Re: [PATCH v7 01/11] rcu: Wake up nocb gp thread on rcu_barrier_entrain()

[Joel Fernandes]

> On Oct 7, 2022, at 7:26 AM, Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
> 
> On Fri, Oct 07, 2022 at 02:47:52AM +0000, Joel Fernandes wrote:
>> On Tue, Oct 04, 2022 at 06:57:59PM -0400, Joel Fernandes wrote:
>> 
>> From: Frederic Weisbecker <frederic@xxxxxxxxxx>
>> Subject: [PATCH] rcu: Wake up nocb gp thread on rcu_barrier_entrain()
>> 
>> In preparation of RCU lazy changes, wake up the RCU nocb gp thread if
>> needed after an entrain. Otherwise, the RCU barrier callback can wait in
>> the queue for several seconds before the lazy callbacks in front of it
>> are serviced.
>> 
>> Reported-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
>> ---
>> kernel/rcu/tree.c      | 11 +++++++++++
>> kernel/rcu/tree.h      |  1 +
>> kernel/rcu/tree_nocb.h |  4 ++++
>> 3 files changed, 16 insertions(+)
>> 
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 5ec97e3f7468..04f33191e5ed 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -3894,6 +3894,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
>> {
>>    unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence);
>>    unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap);
>> +    bool wake_nocb = false;
>> +    bool was_done = false;
>> 
>>    lockdep_assert_held(&rcu_state.barrier_lock);
>>    if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq))
>> @@ -3902,6 +3904,7 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
>>    rdp->barrier_head.func = rcu_barrier_callback;
>>    debug_rcu_head_queue(&rdp->barrier_head);
>>    rcu_nocb_lock(rdp);
>> +    was_done = rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist);
>>    WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
>>    if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
>>        atomic_inc(&rcu_state.barrier_cpu_count);
>> @@ -3909,7 +3912,15 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
>>        debug_rcu_head_unqueue(&rdp->barrier_head);
>>        rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
>>    }
>> +
>> +    /*
>> +     * If bypass list was non-empty, wake up the nocb GP thread otherwise
>> +     * bypass/lazy CBs may not be noticed, and can cause real long delays!
>> +     */
>> +    wake_nocb = was_done && rcu_segcblist_pend_cbs(&rdp->cblist);
>>    rcu_nocb_unlock(rdp);
>> +    if (wake_nocb)
>> +        wake_nocb_gp(rdp, false);
>>    smp_store_release(&rdp->barrier_seq_snap, gseq);
>> }
>> 
>> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
>> index d4a97e40ea9c..41faaf8eb872 100644
>> --- a/kernel/rcu/tree.h
>> +++ b/kernel/rcu/tree.h
>> @@ -443,6 +443,7 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>>                  unsigned long j);
>> static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>>                bool *was_alldone, unsigned long flags);
>> +static bool wake_nocb_gp(struct rcu_data *rdp, bool force);
>> static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
>>                 unsigned long flags);
>> static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
>> index f77a6d7e1356..6caade0683dd 100644
>> --- a/kernel/rcu/tree_nocb.h
>> +++ b/kernel/rcu/tree_nocb.h
>> @@ -1558,6 +1558,10 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
>> {
>> }
>> 
>> +static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
>> +{
> 
> It misses a return value...

Ah, true. I had the return in my tree but not in the posting, sigh.

>> +}
> 
> So I planned to post an updated version (please find below) but I triggered
> a rare TREE01 stall related to rcu_barrier(). I just can't remember if it was
> before or after the patch :-s

Weird.

Thanks,

- Joel




> Anyway I'm holding it off until I get more clues.
> 
> ---
> From: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Date: Tue, 4 Oct 2022 02:41:47 +0000
> Subject: [PATCH] rcu: Fix missing nocb gp wake on rcu_barrier()
> 
> Upon entraining a callback to a NOCB CPU, no further wake up is
> performed on the corresponding nocb_gp kthread. As a result, the callback
> and all the subsequent ones on that CPU may be ignored, at least until
> an RCU_NOCB_WAKE_FORCE timer is ever armed.
> 
> Here is a possible bad scenario:
> 
> 1) CPU 0 is NOCB unlike all other CPUs.
> 2) CPU 0 queues a callback
> 2) The grace period related to that callback elapses
> 3) The callback is moved to the done list (but is not invoked yet),
>   there are no more pending callbacks for CPU 0
> 4) CPU 1 calls rcu_barrier() and sends an IPI to CPU 0
> 5) CPU 0 entrains the callback but doesn't wake up nocb_gp
> 6) CPU 1 blocks forever, unless CPU 0 ever queues enough further
>   callbacks to arm an RCU_NOCB_WAKE_FORCE timer.
> 
> Make sure the necessary wake up is issued if necessary.
> 
> Reported-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
> kernel/rcu/tree.c      | 6 ++++++
> kernel/rcu/tree.h      | 1 +
> kernel/rcu/tree_nocb.h | 5 +++++
> 3 files changed, 12 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 96d678c9cfb6..025f59f6f97f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3914,6 +3914,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> {
>    unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence);
>    unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap);
> +    bool wake_nocb = false;
> +    bool was_alldone = false;
> 
>    lockdep_assert_held(&rcu_state.barrier_lock);
>    if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq))
> @@ -3922,6 +3924,7 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
>    rdp->barrier_head.func = rcu_barrier_callback;
>    debug_rcu_head_queue(&rdp->barrier_head);
>    rcu_nocb_lock(rdp);
> +    was_alldone = rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist);
>    WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
>    if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
>        atomic_inc(&rcu_state.barrier_cpu_count);
> @@ -3929,7 +3932,10 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
>        debug_rcu_head_unqueue(&rdp->barrier_head);
>        rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
>    }
> +    wake_nocb = was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist);
>    rcu_nocb_unlock(rdp);
> +    if (wake_nocb)
> +        wake_nocb_gp(rdp, false);
>    smp_store_release(&rdp->barrier_seq_snap, gseq);
> }
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index d4a97e40ea9c..925dd98f8b23 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -439,6 +439,7 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp);
> static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
> static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
> static void rcu_init_one_nocb(struct rcu_node *rnp);
> +static bool wake_nocb_gp(struct rcu_data *rdp, bool force);
> static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>                  unsigned long j);
> static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index f77a6d7e1356..094fd454b6c3 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1558,6 +1558,11 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
> {
> }
> 
> +static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
> +{
> +    return false;
> +}
> +
> static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>                  unsigned long j)
> {
> -- 
> 2.25.1
> 
> 
>