linux-kernel - Re: [PATCH v2 REPOST] fscache: Use only one fscache_object_cong

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Date:   Mon, 13 Dec 2021 16:10:24 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-cachefs@...hat.com, linux-kernel@...r.kernel.org
Cc:     David Howells <dhowells@...hat.com>, Tejun Heo <tj@...nel.org>,
        Gregor Beck <gregor.beck@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 REPOST] fscache: Use only one fscache_object_cong_wait.

On 2021-12-02 21:52:45 [+0100], To linux-cachefs@...hat.com wrote:
> On 2021-11-18 17:54:44 [+0100], To linux-cachefs@...hat.com wrote:
> > In the commit mentioned below, fscache was converted from slow-work to
> > workqueue. slow_work_enqueue() and slow_work_sleep_till_thread_needed()
> > did not use a per-CPU workqueue. They choose from two global waitqueues
> > depending on the SLOW_WORK_VERY_SLOW bit which was not set so it always
> > one waitqueue.
> > 
> > I can't find out how it is ensured that a waiter on certain CPU is woken
> > up be the other side. My guess is that the timeout in schedule_timeout()
> > ensures that it does not wait forever (or a random wake up).
> > 
> > fscache_object_sleep_till_congested() must be invoked from preemptible
> > context in order for schedule() to work. In this case this_cpu_ptr()
> > should complain with CONFIG_DEBUG_PREEMPT enabled except the thread is
> > bound to one CPU.
> > 
> > wake_up() wakes only one waiter and I'm not sure if it is guaranteed
> > that only one waiter exists.
> > 
> > Replace the per-CPU waitqueue with one global waitqueue.
> > 
> > Fixes: 8b8edefa2fffb ("fscache: convert object to use workqueue instead of slow-work")
> > Reported-by: Gregor Beck <gregor.beck@...il.com>
> > Cc: stable-rt@...r.kernel.org
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> I noticed that -next gained commit
>    608bfec640edb ("fscache: Remove the contents of the fscache driver, pending rewrite")
> 
> which removes slow_work_sleep_till_thread_needed() and the per-CPU
> variable. Since it looks like a bug, what happens stable wise?

a gentle ping.

> > ---
> > Previous post
> >   https://lore.kernel.org/all/20211029083839.xwwt7jgzru3kcpii@linutronix.de/
> > 
> >  fs/fscache/internal.h |    1 -
> >  fs/fscache/main.c     |    6 ------
> >  fs/fscache/object.c   |   13 +++++--------
> >  3 files changed, 5 insertions(+), 15 deletions(-)
> > 
> > --- a/fs/fscache/internal.h
> > +++ b/fs/fscache/internal.h
> > @@ -81,7 +81,6 @@ extern unsigned fscache_debug;
> >  extern struct kobject *fscache_root;
> >  extern struct workqueue_struct *fscache_object_wq;
> >  extern struct workqueue_struct *fscache_op_wq;
> > -DECLARE_PER_CPU(wait_queue_head_t, fscache_object_cong_wait);
> >  
> >  extern unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n);
> >  
> > --- a/fs/fscache/main.c
> > +++ b/fs/fscache/main.c
> > @@ -41,8 +41,6 @@ struct kobject *fscache_root;
> >  struct workqueue_struct *fscache_object_wq;
> >  struct workqueue_struct *fscache_op_wq;
> >  
> > -DEFINE_PER_CPU(wait_queue_head_t, fscache_object_cong_wait);
> > -
> >  /* these values serve as lower bounds, will be adjusted in fscache_init() */
> >  static unsigned fscache_object_max_active = 4;
> >  static unsigned fscache_op_max_active = 2;
> > @@ -138,7 +136,6 @@ unsigned int fscache_hash(unsigned int s
> >  static int __init fscache_init(void)
> >  {
> >  	unsigned int nr_cpus = num_possible_cpus();
> > -	unsigned int cpu;
> >  	int ret;
> >  
> >  	fscache_object_max_active =
> > @@ -161,9 +158,6 @@ static int __init fscache_init(void)
> >  	if (!fscache_op_wq)
> >  		goto error_op_wq;
> >  
> > -	for_each_possible_cpu(cpu)
> > -		init_waitqueue_head(&per_cpu(fscache_object_cong_wait, cpu));
> > -
> >  	ret = fscache_proc_init();
> >  	if (ret < 0)
> >  		goto error_proc;
> > --- a/fs/fscache/object.c
> > +++ b/fs/fscache/object.c
> > @@ -798,6 +798,8 @@ void fscache_object_destroy(struct fscac
> >  }
> >  EXPORT_SYMBOL(fscache_object_destroy);
> >  
> > +static DECLARE_WAIT_QUEUE_HEAD(fscache_object_cong_wait);
> > +
> >  /*
> >   * enqueue an object for metadata-type processing
> >   */
> > @@ -806,16 +808,12 @@ void fscache_enqueue_object(struct fscac
> >  	_enter("{OBJ%x}", object->debug_id);
> >  
> >  	if (fscache_get_object(object, fscache_obj_get_queue) >= 0) {
> > -		wait_queue_head_t *cong_wq =
> > -			&get_cpu_var(fscache_object_cong_wait);
> >  
> >  		if (queue_work(fscache_object_wq, &object->work)) {
> >  			if (fscache_object_congested())
> > -				wake_up(cong_wq);
> > +				wake_up(&fscache_object_cong_wait);
> >  		} else
> >  			fscache_put_object(object, fscache_obj_put_queue);
> > -
> > -		put_cpu_var(fscache_object_cong_wait);
> >  	}
> >  }
> >  
> > @@ -833,16 +831,15 @@ void fscache_enqueue_object(struct fscac
> >   */
> >  bool fscache_object_sleep_till_congested(signed long *timeoutp)
> >  {
> > -	wait_queue_head_t *cong_wq = this_cpu_ptr(&fscache_object_cong_wait);
> >  	DEFINE_WAIT(wait);
> >  
> >  	if (fscache_object_congested())
> >  		return true;
> >  
> > -	add_wait_queue_exclusive(cong_wq, &wait);
> > +	add_wait_queue_exclusive(&fscache_object_cong_wait, &wait);
> >  	if (!fscache_object_congested())
> >  		*timeoutp = schedule_timeout(*timeoutp);
> > -	finish_wait(cong_wq, &wait);
> > +	finish_wait(&fscache_object_cong_wait, &wait);
> >  
> >  	return fscache_object_congested();
> >  }
 
Sebastian