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    SubjectRe: [PATCH 5/5 v2] rcu: Fix dyntick-idle tracing
    On Fri, Apr 07, 2017 at 10:01:11AM -0400, Steven Rostedt wrote:
    > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    >
    > The tracing subsystem started using rcu_irq_entry() and rcu_irq_exit()
    > (with my blessing) to allow the current _rcuidle alternative tracepoint
    > name to be dispensed with while still maintaining good performance.
    > Unfortunately, this causes RCU's dyntick-idle entry code's tracing to
    > appear to RCU like an interrupt that occurs where RCU is not designed
    > to handle interrupts.
    >
    > This commit fixes this problem by moving the zeroing of ->dynticks_nesting
    > after the offending trace_rcu_dyntick() statement, which narrows the
    > window of vulnerability to a pair of adjacent statements that are now
    > marked with comments to that effect.
    >
    > Link: http://lkml.kernel.org/r/20170405193928.GM1600@linux.vnet.ibm.com
    >
    > Reported-by: Steven Rostedt <rostedt@goodmis.org>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    > ---
    > kernel/rcu/tree.c | 48 +++++++++++++++++++++++-------------------------
    > 1 file changed, 23 insertions(+), 25 deletions(-)
    >
    > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    > index 50fee7689e71..8b4d273331e4 100644
    > --- a/kernel/rcu/tree.c
    > +++ b/kernel/rcu/tree.c
    > @@ -57,6 +57,7 @@
    > #include <linux/random.h>
    > #include <linux/trace_events.h>
    > #include <linux/suspend.h>
    > +#include <linux/ftrace.h>
    >
    > #include "tree.h"
    > #include "rcu.h"
    > @@ -771,25 +772,24 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
    > }
    >
    > /*
    > - * rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
    > + * rcu_eqs_enter_common - current CPU is entering an extended quiescent state
    > *
    > - * If the new value of the ->dynticks_nesting counter now is zero,
    > - * we really have entered idle, and must do the appropriate accounting.
    > - * The caller must have disabled interrupts.
    > + * Enter idle, doing appropriate accounting. The caller must have
    > + * disabled interrupts.
    > */
    > -static void rcu_eqs_enter_common(long long oldval, bool user)
    > +static void rcu_eqs_enter_common(bool user)
    > {
    > struct rcu_state *rsp;
    > struct rcu_data *rdp;
    > - RCU_TRACE(struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);)
    > + struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
    >
    > - trace_rcu_dyntick(TPS("Start"), oldval, rdtp->dynticks_nesting);
    > + trace_rcu_dyntick(TPS("Start"), rdtp->dynticks_nesting, 0);
    > if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
    > !user && !is_idle_task(current)) {
    > struct task_struct *idle __maybe_unused =
    > idle_task(smp_processor_id());
    >
    > - trace_rcu_dyntick(TPS("Error on entry: not idle task"), oldval, 0);
    > + trace_rcu_dyntick(TPS("Error on entry: not idle task"), rdtp->dynticks_nesting, 0);
    > rcu_ftrace_dump(DUMP_ORIG);
    > WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
    > current->pid, current->comm,
    > @@ -800,7 +800,10 @@ static void rcu_eqs_enter_common(long long oldval, bool user)
    > do_nocb_deferred_wakeup(rdp);
    > }
    > rcu_prepare_for_idle();
    > - rcu_dynticks_eqs_enter();
    > + stack_tracer_disable();
    > + rdtp->dynticks_nesting = 0; /* Breaks tracing momentarily. */
    > + rcu_dynticks_eqs_enter(); /* After this, tracing works again. */
    > + stack_tracer_enable();

    Hmmm... There is not supposed to be any tracing in this interval,
    and interrupts are disabled. Wouldn't it be better to have something
    that made tracing illegal during this interval?

    Yeah, I am a bit concerned about idle-entry latency...

    Thanx, Paul

    > rcu_dynticks_task_enter();
    >
    > /*
    > @@ -821,19 +824,15 @@ static void rcu_eqs_enter_common(long long oldval, bool user)
    > */
    > static void rcu_eqs_enter(bool user)
    > {
    > - long long oldval;
    > struct rcu_dynticks *rdtp;
    >
    > rdtp = this_cpu_ptr(&rcu_dynticks);
    > - oldval = rdtp->dynticks_nesting;
    > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
    > - (oldval & DYNTICK_TASK_NEST_MASK) == 0);
    > - if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE) {
    > - rdtp->dynticks_nesting = 0;
    > - rcu_eqs_enter_common(oldval, user);
    > - } else {
    > + (rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK) == 0);
    > + if ((rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
    > + rcu_eqs_enter_common(user);
    > + else
    > rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
    > - }
    > }
    >
    > /**
    > @@ -892,19 +891,18 @@ void rcu_user_enter(void)
    > */
    > void rcu_irq_exit(void)
    > {
    > - long long oldval;
    > struct rcu_dynticks *rdtp;
    >
    > RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
    > rdtp = this_cpu_ptr(&rcu_dynticks);
    > - oldval = rdtp->dynticks_nesting;
    > - rdtp->dynticks_nesting--;
    > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
    > - rdtp->dynticks_nesting < 0);
    > - if (rdtp->dynticks_nesting)
    > - trace_rcu_dyntick(TPS("--="), oldval, rdtp->dynticks_nesting);
    > - else
    > - rcu_eqs_enter_common(oldval, true);
    > + rdtp->dynticks_nesting < 1);
    > + if (rdtp->dynticks_nesting <= 1) {
    > + rcu_eqs_enter_common(true);
    > + } else {
    > + trace_rcu_dyntick(TPS("--="), rdtp->dynticks_nesting, rdtp->dynticks_nesting - 1);
    > + rdtp->dynticks_nesting--;
    > + }
    > rcu_sysidle_enter(1);
    > }
    >
    > --
    > 2.10.2
    >
    >

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