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Message-ID: <6058fc59-928f-6dad-126c-221e07714f42@quicinc.com>
Date:   Tue, 19 Jul 2022 14:58:39 +0530
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Subject: Re: [PATCH rcu 3/7] rcu/nocb: Fix NOCB kthreads spawn failure with
 rcu_nocb_rdp_deoffload() direct call
Content-Language: en-US
To:     "Paul E. McKenney" <paulmck@kernel.org>, <rcu@vger.kernel.org>
CC:     <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        <rostedt@goodmis.org>, Zqiang <qiang1.zhang@intel.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Uladzislau Rezki <uladzislau.rezki@sony.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Frederic Weisbecker <frederic@kernel.org>
References: <20220620224455.GA3840881@paulmck-ThinkPad-P17-Gen-1>
 <20220620224503.3841196-3-paulmck@kernel.org>
From:   Neeraj Upadhyay <quic_neeraju@quicinc.com>
In-Reply-To: <20220620224503.3841196-3-paulmck@kernel.org>
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On 6/21/2022 4:14 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@intel.com>
> 
> If the rcuog/o[p] kthreads spawn failed, the offloaded rdp needs to
> be explicitly deoffloaded, otherwise the target rdp is still considered
> offloaded even though nothing actually handles the callbacks.
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---

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


Thanks
Neeraj

>   kernel/rcu/tree_nocb.h | 80 +++++++++++++++++++++++++++++++++---------
>   1 file changed, 64 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index f2f2cab6285a1..4cf9a29bba79d 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -986,10 +986,7 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
>   	}
>   	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
>   
> -	if (wake_gp)
> -		wake_up_process(rdp_gp->nocb_gp_kthread);
> -
> -	return 0;
> +	return wake_gp;
>   }
>   
>   static long rcu_nocb_rdp_deoffload(void *arg)
> @@ -997,9 +994,15 @@ static long rcu_nocb_rdp_deoffload(void *arg)
>   	struct rcu_data *rdp = arg;
>   	struct rcu_segcblist *cblist = &rdp->cblist;
>   	unsigned long flags;
> -	int ret;
> +	int wake_gp;
> +	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
>   
> -	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
> +	/*
> +	 * rcu_nocb_rdp_deoffload() may be called directly if
> +	 * rcuog/o[p] spawn failed, because at this time the rdp->cpu
> +	 * is not online yet.
> +	 */
> +	WARN_ON_ONCE((rdp->cpu != raw_smp_processor_id()) && cpu_online(rdp->cpu));
>   
>   	pr_info("De-offloading %d\n", rdp->cpu);
>   
> @@ -1023,10 +1026,41 @@ static long rcu_nocb_rdp_deoffload(void *arg)
>   	 */
>   	rcu_segcblist_set_flags(cblist, SEGCBLIST_RCU_CORE);
>   	invoke_rcu_core();
> -	ret = rdp_offload_toggle(rdp, false, flags);
> -	swait_event_exclusive(rdp->nocb_state_wq,
> -			      !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
> -							SEGCBLIST_KTHREAD_GP));
> +	wake_gp = rdp_offload_toggle(rdp, false, flags);
> +
> +	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
> +	if (rdp_gp->nocb_gp_kthread) {
> +		if (wake_gp)
> +			wake_up_process(rdp_gp->nocb_gp_kthread);
> +
> +		/*
> +		 * If rcuo[p] kthread spawn failed, directly remove SEGCBLIST_KTHREAD_CB.
> +		 * Just wait SEGCBLIST_KTHREAD_GP to be cleared by rcuog.
> +		 */
> +		if (!rdp->nocb_cb_kthread) {
> +			rcu_nocb_lock_irqsave(rdp, flags);
> +			rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB);
> +			rcu_nocb_unlock_irqrestore(rdp, flags);
> +		}
> +
> +		swait_event_exclusive(rdp->nocb_state_wq,
> +					!rcu_segcblist_test_flags(cblist,
> +					  SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP));
> +	} else {
> +		/*
> +		 * No kthread to clear the flags for us or remove the rdp from the nocb list
> +		 * to iterate. Do it here instead. Locking doesn't look stricly necessary
> +		 * but we stick to paranoia in this rare path.
> +		 */
> +		rcu_nocb_lock_irqsave(rdp, flags);
> +		rcu_segcblist_clear_flags(&rdp->cblist,
> +				SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
> +		rcu_nocb_unlock_irqrestore(rdp, flags);
> +
> +		list_del(&rdp->nocb_entry_rdp);
> +	}
> +	mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> +
>   	/*
>   	 * Lock one last time to acquire latest callback updates from kthreads
>   	 * so we can later handle callbacks locally without locking.
> @@ -1047,7 +1081,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
>   	WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
>   
>   
> -	return ret;
> +	return 0;
>   }
>   
>   int rcu_nocb_cpu_deoffload(int cpu)
> @@ -1079,7 +1113,8 @@ static long rcu_nocb_rdp_offload(void *arg)
>   	struct rcu_data *rdp = arg;
>   	struct rcu_segcblist *cblist = &rdp->cblist;
>   	unsigned long flags;
> -	int ret;
> +	int wake_gp;
> +	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
>   
>   	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
>   	/*
> @@ -1089,6 +1124,9 @@ static long rcu_nocb_rdp_offload(void *arg)
>   	if (!rdp->nocb_gp_rdp)
>   		return -EINVAL;
>   
> +	if (WARN_ON_ONCE(!rdp_gp->nocb_gp_kthread))
> +		return -EINVAL;
> +
>   	pr_info("Offloading %d\n", rdp->cpu);
>   
>   	/*
> @@ -1113,7 +1151,9 @@ static long rcu_nocb_rdp_offload(void *arg)
>   	 *      WRITE flags               READ callbacks
>   	 *      rcu_nocb_unlock()         rcu_nocb_unlock()
>   	 */
> -	ret = rdp_offload_toggle(rdp, true, flags);
> +	wake_gp = rdp_offload_toggle(rdp, true, flags);
> +	if (wake_gp)
> +		wake_up_process(rdp_gp->nocb_gp_kthread);
>   	swait_event_exclusive(rdp->nocb_state_wq,
>   			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
>   			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
> @@ -1126,7 +1166,7 @@ static long rcu_nocb_rdp_offload(void *arg)
>   	rcu_segcblist_clear_flags(cblist, SEGCBLIST_RCU_CORE);
>   	rcu_nocb_unlock_irqrestore(rdp, flags);
>   
> -	return ret;
> +	return 0;
>   }
>   
>   int rcu_nocb_cpu_offload(int cpu)
> @@ -1248,7 +1288,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
>   				"rcuog/%d", rdp_gp->cpu);
>   		if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__)) {
>   			mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> -			return;
> +			goto end;
>   		}
>   		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
>   		if (kthread_prio)
> @@ -1260,12 +1300,20 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
>   	t = kthread_run(rcu_nocb_cb_kthread, rdp,
>   			"rcuo%c/%d", rcu_state.abbr, cpu);
>   	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
> -		return;
> +		goto end;
>   
>   	if (kthread_prio)
>   		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
>   	WRITE_ONCE(rdp->nocb_cb_kthread, t);
>   	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
> +	return;
> +end:
> +	mutex_lock(&rcu_state.barrier_mutex);
> +	if (rcu_rdp_is_offloaded(rdp)) {
> +		rcu_nocb_rdp_deoffload(rdp);
> +		cpumask_clear_cpu(cpu, rcu_nocb_mask);
> +	}
> +	mutex_unlock(&rcu_state.barrier_mutex);
>   }
>   
>   /* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */