linux-kernel - Re: [PATCH 2/6] rcu/nocb: Prepare nocb_cb_wait() to start with a non-offloaded rdp

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Date:   Wed, 1 Dec 2021 14:55:46 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, <rcu@...r.kernel.org>
Subject: Re: [PATCH 2/6] rcu/nocb: Prepare nocb_cb_wait() to start with a
 non-offloaded rdp



On 11/23/2021 6:07 AM, Frederic Weisbecker wrote:
> In order to be able to toggle the offloaded state from cpusets, a nocb
> kthread will need to be created for all possible CPUs whenever the
> "rcu_nocbs=" or "nohz_full=" parameters are passed.
> 
> Therefore nocb_cb_wait() kthread must prepare to start running on a
> de-offloaded rdp. Simply move the sleeping condition in the beginning
> of the kthread callback is enough to prevent from running callbacks
> before the rdp ever becomes offloaded.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>


Thanks
Neeraj

>   kernel/rcu/tree_nocb.h | 23 ++++++++++++-----------
>   1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index cc1165559177..e1cb06840454 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -797,6 +797,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>   	bool can_sleep = true;
>   	struct rcu_node *rnp = rdp->mynode;
>   
> +	do {
> +		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
> +						    nocb_cb_wait_cond(rdp));
> +
> +		// VVV Ensure CB invocation follows _sleep test.
> +		if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
> +			WARN_ON(signal_pending(current));
> +			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
> +		}
> +	} while (!nocb_cb_can_run(rdp));
> +
> +
>   	local_irq_save(flags);
>   	rcu_momentary_dyntick_idle();
>   	local_irq_restore(flags);
> @@ -849,17 +861,6 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>   
>   	if (needwake_state)
>   		swake_up_one(&rdp->nocb_state_wq);
> -
> -	do {
> -		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
> -						    nocb_cb_wait_cond(rdp));
> -
> -		// VVV Ensure CB invocation follows _sleep test.
> -		if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
> -			WARN_ON(signal_pending(current));
> -			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
> -		}
> -	} while (!nocb_cb_can_run(rdp));
>   }
>   
>   /*
>