier(); /* Drop RCU's references to rh before return. */ > > Thanks! > > Gr{oetje,eeting}s, > > Geert > > -- > Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org > > In personal conversations with technical people, I call myself a hacker. But > when I'm talking to journalists I just say "programmer" or something like that. > -- Linus Torvalds >
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 2eda7bece401..ff55c569473c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4712,6 +4712,11 @@ static void rcu_dump_rcu_node_tree(struct rcu_state *rsp) int level = 0; struct rcu_node *rnp; + pr_info("RCU: %s GP kthread: %p state: %d flags: %#x g:%ld c:%ld\n", + rsp->name, rsp->gp_kthread, rsp->gp_state, rsp->gp_flags, + (long)rsp->gpnum, (long)rsp->completed); + pr_info(" jiffies: %#lx GP start: %#lx Last GP activity: %#lx\n", + jiffies, rsp->gp_start, rsp->gp_activity); pr_info("rcu_node tree layout dump\n"); pr_info(" "); rcu_for_each_node_breadth_first(rsp, rnp) {