linux-kernel - Re: [PATCH 4/4] rcu/tree: Use schedule_delayed_work() instead of WQ

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Date:   Mon, 21 Sep 2020 15:27:41 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 4/4] rcu/tree: Use schedule_delayed_work() instead of
 WQ_HIGHPRI queue

On Sun, Sep 20, 2020 at 08:06:38AM -0700, Paul E. McKenney wrote:
> On Fri, Sep 18, 2020 at 09:48:17PM +0200, Uladzislau Rezki (Sony) wrote:
> > Recently the separate worker thread has been introduced to
> > maintain the local page cache from the regular kernel context,
> > instead of kvfree_rcu() contexts. That was done because a caller
> > of the k[v]free_rcu() can be any context type what is a problem
> > from the allocation point of view.
> > 
> > >From the other hand, the lock-less way of obtaining a page has
> > been introduced and directly injected to the k[v]free_rcu() path.
> > 
> > Therefore it is not important anymore to use a high priority "wq"
> > for the external job that used to fill a page cache ASAP when it
> > was empty.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> 
> And I needed to apply the patch below to make this one pass rcutorture
> scenarios SRCU-P and TREE05.  Repeat by:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 3 --configs "SRCU-P TREE05" --trust-make
> 
> Without the patch below, the system hangs very early in boot.
> 
> Please let me know if some other fix would be better.
> 
> 						Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8ce1ea4..2424e2a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3481,7 +3481,8 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  	success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
>  	if (!success) {
>  		// Use delayed work, so we do not deadlock with rq->lock.
> -		if (!atomic_xchg(&krcp->work_in_progress, 1))
> +		if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
> +		    !atomic_xchg(&krcp->work_in_progress, 1))
>  			schedule_delayed_work(&krcp->page_cache_work, 1);
>  
>  		if (head == NULL)
I will double check!

--
Vlad Rezki