Messages in this thread | | | Date | Sat, 17 Apr 2021 21:46:16 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/2] srcu: Fix broken node geometry after early ssp init |
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On Sat, Apr 17, 2021 at 03:16:49PM +0200, Frederic Weisbecker wrote: > On Wed, Apr 14, 2021 at 08:55:38AM -0700, Paul E. McKenney wrote: > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > > > index 75ed367d5b60..24db97cbf76b 100644 > > > --- a/kernel/rcu/rcu.h > > > +++ b/kernel/rcu/rcu.h > > > @@ -278,6 +278,7 @@ extern void resched_cpu(int cpu); > > > extern int rcu_num_lvls; > > > extern int num_rcu_lvl[]; > > > extern int rcu_num_nodes; > > > +extern bool rcu_geometry_initialized; > > > > Can this be a static local variable inside rcu_init_geometry()? > > > > After all, init_srcu_struct() isn't called all that often, and its overhead > > is such that an extra function call and check is going to hurt it. This > > of course requires removing __init from rcu_init_geometry(), but it is not > > all that large, so why not just remove the __init? > > > > But if we really are worried about reclaiming rcu_init_geometry()'s > > instructions (maybe we are?), then rcu_init_geometry() can be split > > into a function that just does the check (which is not __init) and the > > remainder of the function, which could remain __init. > > There you go:
Queued, thank you!
Thanx, Paul
> --- > From: Frederic Weisbecker <frederic@kernel.org> > Date: Wed, 31 Mar 2021 16:10:36 +0200 > Subject: [PATCH] srcu: Fix broken node geometry after early ssp init > > An ssp initialized before rcu_init_geometry() will have its snp hierarchy > based on CONFIG_NR_CPUS. > > Once rcu_init_geometry() is called, the nodes distribution is shrinked > and optimized toward meeting the actual possible number of CPUs detected > on boot. > > Later on, the ssp that was initialized before rcu_init_geometry() is > confused and sometimes refers to its initial CONFIG_NR_CPUS based node > hierarchy, sometimes to the new num_possible_cpus() based one instead. > For example each of its sdp->mynode remain backward and refer to the > early node leaves that may not exist anymore. On the other hand the > srcu_for_each_node_breadth_first() refers to the new node hierarchy. > > There are at least two bad possible outcomes to this: > > 1) a) A callback enqueued early on an sdp is recorded pending on > sdp->mynode->srcu_data_have_cbs in srcu_funnel_gp_start() with > sdp->mynode pointing to a deep leaf (say 3 levels). > > b) The grace period ends after rcu_init_geometry() which shrinks the > nodes level to a single one. srcu_gp_end() walks through the new > snp hierarchy without ever reaching the old leaves so the callback > is never executed. > > This is easily reproduced on an 8 CPUs machine with > CONFIG_NR_CPUS >= 32 and "rcupdate.rcu_self_test=1". The > srcu_barrier() after early tests verification never completes and > the boot hangs: > > [ 5413.141029] INFO: task swapper/0:1 blocked for more than 4915 seconds. > [ 5413.147564] Not tainted 5.12.0-rc4+ #28 > [ 5413.151927] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > [ 5413.159753] task:swapper/0 state:D stack: 0 pid: 1 ppid: 0 flags:0x00004000 > [ 5413.168099] Call Trace: > [ 5413.170555] __schedule+0x36c/0x930 > [ 5413.174057] ? wait_for_completion+0x88/0x110 > [ 5413.178423] schedule+0x46/0xf0 > [ 5413.181575] schedule_timeout+0x284/0x380 > [ 5413.185591] ? wait_for_completion+0x88/0x110 > [ 5413.189957] ? mark_held_locks+0x61/0x80 > [ 5413.193882] ? mark_held_locks+0x61/0x80 > [ 5413.197809] ? _raw_spin_unlock_irq+0x24/0x50 > [ 5413.202173] ? wait_for_completion+0x88/0x110 > [ 5413.206535] wait_for_completion+0xb4/0x110 > [ 5413.210724] ? srcu_torture_stats_print+0x110/0x110 > [ 5413.215610] srcu_barrier+0x187/0x200 > [ 5413.219277] ? rcu_tasks_verify_self_tests+0x50/0x50 > [ 5413.224244] ? rdinit_setup+0x2b/0x2b > [ 5413.227907] rcu_verify_early_boot_tests+0x2d/0x40 > [ 5413.232700] do_one_initcall+0x63/0x310 > [ 5413.236541] ? rdinit_setup+0x2b/0x2b > [ 5413.240207] ? rcu_read_lock_sched_held+0x52/0x80 > [ 5413.244912] kernel_init_freeable+0x253/0x28f > [ 5413.249273] ? rest_init+0x250/0x250 > [ 5413.252846] kernel_init+0xa/0x110 > [ 5413.256257] ret_from_fork+0x22/0x30 > > 2) An ssp that gets initialized before rcu_init_geometry() and used > afterward will always have stale rdp->mynode references, resulting in > callbacks to be missed in srcu_gp_end(), just like in the previous > scenario. > > Solve this with initializing nodes geometry whenever an struct srcu_state > happens to be initialized before rcu_init(). This way we make sure the > RCU nodes hierarchy is properly built and distributed before the nodes > of an struct srcu_state are allocated. > > Suggested-by: Paul E. McKenney <paulmck@kernel.org> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > Cc: Boqun Feng <boqun.feng@gmail.com> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > Cc: Neeraj Upadhyay <neeraju@codeaurora.org> > Cc: Josh Triplett <josh@joshtriplett.org> > Cc: Joel Fernandes <joel@joelfernandes.org> > Cc: Uladzislau Rezki <urezki@gmail.com> > --- > kernel/rcu/rcu.h | 2 ++ > kernel/rcu/srcutree.c | 3 +++ > kernel/rcu/tree.c | 17 ++++++++++++++++- > 3 files changed, 21 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > index 75ed367d5b60..edba5976ca17 100644 > --- a/kernel/rcu/rcu.h > +++ b/kernel/rcu/rcu.h > @@ -308,6 +308,8 @@ static inline void rcu_init_levelspread(int *levelspread, const int *levelcnt) > } > } > > +extern void rcu_init_geometry(void); > + > /* Returns a pointer to the first leaf rcu_node structure. */ > #define rcu_first_leaf_node() (rcu_state.level[rcu_num_lvls - 1]) > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index 55bc9473562b..9efd5eeb5dd5 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -90,6 +90,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp) > struct srcu_node *snp; > struct srcu_node *snp_first; > > + /* First make sure the nodes hierarchy is properly built */ > + rcu_init_geometry(); > + > /* Work out the overall tree geometry. */ > ssp->level[0] = &ssp->node[0]; > for (i = 1; i < rcu_num_lvls; i++) > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 51600ef3fb0c..906f81786ca5 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -4617,11 +4617,26 @@ static void __init rcu_init_one(void) > * replace the definitions in tree.h because those are needed to size > * the ->node array in the rcu_state structure. > */ > -static void __init rcu_init_geometry(void) > +void rcu_init_geometry(void) > { > ulong d; > int i; > + static unsigned long old_nr_cpu_ids; > int rcu_capacity[RCU_NUM_LVLS]; > + static bool initialized; > + > + if (initialized) { > + /* > + * Arrange for warning if rcu_init_geometry() was called before > + * setup_nr_cpu_ids(). We may miss cases when > + * nr_cpus_ids == NR_CPUS but that shouldn't matter too much. > + */ > + WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids); > + return; > + } > + > + old_nr_cpu_ids = nr_cpu_ids; > + initialized = true; > > /* > * Initialize any unspecified boot parameters. > -- > 2.25.1 >
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