linux-kernel - RE: [PATCH] rcu-tasks: Make shrink down to a single callback queue safely

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Date:   Fri, 2 Dec 2022 23:35:09 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu-tasks: Make shrink down to a single callback queue
 safely

On Wed, Nov 30, 2022 at 01:12:53PM +0800, Zqiang wrote:
> Assume that the current RCU-task belongs to per-CPU callback queuing
> mode and the rcu_task_cb_adjust is true.
> 
>       CPU0					CPU1
> 
> rcu_tasks_need_gpcb()
>   ncbsnz == 0 and
>   ncbs < rcu_task_collapse_lim
> 
> 					      invoke call_rcu_tasks_generic()
> 						 enqueue callback to CPU1
> 					        (CPU1 n_cbs not equal zero)
> 
>   if (rcu_task_cb_adjust &&
>   ncbs <= rcu_task_collapse_lim)
>     if (rtp->percpu_enqueue_lim > 1)
>       rtp->percpu_enqueue_lim = 1;
>       rtp->percpu_dequeue_gpseq =
>       get_state_synchronize_rcu();
> 
> 
>   A full RCU grace period has passed
>
>
>I don't see how this grace period can elapse.  The rcu_tasks_need_gpcb()
>function is invoked only from rcu_tasks_one_gp(), and while holding
>->tasks_gp_mutex.


Hi Paul

I mean that It's the RCU grace period instead of the RCU task grace period.

rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();

get_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);

There is a window period between these two calls,  preemption may occur.
A full RCU grace period may have passed.
when we run it again we find get_state_synchronize_rcu() == true.

Thanks
Zqiang


>
>What am I missing here?
>
>							Thanx, Paul

>   if (rcu_task_cb_adjust && !ncbsnz &&
>   poll_state_synchronize_rcu(
>     rtp->percpu_dequeue_gpseq) == true
>     if (rtp->percpu_enqueue_lim <
> 	rtp->percpu_dequeue_lim)
>         rtp->percpu_dequeue_lim = 1
>     for (cpu = rtp->percpu_dequeue_lim;
>         cpu < nr_cpu_ids; cpu++)
>         find CPU1 n_cbs is not zero
>         trigger warning
> 
> The above scenario will not only trigger WARN_ONCE(), but also set the
> rcu_tasks structure's->percpu_dequeue_lim is one when CPU1 still have
> callbacks, which will cause the callback of CPU1 to have no chance to be
> called.
> 
> This commit add per-cpu callback check(except CPU0) before set the rcu_tasks
> structure's->percpu_dequeue_lim to one, if other CPUs(except CPU0) still have
> callback, not set the rcu_tasks structure's->percpu_dequeue_lim to one, set it
> until the all CPUs(except CPU0) has no callback.
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---
>  kernel/rcu/tasks.h | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index e4f7d08bde64..690af479074f 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -433,14 +433,17 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
>  	    poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq)) {
>  		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
>  		if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
> +			for (cpu = rtp->percpu_enqueue_lim; cpu < nr_cpu_ids; cpu++) {
> +				struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> +
> +				if(rcu_segcblist_n_cbs(&rtpcp->cblist)) {
> +					raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
> +					return needgpcb;
> +				}
> +			}
>  			WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
>  			pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
>  		}
> -		for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
> -			struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> -
> -			WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
> -		}
>  		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
>  	}
>  
> -- 
> 2.25.1
>