On Tue, Jul 23, 2024 at 12:38:35PM -0700, Paul E. McKenney wrote: > On Thu, Jul 18, 2024 at 03:04:49PM -0700, Paul E. McKenney wrote: > > On Mon, Jul 15, 2024 at 04:23:24PM -0700, JP Kobryn wrote: > > > Using a higher value for the initial gp sequence counters allows for > > > wrapping to occur faster. It can help with surfacing any issues that may > > > be happening as a result of the wrap around. > > > > > > Signed-off-by: JP Kobryn <inwardvessel@xxxxxxxxx> > > > > Much nicer, thank you! > > > > Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Unfortunately, extended testing [1] triggered this warning: > > do_rtws_sync: Cookie check 3 failed srcu_torture_synchronize+0x0/0x10() online 0-3. > WARNING: CPU: 2 PID: 57 at kernel/rcu/rcutorture.c:1347 do_rtws_sync.constprop.0+0x1e3/0x250 > > This is in the following code: > > 1 dopoll = cur_ops->get_gp_state && cur_ops->poll_gp_state && !(r & 0x300); > 2 dopoll_full = cur_ops->get_gp_state_full && cur_ops->poll_gp_state_full && !(r & 0xc00); > 3 if (dopoll || dopoll_full) > 4 cpus_read_lock(); > 5 if (dopoll) > 6 cookie = cur_ops->get_gp_state(); > 7 if (dopoll_full) > 8 cur_ops->get_gp_state_full(&cookie_full); > 9 if (cur_ops->poll_need_2gp && cur_ops->poll_need_2gp(dopoll, dopoll_full)) > 10 sync(); > 11 sync(); > 12 WARN_ONCE(dopoll && !cur_ops->poll_gp_state(cookie), > 13 "%s: Cookie check 3 failed %pS() online %*pbl.", > 14 __func__, sync, cpumask_pr_args(cpu_online_mask)); > 15 WARN_ONCE(dopoll_full && !cur_ops->poll_gp_state_full(&cookie_full), > 16 "%s: Cookie check 4 failed %pS() online %*pbl", > 17 __func__, sync, cpumask_pr_args(cpu_online_mask)); > 18 if (dopoll || dopoll_full) > 19 cpus_read_unlock(); > > The cookie collected from get_state_synchronize_srcu() at line 6 > apparently had not yet expired by line 12. > > Adding some debugging got me this: > > do_rtws_sync: Cookie 4/-388 check 3 failed srcu_torture_synchronize+0x0/0x10() online 0-3. > WARNING: CPU: 2 PID: 57 at kernel/rcu/rcutorture.c:1347 do_rtws_sync.constprop.0+0x1e3/0x250 > > The "4/-388" is the value returned by get_state_synchronize_srcu() > (which that ->->get_gp_state() points to) at line 6, namely "4", and that > returned by another call to that same function at line 12, namely -388. > > What is happening is that this rcutorture scenario uses an srcu_struct > structure from DEFINE_STATIC_SRCU(), which initializes the various > grace-period sequence numbers to zero. Therefore, the first call to > get_state_synchronize_srcu() returns 4 (the number of the first grace > period following the mythical grace period #0). But the intervening > call to synchronize_srcu() (which that sync() points to) invokes > check_init_srcu_struct(), which initializes all of those grace-period > squence numbers to negative numbers. Which means that the call to > poll_state_synchronize_srcu() on line 12 (which ->poll_gp_state() points > to) sees a negative grace-period sequence number, and concludes that the > grace period corresponding to that positive-4-values cookie corresponds > to a grace period that has not yet expired. > > My guess is that we will have to do this the hard way, by making > DEFINE_STATIC_SRCU() another counter to an impossible value, for example, > ->srcu_gp_seq_needed_exp to 0x1. Then get_state_synchronize_srcu() > can check for that value, returning (SRCU_GP_SEQ_INITIAL_VAL + > RCU_SEQ_STATE_MASK + 1) or similar. > > Note that start_poll_synchronize_rcu() does not (repeat, *not*) need > adjustment because it already invokes check_init_srcu_struct(). > > But is there a better way? > Though exporting the RCU_SEQ_CTR macros and initial values isn't great, given this scenario, maybe we can go back to the approach taken by JP in his initial patch [1], to initialize the static SRCU initial gp_seq state with SRCU_GP_SEQ_INITIAL_VAL. Does that work? - Neeraj [1] https://lore.kernel.org/rcu/20240712005629.2980-1-inwardvessel@xxxxxxxxx/ > For more detail, there is [2]. And welcome to the exciting world of RCU! > This is why we have long and repeated rcutorture runs. Though "long" > does not help here because this happens at boot or not at all. ;-) > > Thanx, Paul > > [1] tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 90s --configs "400*SRCU-N" --bootargs "rcutorture.gp_sync=1 rcutorture.nfakewriters=70" > [2] https://docs.google.com/document/d/1FHVI1-kjCgLWajSVq8MlBtoc0xxoZNsZYuQsUzW7SIY/edit?usp=sharing > > > > --- > > > kernel/rcu/srcutree.c | 10 +++++++--- > > > 1 file changed, 7 insertions(+), 3 deletions(-) > > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > > index bc4b58b0204e..907c4a484503 100644 > > > --- a/kernel/rcu/srcutree.c > > > +++ b/kernel/rcu/srcutree.c > > > @@ -30,6 +30,9 @@ > > > #include "rcu.h" > > > #include "rcu_segcblist.h" > > > > > > +/* Start with a gp sequence value that allows wrapping to occur faster. */ > > > +#define SRCU_GP_SEQ_INITIAL_VAL ((0UL - 100UL) << RCU_SEQ_CTR_SHIFT) > > > + > > > /* Holdoff in nanoseconds for auto-expediting. */ > > > #define DEFAULT_SRCU_EXP_HOLDOFF (25 * 1000) > > > static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF; > > > @@ -247,7 +250,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static) > > > mutex_init(&ssp->srcu_sup->srcu_cb_mutex); > > > mutex_init(&ssp->srcu_sup->srcu_gp_mutex); > > > ssp->srcu_idx = 0; > > > - ssp->srcu_sup->srcu_gp_seq = 0; > > > + ssp->srcu_sup->srcu_gp_seq = SRCU_GP_SEQ_INITIAL_VAL; > > > ssp->srcu_sup->srcu_barrier_seq = 0; > > > mutex_init(&ssp->srcu_sup->srcu_barrier_mutex); > > > atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0); > > > @@ -258,7 +261,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static) > > > if (!ssp->sda) > > > goto err_free_sup; > > > init_srcu_struct_data(ssp); > > > - ssp->srcu_sup->srcu_gp_seq_needed_exp = 0; > > > + ssp->srcu_sup->srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL; > > > ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns(); > > > if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) { > > > if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC)) > > > @@ -266,7 +269,8 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static) > > > WRITE_ONCE(ssp->srcu_sup->srcu_size_state, SRCU_SIZE_BIG); > > > } > > > ssp->srcu_sup->srcu_ssp = ssp; > > > - smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, 0); /* Init done. */ > > > + smp_store_release(&ssp->srcu_sup->srcu_gp_seq_needed, > > > + SRCU_GP_SEQ_INITIAL_VAL); /* Init done. */ > > > return 0; > > > > > > err_free_sda: > > > -- > > > 2.45.2 > > > > >
