linux-kernel - Re: [PATCH 01/16] rcu/tree: Add a work to allocate pages from regular context

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Date:   Wed, 4 Nov 2020 19:38:47 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 01/16] rcu/tree: Add a work to allocate pages from
 regular context

> > > >   * This is a per-CPU structure.  The reason that it is not included in
> > > > @@ -3100,6 +3103,11 @@ struct kfree_rcu_cpu {
> > > >  	bool monitor_todo;
> > > >  	bool initialized;
> > > >  	int count;
> > > > +
> > > > +	struct work_struct page_cache_work;
> > > > +	atomic_t work_in_progress;
> > > 
> > > Does it need to be atomic? run_page_cache_work() is only called under a lock.
> > > You can use xchg() there. And when you do the atomic_set, you can use
> > > WRITE_ONCE as it is a data-race.
> > > 
> > We can use xchg together with *_ONCE() macro. Could you please clarify what
> > is your concern about using atomic_t? Both xchg() and atomic_xchg() guarantee
> > atamarity. Same as WRITE_ONCE() or atomic_set().
> 
> Right, whether there's lock or not does not matter as xchg() is also
> atomic-swap.
> 
> atomic_t is a more complex type though, I would directly use int since
> atomic_t is not needed here and there's no lost-update issue here. It could
> be matter of style as well.
> 
> BTW I did think atomic_xchg() adds additional memory barriers
> but I could not find that to be the case in the implementation. Is that not
> the case? Docs says "atomic_xchg must provide explicit memory barriers around
> the operation.".
> 
In most of the systems atmoc_xchg() is same as xchg() and atomic_set()
is same as WRITE_ONCE(). But there are exceptions, for example "parisc"

*** arch/parisc/include/asm/atomic.h:
<snip>
...
#define _atomic_spin_lock_irqsave(l,f) do { \
    arch_spinlock_t *s = ATOMIC_HASH(l); \
    local_irq_save(f);   \
    arch_spin_lock(s);   \
} while(0)
...
static __inline__ void atomic_set(atomic_t *v, int i)
{
     unsigned long flags;
     _atomic_spin_lock_irqsave(v, flags);

     v->counter = i;

     _atomic_spin_unlock_irqrestore(v, flags);
}
<snip>

I will switch to xchg() and WRITE_ONCE(), because of such specific ARCHs.

> > > > @@ -4449,24 +4482,14 @@ static void __init kfree_rcu_batch_init(void)
> > > >  
> > > >  	for_each_possible_cpu(cpu) {
> > > >  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > > > -		struct kvfree_rcu_bulk_data *bnode;
> > > >  
> > > >  		for (i = 0; i < KFREE_N_BATCHES; i++) {
> > > >  			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
> > > >  			krcp->krw_arr[i].krcp = krcp;
> > > >  		}
> > > >  
> > > > -		for (i = 0; i < rcu_min_cached_objs; i++) {
> > > > -			bnode = (struct kvfree_rcu_bulk_data *)
> > > > -				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > > > -
> > > > -			if (bnode)
> > > > -				put_cached_bnode(krcp, bnode);
> > > > -			else
> > > > -				pr_err("Failed to preallocate for %d CPU!\n", cpu);
> > > > -		}
> > > > -
> > > >  		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> > > > +		INIT_WORK(&krcp->page_cache_work, fill_page_cache_func);
> > > >  		krcp->initialized = true;
> > > 
> > > During initialization, is it not better to still pre-allocate? That way you
> > > don't have to wait to get into a situation where you need to initially
> > > allocate.
> > > 
> > Since we have a worker that does it when a cache is empty there is no
> > a high need in doing it during initialization phase. If we can reduce
> > an amount of code it is always good :)
> 
> I am all for not having more code than needed. But you would hit
> synchronize_rcu() slow path immediately on first headless kfree_rcu() right?
> That seems like a step back from the current code :)
> 
As for slow path and hitting the synchronize_rcu() immediately. Yes, a slow 
hit "counter" will be increased by 1, the difference between two variants
will be N and N + 1 times. I do not consider N + 1 as a big difference and
impact on performance.

Should we guarantee that a first user does not hit a fallback path that
invokes synchronize_rcu()? If not, i would rather remove redundant code.

Any thoughts here?

Thanks!

--
Vlad Rezki