linux-kernel - Re: [PATCH v2] mm/slab.c: use list_{empty_careful,last_entry} in drain

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Message-ID: <20151205023627.GA9812@bogon>
Date:	Sat, 5 Dec 2015 10:36:27 +0800
From:	Geliang Tang <geliangtang@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Geliang Tang <geliangtang@....com>
Subject: Re: [PATCH v2] mm/slab.c: use list_{empty_careful,last_entry} in
 drain_freelist

On Fri, Dec 04, 2015 at 10:16:38AM -0600, Christoph Lameter wrote:
> On Fri, 4 Dec 2015, Geliang Tang wrote:
> 
> > On Thu, Dec 03, 2015 at 08:53:21AM -0600, Christoph Lameter wrote:
> > > On Thu, 3 Dec 2015, Geliang Tang wrote:
> > >
> > > >  	while (nr_freed < tofree && !list_empty(&n->slabs_free)) {
> > > >
> > > >  		spin_lock_irq(&n->list_lock);
> > > > -		p = n->slabs_free.prev;
> > > > -		if (p == &n->slabs_free) {
> > > > +		if (list_empty_careful(&n->slabs_free)) {
> > >
> > > We have taken the lock. Why do we need to be "careful"? list_empty()
> > > shoudl work right?
> >
> > Yes. list_empty() is OK.
> >
> > >
> > > >  			spin_unlock_irq(&n->list_lock);
> > > >  			goto out;
> > > >  		}
> > > >
> > > > -		page = list_entry(p, struct page, lru);
> > > > +		page = list_last_entry(&n->slabs_free, struct page, lru);
> > >
> > > last???
> >
> > The original code delete the page from the tail of slabs_free list.
> 
> Maybe make the code clearer by using another method to get the page
> pointer?
> 
> > >
> > > Would the the other new function that returns NULL on the empty list or
> > > the pointer not be useful here too and save some code?
> >
> > Sorry, I don't really understand what do you mean. Can you please specify
> > it a little bit?
> 
> I take that back. list_empty is the best choice here.

If we use list_empty(), there will be two list_empty() in the code:

        while (nr_freed < tofree && !list_empty(&n->slabs_free)) {
                spin_lock_irq(&n->list_lock);
                if (list_empty(&n->slabs_free)) {
                        spin_unlock_irq(&n->list_lock);
                        goto out; 
                }
                page = list_last_entry(&n->slabs_free, struct page, lru);
                list_del(&page->lru);
                spin_unlock_irq(&n->list_lock);
        }

Or can we drop the first list_empty() like this? It will function the same as the above code.

        while (nr_freed < tofree) {
                spin_lock_irq(&n->list_lock);
                if (list_empty(&n->slabs_free)) {
                        spin_unlock_irq(&n->list_lock);
                        goto out; 
                }
                page = list_last_entry(&n->slabs_free, struct page, lru);
                list_del(&page->lru);
                spin_unlock_irq(&n->list_lock);
        }

Please let me know which one is better?

Thanks.

- Geliang

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