* [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
[not found] <20200214235536.GA13364@paulmck-ThinkPad-P72>
@ 2020-02-14 23:56 ` paulmck
2020-02-25 10:24 ` Boqun Feng
0 siblings, 1 reply; 6+ messages in thread
From: paulmck @ 2020-02-14 23:56 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, kernel-team, mingo, jiangshanlai, dipankar, akpm,
mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, Paul E. McKenney, # 5 . 5 . x
From: "Paul E. McKenney" <paulmck@kernel.org>
Currently, rcu_barrier() ignores offline CPUs, However, it is possible
for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
must wait for those callbacks. This commit therefore makes rcu_barrier()
directly invoke the rcu_barrier_func() with interrupts disabled for such
CPUs. This requires passing the CPU number into this function so that
it can entrain the rcu_barrier() callback onto the correct CPU's callback
list, given that the code must instead execute on the current CPU.
While in the area, this commit fixes a bug where the first CPU's callback
might have been invoked before rcu_segcblist_entrain() returned, which
would also result in an early wakeup.
Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: <stable@vger.kernel.org> # 5.5.x
---
include/trace/events/rcu.h | 1 +
kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
2 files changed, 21 insertions(+), 12 deletions(-)
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 5e49b06..d56d54c 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
* "Begin": rcu_barrier() started.
* "EarlyExit": rcu_barrier() piggybacked, thus early exit.
* "Inc1": rcu_barrier() piggyback check counter incremented.
+ * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
* "OnlineQ": rcu_barrier() found online CPU with callbacks.
* "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
* "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d15041f..160643e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
/*
* Called with preemption disabled, and from cross-cpu IRQ context.
*/
-static void rcu_barrier_func(void *unused)
+static void rcu_barrier_func(void *cpu_in)
{
- struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
+ uintptr_t cpu = (uintptr_t)cpu_in;
+ struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
rdp->barrier_head.func = rcu_barrier_callback;
@@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
*/
void rcu_barrier(void)
{
- int cpu;
+ uintptr_t cpu;
struct rcu_data *rdp;
unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
@@ -3150,13 +3151,14 @@ void rcu_barrier(void)
rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
/*
- * Initialize the count to one rather than to zero in order to
- * avoid a too-soon return to zero in case of a short grace period
- * (or preemption of this task). Exclude CPU-hotplug operations
- * to ensure that no offline CPU has callbacks queued.
+ * Initialize the count to two rather than to zero in order
+ * to avoid a too-soon return to zero in case of an immediate
+ * invocation of the just-enqueued callback (or preemption of
+ * this task). Exclude CPU-hotplug operations to ensure that no
+ * offline non-offloaded CPU has callbacks queued.
*/
init_completion(&rcu_state.barrier_completion);
- atomic_set(&rcu_state.barrier_cpu_count, 1);
+ atomic_set(&rcu_state.barrier_cpu_count, 2);
get_online_cpus();
/*
@@ -3166,13 +3168,19 @@ void rcu_barrier(void)
*/
for_each_possible_cpu(cpu) {
rdp = per_cpu_ptr(&rcu_data, cpu);
- if (!cpu_online(cpu) &&
+ if (cpu_is_offline(cpu) &&
!rcu_segcblist_is_offloaded(&rdp->cblist))
continue;
- if (rcu_segcblist_n_cbs(&rdp->cblist)) {
+ if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
rcu_barrier_trace(TPS("OnlineQ"), cpu,
rcu_state.barrier_sequence);
- smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
+ smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
+ } else if (cpu_is_offline(cpu)) {
+ rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
+ rcu_state.barrier_sequence);
+ local_irq_disable();
+ rcu_barrier_func((void *)cpu);
+ local_irq_enable();
} else {
rcu_barrier_trace(TPS("OnlineNQ"), cpu,
rcu_state.barrier_sequence);
@@ -3184,7 +3192,7 @@ void rcu_barrier(void)
* Now that we have an rcu_barrier_callback() callback on each
* CPU, and thus each counted, remove the initial count.
*/
- if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
+ if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
complete(&rcu_state.barrier_completion);
/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
--
2.9.5
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
2020-02-14 23:56 ` [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs paulmck
@ 2020-02-25 10:24 ` Boqun Feng
2020-02-26 3:14 ` Paul E. McKenney
0 siblings, 1 reply; 6+ messages in thread
From: Boqun Feng @ 2020-02-25 10:24 UTC (permalink / raw)
To: paulmck
Cc: rcu, linux-kernel, kernel-team, mingo, jiangshanlai, dipankar,
akpm, mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, # 5 . 5 . x
Hi Paul,
On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
>
> Currently, rcu_barrier() ignores offline CPUs, However, it is possible
> for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
> must wait for those callbacks. This commit therefore makes rcu_barrier()
> directly invoke the rcu_barrier_func() with interrupts disabled for such
> CPUs. This requires passing the CPU number into this function so that
> it can entrain the rcu_barrier() callback onto the correct CPU's callback
> list, given that the code must instead execute on the current CPU.
>
> While in the area, this commit fixes a bug where the first CPU's callback
> might have been invoked before rcu_segcblist_entrain() returned, which
> would also result in an early wakeup.
>
> Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: <stable@vger.kernel.org> # 5.5.x
> ---
> include/trace/events/rcu.h | 1 +
> kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
> 2 files changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 5e49b06..d56d54c 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
> * "Begin": rcu_barrier() started.
> * "EarlyExit": rcu_barrier() piggybacked, thus early exit.
> * "Inc1": rcu_barrier() piggyback check counter incremented.
> + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
> * "OnlineQ": rcu_barrier() found online CPU with callbacks.
> * "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
> * "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d15041f..160643e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> /*
> * Called with preemption disabled, and from cross-cpu IRQ context.
> */
> -static void rcu_barrier_func(void *unused)
> +static void rcu_barrier_func(void *cpu_in)
> {
> - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> + uintptr_t cpu = (uintptr_t)cpu_in;
> + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>
> rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
> rdp->barrier_head.func = rcu_barrier_callback;
> @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
> */
> void rcu_barrier(void)
> {
> - int cpu;
> + uintptr_t cpu;
> struct rcu_data *rdp;
> unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
>
> @@ -3150,13 +3151,14 @@ void rcu_barrier(void)
> rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
>
> /*
> - * Initialize the count to one rather than to zero in order to
> - * avoid a too-soon return to zero in case of a short grace period
> - * (or preemption of this task). Exclude CPU-hotplug operations
> - * to ensure that no offline CPU has callbacks queued.
> + * Initialize the count to two rather than to zero in order
> + * to avoid a too-soon return to zero in case of an immediate
> + * invocation of the just-enqueued callback (or preemption of
> + * this task). Exclude CPU-hotplug operations to ensure that no
> + * offline non-offloaded CPU has callbacks queued.
> */
> init_completion(&rcu_state.barrier_completion);
> - atomic_set(&rcu_state.barrier_cpu_count, 1);
> + atomic_set(&rcu_state.barrier_cpu_count, 2);
> get_online_cpus();
>
> /*
> @@ -3166,13 +3168,19 @@ void rcu_barrier(void)
> */
> for_each_possible_cpu(cpu) {
> rdp = per_cpu_ptr(&rcu_data, cpu);
> - if (!cpu_online(cpu) &&
> + if (cpu_is_offline(cpu) &&
> !rcu_segcblist_is_offloaded(&rdp->cblist))
> continue;
> - if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
> rcu_barrier_trace(TPS("OnlineQ"), cpu,
> rcu_state.barrier_sequence);
> - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
> + } else if (cpu_is_offline(cpu)) {
I wonder whether this should be:
else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))
? Because I think we only want to queue the barrier call back if there
are callbacks for a particular CPU. Am I missing something subtle?
Regards,
Boqun
> + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
> + rcu_state.barrier_sequence);
> + local_irq_disable();
> + rcu_barrier_func((void *)cpu);
> + local_irq_enable();
> } else {
> rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> rcu_state.barrier_sequence);
> @@ -3184,7 +3192,7 @@ void rcu_barrier(void)
> * Now that we have an rcu_barrier_callback() callback on each
> * CPU, and thus each counted, remove the initial count.
> */
> - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
> + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
> complete(&rcu_state.barrier_completion);
>
> /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
> --
> 2.9.5
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
2020-02-25 10:24 ` Boqun Feng
@ 2020-02-26 3:14 ` Paul E. McKenney
2020-02-26 4:18 ` Paul E. McKenney
2020-02-26 6:14 ` Boqun Feng
0 siblings, 2 replies; 6+ messages in thread
From: Paul E. McKenney @ 2020-02-26 3:14 UTC (permalink / raw)
To: Boqun Feng
Cc: rcu, linux-kernel, kernel-team, mingo, jiangshanlai, dipankar,
akpm, mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, # 5 . 5 . x
On Tue, Feb 25, 2020 at 06:24:36PM +0800, Boqun Feng wrote:
> Hi Paul,
>
> On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> >
> > Currently, rcu_barrier() ignores offline CPUs, However, it is possible
> > for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
> > must wait for those callbacks. This commit therefore makes rcu_barrier()
> > directly invoke the rcu_barrier_func() with interrupts disabled for such
> > CPUs. This requires passing the CPU number into this function so that
> > it can entrain the rcu_barrier() callback onto the correct CPU's callback
> > list, given that the code must instead execute on the current CPU.
> >
> > While in the area, this commit fixes a bug where the first CPU's callback
> > might have been invoked before rcu_segcblist_entrain() returned, which
> > would also result in an early wakeup.
> >
> > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: <stable@vger.kernel.org> # 5.5.x
> > ---
> > include/trace/events/rcu.h | 1 +
> > kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
> > 2 files changed, 21 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > index 5e49b06..d56d54c 100644
> > --- a/include/trace/events/rcu.h
> > +++ b/include/trace/events/rcu.h
> > @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
> > * "Begin": rcu_barrier() started.
> > * "EarlyExit": rcu_barrier() piggybacked, thus early exit.
> > * "Inc1": rcu_barrier() piggyback check counter incremented.
> > + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
> > * "OnlineQ": rcu_barrier() found online CPU with callbacks.
> > * "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
> > * "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index d15041f..160643e 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> > /*
> > * Called with preemption disabled, and from cross-cpu IRQ context.
> > */
> > -static void rcu_barrier_func(void *unused)
> > +static void rcu_barrier_func(void *cpu_in)
> > {
> > - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> > + uintptr_t cpu = (uintptr_t)cpu_in;
> > + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> >
> > rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
> > rdp->barrier_head.func = rcu_barrier_callback;
> > @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
> > */
> > void rcu_barrier(void)
> > {
> > - int cpu;
> > + uintptr_t cpu;
> > struct rcu_data *rdp;
> > unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
> >
> > @@ -3150,13 +3151,14 @@ void rcu_barrier(void)
> > rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
> >
> > /*
> > - * Initialize the count to one rather than to zero in order to
> > - * avoid a too-soon return to zero in case of a short grace period
> > - * (or preemption of this task). Exclude CPU-hotplug operations
> > - * to ensure that no offline CPU has callbacks queued.
> > + * Initialize the count to two rather than to zero in order
> > + * to avoid a too-soon return to zero in case of an immediate
> > + * invocation of the just-enqueued callback (or preemption of
> > + * this task). Exclude CPU-hotplug operations to ensure that no
> > + * offline non-offloaded CPU has callbacks queued.
> > */
> > init_completion(&rcu_state.barrier_completion);
> > - atomic_set(&rcu_state.barrier_cpu_count, 1);
> > + atomic_set(&rcu_state.barrier_cpu_count, 2);
> > get_online_cpus();
> >
> > /*
> > @@ -3166,13 +3168,19 @@ void rcu_barrier(void)
> > */
> > for_each_possible_cpu(cpu) {
> > rdp = per_cpu_ptr(&rcu_data, cpu);
> > - if (!cpu_online(cpu) &&
> > + if (cpu_is_offline(cpu) &&
> > !rcu_segcblist_is_offloaded(&rdp->cblist))
> > continue;
> > - if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> > + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
> > rcu_barrier_trace(TPS("OnlineQ"), cpu,
> > rcu_state.barrier_sequence);
> > - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> > + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
> > + } else if (cpu_is_offline(cpu)) {
>
> I wonder whether this should be:
>
> else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))
>
> ? Because I think we only want to queue the barrier call back if there
> are callbacks for a particular CPU. Am I missing something subtle?
I don't believe that you are missing anything at all!
Thank you very much -- this bug would not have shown up in any validation
setup that I am aware of. ;-)
Thanx, Paul
> Regards,
> Boqun
>
> > + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
> > + rcu_state.barrier_sequence);
> > + local_irq_disable();
> > + rcu_barrier_func((void *)cpu);
> > + local_irq_enable();
> > } else {
> > rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> > rcu_state.barrier_sequence);
> > @@ -3184,7 +3192,7 @@ void rcu_barrier(void)
> > * Now that we have an rcu_barrier_callback() callback on each
> > * CPU, and thus each counted, remove the initial count.
> > */
> > - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
> > + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
> > complete(&rcu_state.barrier_completion);
> >
> > /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
> > --
> > 2.9.5
> >
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
2020-02-26 3:14 ` Paul E. McKenney
@ 2020-02-26 4:18 ` Paul E. McKenney
2020-02-26 6:14 ` Boqun Feng
1 sibling, 0 replies; 6+ messages in thread
From: Paul E. McKenney @ 2020-02-26 4:18 UTC (permalink / raw)
To: Boqun Feng
Cc: rcu, linux-kernel, kernel-team, mingo, jiangshanlai, dipankar,
akpm, mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, # 5 . 5 . x
On Tue, Feb 25, 2020 at 07:14:55PM -0800, Paul E. McKenney wrote:
> On Tue, Feb 25, 2020 at 06:24:36PM +0800, Boqun Feng wrote:
> > Hi Paul,
> >
> > On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
> > > From: "Paul E. McKenney" <paulmck@kernel.org>
> > >
> > > Currently, rcu_barrier() ignores offline CPUs, However, it is possible
> > > for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
> > > must wait for those callbacks. This commit therefore makes rcu_barrier()
> > > directly invoke the rcu_barrier_func() with interrupts disabled for such
> > > CPUs. This requires passing the CPU number into this function so that
> > > it can entrain the rcu_barrier() callback onto the correct CPU's callback
> > > list, given that the code must instead execute on the current CPU.
> > >
> > > While in the area, this commit fixes a bug where the first CPU's callback
> > > might have been invoked before rcu_segcblist_entrain() returned, which
> > > would also result in an early wakeup.
> > >
> > > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > Cc: <stable@vger.kernel.org> # 5.5.x
> > > ---
> > > include/trace/events/rcu.h | 1 +
> > > kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
> > > 2 files changed, 21 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > > index 5e49b06..d56d54c 100644
> > > --- a/include/trace/events/rcu.h
> > > +++ b/include/trace/events/rcu.h
> > > @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
> > > * "Begin": rcu_barrier() started.
> > > * "EarlyExit": rcu_barrier() piggybacked, thus early exit.
> > > * "Inc1": rcu_barrier() piggyback check counter incremented.
> > > + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
> > > * "OnlineQ": rcu_barrier() found online CPU with callbacks.
> > > * "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
> > > * "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index d15041f..160643e 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> > > /*
> > > * Called with preemption disabled, and from cross-cpu IRQ context.
> > > */
> > > -static void rcu_barrier_func(void *unused)
> > > +static void rcu_barrier_func(void *cpu_in)
> > > {
> > > - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> > > + uintptr_t cpu = (uintptr_t)cpu_in;
> > > + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > >
> > > rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
> > > rdp->barrier_head.func = rcu_barrier_callback;
> > > @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
> > > */
> > > void rcu_barrier(void)
> > > {
> > > - int cpu;
> > > + uintptr_t cpu;
> > > struct rcu_data *rdp;
> > > unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
> > >
> > > @@ -3150,13 +3151,14 @@ void rcu_barrier(void)
> > > rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
> > >
> > > /*
> > > - * Initialize the count to one rather than to zero in order to
> > > - * avoid a too-soon return to zero in case of a short grace period
> > > - * (or preemption of this task). Exclude CPU-hotplug operations
> > > - * to ensure that no offline CPU has callbacks queued.
> > > + * Initialize the count to two rather than to zero in order
> > > + * to avoid a too-soon return to zero in case of an immediate
> > > + * invocation of the just-enqueued callback (or preemption of
> > > + * this task). Exclude CPU-hotplug operations to ensure that no
> > > + * offline non-offloaded CPU has callbacks queued.
> > > */
> > > init_completion(&rcu_state.barrier_completion);
> > > - atomic_set(&rcu_state.barrier_cpu_count, 1);
> > > + atomic_set(&rcu_state.barrier_cpu_count, 2);
> > > get_online_cpus();
> > >
> > > /*
> > > @@ -3166,13 +3168,19 @@ void rcu_barrier(void)
> > > */
> > > for_each_possible_cpu(cpu) {
> > > rdp = per_cpu_ptr(&rcu_data, cpu);
> > > - if (!cpu_online(cpu) &&
> > > + if (cpu_is_offline(cpu) &&
> > > !rcu_segcblist_is_offloaded(&rdp->cblist))
> > > continue;
> > > - if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> > > + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
> > > rcu_barrier_trace(TPS("OnlineQ"), cpu,
> > > rcu_state.barrier_sequence);
> > > - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> > > + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
> > > + } else if (cpu_is_offline(cpu)) {
> >
> > I wonder whether this should be:
> >
> > else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))
> >
> > ? Because I think we only want to queue the barrier call back if there
> > are callbacks for a particular CPU. Am I missing something subtle?
>
> I don't believe that you are missing anything at all!
>
> Thank you very much -- this bug would not have shown up in any validation
> setup that I am aware of. ;-)
And with additional adjustment to make tracing accurate.
Thanx, Paul
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
2020-02-26 3:14 ` Paul E. McKenney
2020-02-26 4:18 ` Paul E. McKenney
@ 2020-02-26 6:14 ` Boqun Feng
2020-02-26 15:02 ` Paul E. McKenney
1 sibling, 1 reply; 6+ messages in thread
From: Boqun Feng @ 2020-02-26 6:14 UTC (permalink / raw)
To: Paul E. McKenney
Cc: rcu, linux-kernel, kernel-team, mingo, jiangshanlai, dipankar,
akpm, mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, # 5 . 5 . x
On Tue, Feb 25, 2020 at 07:14:55PM -0800, Paul E. McKenney wrote:
> On Tue, Feb 25, 2020 at 06:24:36PM +0800, Boqun Feng wrote:
> > Hi Paul,
> >
> > On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
> > > From: "Paul E. McKenney" <paulmck@kernel.org>
> > >
> > > Currently, rcu_barrier() ignores offline CPUs, However, it is possible
> > > for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
> > > must wait for those callbacks. This commit therefore makes rcu_barrier()
> > > directly invoke the rcu_barrier_func() with interrupts disabled for such
> > > CPUs. This requires passing the CPU number into this function so that
> > > it can entrain the rcu_barrier() callback onto the correct CPU's callback
> > > list, given that the code must instead execute on the current CPU.
> > >
> > > While in the area, this commit fixes a bug where the first CPU's callback
> > > might have been invoked before rcu_segcblist_entrain() returned, which
> > > would also result in an early wakeup.
> > >
> > > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > Cc: <stable@vger.kernel.org> # 5.5.x
> > > ---
> > > include/trace/events/rcu.h | 1 +
> > > kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
> > > 2 files changed, 21 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > > index 5e49b06..d56d54c 100644
> > > --- a/include/trace/events/rcu.h
> > > +++ b/include/trace/events/rcu.h
> > > @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
> > > * "Begin": rcu_barrier() started.
> > > * "EarlyExit": rcu_barrier() piggybacked, thus early exit.
> > > * "Inc1": rcu_barrier() piggyback check counter incremented.
> > > + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
> > > * "OnlineQ": rcu_barrier() found online CPU with callbacks.
> > > * "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
> > > * "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index d15041f..160643e 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> > > /*
> > > * Called with preemption disabled, and from cross-cpu IRQ context.
> > > */
> > > -static void rcu_barrier_func(void *unused)
> > > +static void rcu_barrier_func(void *cpu_in)
> > > {
> > > - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> > > + uintptr_t cpu = (uintptr_t)cpu_in;
> > > + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > >
> > > rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
> > > rdp->barrier_head.func = rcu_barrier_callback;
> > > @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
> > > */
> > > void rcu_barrier(void)
> > > {
> > > - int cpu;
> > > + uintptr_t cpu;
> > > struct rcu_data *rdp;
> > > unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
> > >
> > > @@ -3150,13 +3151,14 @@ void rcu_barrier(void)
> > > rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
> > >
> > > /*
> > > - * Initialize the count to one rather than to zero in order to
> > > - * avoid a too-soon return to zero in case of a short grace period
> > > - * (or preemption of this task). Exclude CPU-hotplug operations
> > > - * to ensure that no offline CPU has callbacks queued.
> > > + * Initialize the count to two rather than to zero in order
> > > + * to avoid a too-soon return to zero in case of an immediate
> > > + * invocation of the just-enqueued callback (or preemption of
> > > + * this task). Exclude CPU-hotplug operations to ensure that no
> > > + * offline non-offloaded CPU has callbacks queued.
> > > */
> > > init_completion(&rcu_state.barrier_completion);
> > > - atomic_set(&rcu_state.barrier_cpu_count, 1);
> > > + atomic_set(&rcu_state.barrier_cpu_count, 2);
> > > get_online_cpus();
> > >
> > > /*
> > > @@ -3166,13 +3168,19 @@ void rcu_barrier(void)
> > > */
> > > for_each_possible_cpu(cpu) {
> > > rdp = per_cpu_ptr(&rcu_data, cpu);
> > > - if (!cpu_online(cpu) &&
> > > + if (cpu_is_offline(cpu) &&
> > > !rcu_segcblist_is_offloaded(&rdp->cblist))
> > > continue;
> > > - if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> > > + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
> > > rcu_barrier_trace(TPS("OnlineQ"), cpu,
> > > rcu_state.barrier_sequence);
> > > - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> > > + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
> > > + } else if (cpu_is_offline(cpu)) {
> >
> > I wonder whether this should be:
> >
> > else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))
> >
> > ? Because I think we only want to queue the barrier call back if there
> > are callbacks for a particular CPU. Am I missing something subtle?
>
> I don't believe that you are missing anything at all!
>
> Thank you very much -- this bug would not have shown up in any validation
> setup that I am aware of. ;-)
>
> Thanx, Paul
>
> > Regards,
> > Boqun
> >
> > > + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
> > > + rcu_state.barrier_sequence);
> > > + local_irq_disable();
> > > + rcu_barrier_func((void *)cpu);
> > > + local_irq_enable();
Another (interesting) thing I found here is that we actually don't need
the irq-off section to call rcu_barrier_func() in this branch. Because
the target CPU is offlined, so only the cblist is only accessed at two
places, IIUC, one is the rcuo kthread and one is here (in
rcu_barrier()), and both places are in the process context rather than
irq context, so irq-off is not required to prevent the deadlock.
But yes, I know, if we drop the local_irq_disable/enable() pair here,
it will make lockdep very unhappy ;-)
Regards,
Boqun
> > > } else {
> > > rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> > > rcu_state.barrier_sequence);
> > > @@ -3184,7 +3192,7 @@ void rcu_barrier(void)
> > > * Now that we have an rcu_barrier_callback() callback on each
> > > * CPU, and thus each counted, remove the initial count.
> > > */
> > > - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
> > > + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
> > > complete(&rcu_state.barrier_completion);
> > >
> > > /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
> > > --
> > > 2.9.5
> > >
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
2020-02-26 6:14 ` Boqun Feng
@ 2020-02-26 15:02 ` Paul E. McKenney
0 siblings, 0 replies; 6+ messages in thread
From: Paul E. McKenney @ 2020-02-26 15:02 UTC (permalink / raw)
To: Boqun Feng
Cc: rcu, linux-kernel, kernel-team, mingo, jiangshanlai, dipankar,
akpm, mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, # 5 . 5 . x
On Wed, Feb 26, 2020 at 02:14:30PM +0800, Boqun Feng wrote:
> On Tue, Feb 25, 2020 at 07:14:55PM -0800, Paul E. McKenney wrote:
> > On Tue, Feb 25, 2020 at 06:24:36PM +0800, Boqun Feng wrote:
> > > Hi Paul,
> > >
> > > On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
> > > > From: "Paul E. McKenney" <paulmck@kernel.org>
> > > >
> > > > Currently, rcu_barrier() ignores offline CPUs, However, it is possible
> > > > for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
> > > > must wait for those callbacks. This commit therefore makes rcu_barrier()
> > > > directly invoke the rcu_barrier_func() with interrupts disabled for such
> > > > CPUs. This requires passing the CPU number into this function so that
> > > > it can entrain the rcu_barrier() callback onto the correct CPU's callback
> > > > list, given that the code must instead execute on the current CPU.
> > > >
> > > > While in the area, this commit fixes a bug where the first CPU's callback
> > > > might have been invoked before rcu_segcblist_entrain() returned, which
> > > > would also result in an early wakeup.
> > > >
> > > > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> > > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > > Cc: <stable@vger.kernel.org> # 5.5.x
> > > > ---
> > > > include/trace/events/rcu.h | 1 +
> > > > kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
> > > > 2 files changed, 21 insertions(+), 12 deletions(-)
> > > >
> > > > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > > > index 5e49b06..d56d54c 100644
> > > > --- a/include/trace/events/rcu.h
> > > > +++ b/include/trace/events/rcu.h
> > > > @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
> > > > * "Begin": rcu_barrier() started.
> > > > * "EarlyExit": rcu_barrier() piggybacked, thus early exit.
> > > > * "Inc1": rcu_barrier() piggyback check counter incremented.
> > > > + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
> > > > * "OnlineQ": rcu_barrier() found online CPU with callbacks.
> > > > * "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
> > > > * "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index d15041f..160643e 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> > > > /*
> > > > * Called with preemption disabled, and from cross-cpu IRQ context.
> > > > */
> > > > -static void rcu_barrier_func(void *unused)
> > > > +static void rcu_barrier_func(void *cpu_in)
> > > > {
> > > > - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> > > > + uintptr_t cpu = (uintptr_t)cpu_in;
> > > > + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > > >
> > > > rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
> > > > rdp->barrier_head.func = rcu_barrier_callback;
> > > > @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
> > > > */
> > > > void rcu_barrier(void)
> > > > {
> > > > - int cpu;
> > > > + uintptr_t cpu;
> > > > struct rcu_data *rdp;
> > > > unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
> > > >
> > > > @@ -3150,13 +3151,14 @@ void rcu_barrier(void)
> > > > rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
> > > >
> > > > /*
> > > > - * Initialize the count to one rather than to zero in order to
> > > > - * avoid a too-soon return to zero in case of a short grace period
> > > > - * (or preemption of this task). Exclude CPU-hotplug operations
> > > > - * to ensure that no offline CPU has callbacks queued.
> > > > + * Initialize the count to two rather than to zero in order
> > > > + * to avoid a too-soon return to zero in case of an immediate
> > > > + * invocation of the just-enqueued callback (or preemption of
> > > > + * this task). Exclude CPU-hotplug operations to ensure that no
> > > > + * offline non-offloaded CPU has callbacks queued.
> > > > */
> > > > init_completion(&rcu_state.barrier_completion);
> > > > - atomic_set(&rcu_state.barrier_cpu_count, 1);
> > > > + atomic_set(&rcu_state.barrier_cpu_count, 2);
> > > > get_online_cpus();
> > > >
> > > > /*
> > > > @@ -3166,13 +3168,19 @@ void rcu_barrier(void)
> > > > */
> > > > for_each_possible_cpu(cpu) {
> > > > rdp = per_cpu_ptr(&rcu_data, cpu);
> > > > - if (!cpu_online(cpu) &&
> > > > + if (cpu_is_offline(cpu) &&
> > > > !rcu_segcblist_is_offloaded(&rdp->cblist))
> > > > continue;
> > > > - if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> > > > + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
> > > > rcu_barrier_trace(TPS("OnlineQ"), cpu,
> > > > rcu_state.barrier_sequence);
> > > > - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> > > > + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
> > > > + } else if (cpu_is_offline(cpu)) {
> > >
> > > I wonder whether this should be:
> > >
> > > else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))
> > >
> > > ? Because I think we only want to queue the barrier call back if there
> > > are callbacks for a particular CPU. Am I missing something subtle?
> >
> > I don't believe that you are missing anything at all!
> >
> > Thank you very much -- this bug would not have shown up in any validation
> > setup that I am aware of. ;-)
> >
> > Thanx, Paul
> >
> > > Regards,
> > > Boqun
> > >
> > > > + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
> > > > + rcu_state.barrier_sequence);
> > > > + local_irq_disable();
> > > > + rcu_barrier_func((void *)cpu);
> > > > + local_irq_enable();
>
> Another (interesting) thing I found here is that we actually don't need
> the irq-off section to call rcu_barrier_func() in this branch. Because
> the target CPU is offlined, so only the cblist is only accessed at two
> places, IIUC, one is the rcuo kthread and one is here (in
> rcu_barrier()), and both places are in the process context rather than
> irq context, so irq-off is not required to prevent the deadlock.
>
> But yes, I know, if we drop the local_irq_disable/enable() pair here,
> it will make lockdep very unhappy ;-)
And acquiring ->nocb_lock with interrupts enabled would be rather scary.
And probably would be an accident waiting to happen. So I am happy to
disable interrupts on this path, given that it should be infrequent,
only being executed for a short time after a no-CBs CPU goes offline.
Much nicer to let lockdep do its thing than to have to second-guess it
on every change that involves acquiring ->nocb_lock in an interrupt
handler! ;-)
Thanx, Paul
> Regards,
> Boqun
>
> > > > } else {
> > > > rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> > > > rcu_state.barrier_sequence);
> > > > @@ -3184,7 +3192,7 @@ void rcu_barrier(void)
> > > > * Now that we have an rcu_barrier_callback() callback on each
> > > > * CPU, and thus each counted, remove the initial count.
> > > > */
> > > > - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
> > > > + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
> > > > complete(&rcu_state.barrier_completion);
> > > >
> > > > /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
> > > > --
> > > > 2.9.5
> > > >
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2020-02-26 15:02 UTC | newest]
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2020-02-14 23:56 ` [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs paulmck
2020-02-25 10:24 ` Boqun Feng
2020-02-26 3:14 ` Paul E. McKenney
2020-02-26 4:18 ` Paul E. McKenney
2020-02-26 6:14 ` Boqun Feng
2020-02-26 15:02 ` Paul E. McKenney
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