Messages in this thread | | | Date | Wed, 20 Jun 2018 09:49:02 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks |
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On Thu, Jun 21, 2018 at 01:05:22AM +0900, Byungchul Park wrote: > On Wed, Jun 20, 2018 at 11:58 PM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > On Wed, Jun 20, 2018 at 05:47:20PM +0900, Byungchul Park wrote: > >> Hello folks, > >> > >> I'm careful in saying that ->dynticks_nmi_nesting can be removed but I > >> think it's possible since the only thing we are interested in with > >> regard to ->dynticks_nesting or ->dynticks_nmi_nesting is whether rcu is > >> idle or not. > > > > Please keep in mind that NMIs cannot be masked, which means that the > > rcu_nmi_enter() and rcu_nmi_exit() pair can be invoked at any point in > > the process, between any consecutive pair of instructions. The saving
And yes, I should have looked at this patch more closely before replying. But please see below.
> I believe I understand what NMI is and why you introduced > ->dynticks_nmi_nesting. Or am I missing something?
Perhaps the fact that there are architectures that can enter interrupt handlers and never leave them when the CPU is non-idle. One example of this is the usermode upcalls in the comment that you removed.
Or have all the architectures been modified so that each and every call to rcu_irq_enter() and to rcu_irq_exit() are now properly paired and nested?
Proper nesting and pairing was -not- present in the past, hence the special updates (AKA "crowbar") to the counters when transitioning to and from idle.
If proper nesting and pairing of rcu_irq_enter() and rcu_irq_exit() is now fully in force across all architectures and configurations, the commit log needs to say this, preferably pointing to the corresponding commits that made this change.
> > grace is that these two functions restore state, but you cannot make them. > > After all, NMI does stand for non-maskable interrupt. > > Excuse me, but I think I've considered that all. Could you show me > what problem can happen with this?
There is a call to rcu_irq_enter() without a corresponding call to rcu_irq_exit(), so that the ->dynticks_nesting counter never goes back to zero so that the next time this CPU goes idle, RCU thinks that the CPU is still non-idle. This can result in excessively long grace periods and needless IPIs to idle CPUs.
> These two functions, rcu_nmi_enter() and rcu_nmi_exit(), would still > save and restore the state with ->dynticks_nesting.
As far as I know, rcu_nmi_enter() and rcu_nmi_exit() -are- properly paired and nested across all architectures and configurations, so yes, they do act more naturally.
> Even though I made ->dynticks_nesting shared between NMI and > other contexts entering or exiting eqs, I believe it's not a problem > because anyway the variable would be updated and finally restored > in a *nested* manner.
But only if calls to rcu_irq_enter() and rcu_irq_exit() are now always properly paired and nested, which was definitely -not- the case last I looked.
> > At first glance, the code below does -not- take this into account. > > Excuse me, but could explain it more? I don't understand your point :( > > > What am I missing that would make this change safe? (You would need to > > convince both me and Andy Lutomirski, who I have added on CC.) > > Thanks for doing that. > > >> And I'm also afraid if the assumption is correct for every archs which I > >> based on, that is, an assignment operation on a single aligned word is > >> atomic in terms of instruction. > > > > The key point is that both ->dynticks_nesting and ->dynticks_nmi_nesting > > are accessed only by the corresponding CPU (other than debug prints). > > Load and store tearing should therefore not be a problem. Are there > > Right. But I thought it can be a problem between NMI and other contexts > because I made ->dynticks_nesting shared between NMI and others. > > > other reasons to promote to READ_ONCE() and WRITE_ONCE()? If there are, > > a separate patch doing that promotion would be good. > > But the promotion is meaningless without making ->dynticks_nesting > shared as you said. I'm afraid it's too dependent on this patch to > separate it. > > I'm sorry I don't understand your point. It would be very appreciated if > you explain it more about what I'm missing or your point :(
OK, so I can further consider this pair of patches only if all architectures now properly pair and nest rcu_irq_enter() and rcu_irq_exit(). It would be very good if they did, but actually testing back in the day showed that they did not. If that has changed, that would be a very good thing, but if not, this patch injects bugs.
Thanx, Paul
> >> Thoughs? > >> > >> ----->8----- > >> >From 84970b33eb06c3bb1bebbb1754db405c0fc50fbe Mon Sep 17 00:00:00 2001 > >> From: Byungchul Park <byungchul.park@lge.com> > >> Date: Wed, 20 Jun 2018 16:01:20 +0900 > >> Subject: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks > >> > >> The only thing we are interested in with regard to ->dynticks_nesting or > >> ->dynticks_nmi_nesting is whether rcu is idle or not, which can be > >> handled only using ->dynticks_nesting though. ->dynticks_nmi_nesting is > >> unnecessary but to make the code more complicated. > >> > >> This patch makes both rcu_eqs_{enter,exit}() and rcu_nmi_{enter,exit}() > >> count up and down a single variable, ->dynticks_nesting to keep how many > >> rcu non-idle sections have been nested. > >> > >> As a result, no matter who made the variable be non-zero, it's anyway > >> non-idle, and it can be considered as just having been idle once the > >> variable is equal to zero. So tricky code can be removed. > >> > >> In addition, it was assumed that an assignment operation on a single > >> aligned word is atomic so that ->dynticks_nesting can be safely assigned > >> within both nmi context and others concurrently. > >> > >> Signed-off-by: Byungchul Park <byungchul.park@lge.com> > >> --- > >> kernel/rcu/tree.c | 76 ++++++++++++++++++++---------------------------- > >> kernel/rcu/tree.h | 4 +-- > >> kernel/rcu/tree_plugin.h | 4 +-- > >> 3 files changed, 35 insertions(+), 49 deletions(-) > >> > >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > >> index 59ae94e..61f203a 100644 > >> --- a/kernel/rcu/tree.c > >> +++ b/kernel/rcu/tree.c > >> @@ -260,7 +260,6 @@ void rcu_bh_qs(void) > >> > >> static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = { > >> .dynticks_nesting = 1, > >> - .dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE, > >> .dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR), > >> }; > >> > >> @@ -694,10 +693,6 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp) > >> /* > >> * Enter an RCU extended quiescent state, which can be either the > >> * idle loop or adaptive-tickless usermode execution. > >> - * > >> - * We crowbar the ->dynticks_nmi_nesting field to zero to allow for > >> - * the possibility of usermode upcalls having messed up our count > >> - * of interrupt nesting level during the prior busy period. > >> */ > >> static void rcu_eqs_enter(bool user) > >> { > >> @@ -706,11 +701,11 @@ static void rcu_eqs_enter(bool user) > >> struct rcu_dynticks *rdtp; > >> > >> rdtp = this_cpu_ptr(&rcu_dynticks); > >> - WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); > >> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && > >> rdtp->dynticks_nesting == 0); > >> if (rdtp->dynticks_nesting != 1) { > >> - rdtp->dynticks_nesting--; > >> + WRITE_ONCE(rdtp->dynticks_nesting, /* No store tearing. */ > >> + rdtp->dynticks_nesting - 1); > >> return; > >> } > >> > >> @@ -767,7 +762,7 @@ void rcu_user_enter(void) > >> * rcu_nmi_exit - inform RCU of exit from NMI context > >> * > >> * If we are returning from the outermost NMI handler that interrupted an > >> - * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting > >> + * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nesting > >> * to let the RCU grace-period handling know that the CPU is back to > >> * being RCU-idle. > >> * > >> @@ -779,21 +774,21 @@ void rcu_nmi_exit(void) > >> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > >> > >> /* > >> - * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks. > >> + * Check for ->dynticks_nesting underflow and bad ->dynticks. > >> * (We are exiting an NMI handler, so RCU better be paying attention > >> * to us!) > >> */ > >> - WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0); > >> + WARN_ON_ONCE(rdtp->dynticks_nesting <= 0); > >> WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()); > >> > >> /* > >> * If the nesting level is not 1, the CPU wasn't RCU-idle, so > >> * leave it in non-RCU-idle state. > >> */ > >> - if (rdtp->dynticks_nmi_nesting != 1) { > >> - trace_rcu_dyntick(TPS("--="), rdtp->dynticks_nmi_nesting, rdtp->dynticks_nmi_nesting - 2, rdtp->dynticks); > >> - WRITE_ONCE(rdtp->dynticks_nmi_nesting, /* No store tearing. */ > >> - rdtp->dynticks_nmi_nesting - 2); > >> + if (rdtp->dynticks_nesting != 1) { > >> + trace_rcu_dyntick(TPS("--="), rdtp->dynticks_nesting, rdtp->dynticks_nesting - 1, rdtp->dynticks); > >> + WRITE_ONCE(rdtp->dynticks_nesting, /* No store tearing. */ > >> + rdtp->dynticks_nesting - 1); > >> return; > >> } > >> > >> @@ -803,8 +798,8 @@ void rcu_nmi_exit(void) > >> } > >> > >> /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */ > >> - trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0, rdtp->dynticks); > >> - WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */ > >> + trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nesting, 0, rdtp->dynticks); > >> + WRITE_ONCE(rdtp->dynticks_nesting, 0); /* Avoid store tearing. */ > >> rcu_dynticks_eqs_enter(); > >> } > >> > >> @@ -851,10 +846,6 @@ void rcu_irq_exit_irqson(void) > >> /* > >> * Exit an RCU extended quiescent state, which can be either the > >> * idle loop or adaptive-tickless usermode execution. > >> - * > >> - * We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to > >> - * allow for the possibility of usermode upcalls messing up our count of > >> - * interrupt nesting level during the busy period that is just now starting. > >> */ > >> static void rcu_eqs_exit(bool user) > >> { > >> @@ -866,7 +857,8 @@ static void rcu_eqs_exit(bool user) > >> oldval = rdtp->dynticks_nesting; > >> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0); > >> if (oldval) { > >> - rdtp->dynticks_nesting++; > >> + WRITE_ONCE(rdtp->dynticks_nesting, /* No store tearing. */ > >> + rdtp->dynticks_nesting + 1); > >> return; > >> } > >> rcu_dynticks_task_exit(); > >> @@ -875,7 +867,6 @@ static void rcu_eqs_exit(bool user) > >> trace_rcu_dyntick(TPS("End"), rdtp->dynticks_nesting, 1, rdtp->dynticks); > >> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current)); > >> WRITE_ONCE(rdtp->dynticks_nesting, 1); > >> - WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE); > >> } > >> > >> /** > >> @@ -915,11 +906,11 @@ void rcu_user_exit(void) > >> /** > >> * rcu_nmi_enter - inform RCU of entry to NMI context > >> * > >> - * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and > >> - * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know > >> - * that the CPU is active. This implementation permits nested NMIs, as > >> - * long as the nesting level does not overflow an int. (You will probably > >> - * run out of stack space first.) > >> + * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks. And > >> + * then update rdtp->dynticks_nesting to let the RCU grace-period handling > >> + * know that the CPU is active. This implementation permits nested NMIs, > >> + * as long as the nesting level does not overflow an int. (You will > >> + * probably run out of stack space first.) > >> * > >> * If you add or remove a call to rcu_nmi_enter(), be sure to test > >> * with CONFIG_RCU_EQS_DEBUG=y. > >> @@ -927,33 +918,29 @@ void rcu_user_exit(void) > >> void rcu_nmi_enter(void) > >> { > >> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > >> - long incby = 2; > >> > >> /* Complain about underflow. */ > >> - WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 0); > >> + WARN_ON_ONCE(rdtp->dynticks_nesting < 0); > >> > >> - /* > >> - * If idle from RCU viewpoint, atomically increment ->dynticks > >> - * to mark non-idle and increment ->dynticks_nmi_nesting by one. > >> - * Otherwise, increment ->dynticks_nmi_nesting by two. This means > >> - * if ->dynticks_nmi_nesting is equal to one, we are guaranteed > >> - * to be in the outermost NMI handler that interrupted an RCU-idle > >> - * period (observation due to Andy Lutomirski). > >> - */ > >> if (rcu_dynticks_curr_cpu_in_eqs()) { > >> rcu_dynticks_eqs_exit(); > >> - incby = 1; > >> > >> if (!in_nmi()) { > >> rcu_dynticks_task_exit(); > >> rcu_cleanup_after_idle(); > >> } > >> } > >> - trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="), > >> - rdtp->dynticks_nmi_nesting, > >> - rdtp->dynticks_nmi_nesting + incby, rdtp->dynticks); > >> - WRITE_ONCE(rdtp->dynticks_nmi_nesting, /* Prevent store tearing. */ > >> - rdtp->dynticks_nmi_nesting + incby); > >> + > >> + trace_rcu_dyntick(rdtp->dynticks_nesting ? TPS("++=") : TPS("Endirq"), > >> + rdtp->dynticks_nesting, > >> + rdtp->dynticks_nesting + 1, rdtp->dynticks); > >> + /* > >> + * If ->dynticks_nesting is equal to one on rcu_nmi_exit(), we are > >> + * guaranteed to be in the outermost NMI handler that interrupted > >> + * an RCU-idle period (observation due to Andy Lutomirski). > >> + */ > >> + WRITE_ONCE(rdtp->dynticks_nesting, /* Prevent store tearing. */ > >> + rdtp->dynticks_nesting + 1); > >> barrier(); > >> } > >> > >> @@ -1089,8 +1076,7 @@ bool rcu_lockdep_current_cpu_online(void) > >> */ > >> static int rcu_is_cpu_rrupt_from_idle(void) > >> { > >> - return __this_cpu_read(rcu_dynticks.dynticks_nesting) <= 0 && > >> - __this_cpu_read(rcu_dynticks.dynticks_nmi_nesting) <= 1; > >> + return __this_cpu_read(rcu_dynticks.dynticks_nesting) <= 1; > >> } > >> > >> /* > >> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > >> index 4e74df7..071afe4 100644 > >> --- a/kernel/rcu/tree.h > >> +++ b/kernel/rcu/tree.h > >> @@ -38,8 +38,8 @@ > >> * Dynticks per-CPU state. > >> */ > >> struct rcu_dynticks { > >> - long dynticks_nesting; /* Track process nesting level. */ > >> - long dynticks_nmi_nesting; /* Track irq/NMI nesting level. */ > >> + long dynticks_nesting __attribute__((aligned(sizeof(long)))); > >> + /* Track process nesting level. */ > >> atomic_t dynticks; /* Even value for idle, else odd. */ > >> bool rcu_need_heavy_qs; /* GP old, need heavy quiescent state. */ > >> unsigned long rcu_qs_ctr; /* Light universal quiescent state ctr. */ > >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > >> index c1b17f5..0c57e50 100644 > >> --- a/kernel/rcu/tree_plugin.h > >> +++ b/kernel/rcu/tree_plugin.h > >> @@ -1801,7 +1801,7 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu) > >> } > >> print_cpu_stall_fast_no_hz(fast_no_hz, cpu); > >> delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq); > >> - pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n", > >> + pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld softirq=%u/%u fqs=%ld %s\n", > >> cpu, > >> "O."[!!cpu_online(cpu)], > >> "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)], > >> @@ -1811,7 +1811,7 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu) > >> "!."[!delta], > >> ticks_value, ticks_title, > >> rcu_dynticks_snap(rdtp) & 0xfff, > >> - rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting, > >> + rdtp->dynticks_nesting, > >> rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu), > >> READ_ONCE(rsp->n_force_qs) - rsp->n_force_qs_gpstart, > >> fast_no_hz); > >> -- > >> 1.9.1 > >> > > > > > > -- > Thanks, > Byungchul >
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