linux-kernel - Re: [PATCH] Implement slub fastpath in terms of freebase and freeoffset

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Date:	Thu, 28 Feb 2008 20:56:21 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Implement slub fastpath in terms of freebase and
	freeoffset

* Christoph Lameter (clameter@....com) wrote:
> 
> > Index: linux-2.6-lttng/mm/slub.c
> > ===================================================================
> > --- linux-2.6-lttng.orig/mm/slub.c	2008-02-27 20:41:14.000000000 -0500
> > +++ linux-2.6-lttng/mm/slub.c	2008-02-28 18:22:14.000000000 -0500
> > @@ -138,6 +138,22 @@ static inline void ClearSlabDebug(struct
> >  	page->flags &= ~SLABDEBUG;
> >  }
> >  
> > +static inline void **get_freelist_ptr(struct kmem_cache_cpu *c)
> > +{
> > +	return (void **)((unsigned long)page_address(c->page)
> > +		| (c->freeoffset & c->off_mask));
> > +}
> 
> Hmmmm... page_address() is an expensive operation.
> 

Hrm, this is why I kept a separate "freebase", as a sort of
page_address() cache.

> > @@ -1779,21 +1815,21 @@ static __always_inline void slab_free(st
> >  	struct kmem_cache_cpu *c;
> >  
> >  #ifdef SLUB_FASTPATH
> > -	void **freelist;
> > +	unsigned long freeoffset, newoffset;
> >  
> >  	c = get_cpu_slab(s, raw_smp_processor_id());
> >  	debug_check_no_locks_freed(object, s->objsize);
> >  	do {
> > -		freelist = c->freelist;
> > +		freeoffset = c->freeoffset;
> >  		barrier();
> >  		/*
> >  		 * If the compiler would reorder the retrieval of c->page to
> > -		 * come before c->freelist then an interrupt could
> > -		 * change the cpu slab before we retrieve c->freelist. We
> > -		 * could be matching on a page no longer active and put the
> > -		 * object onto the freelist of the wrong slab.
> > +		 * come before c->freeoffset then an interrupt could change the
> > +		 * cpu slab before we retrieve c->freelist. We could be matching
> > +		 * on a page no longer active and put the object onto the
> > +		 * freelist of the wrong slab.
> >  		 *
> > -		 * On the other hand: If we already have the freelist pointer
> > +		 * On the other hand: If we already have the freeoffset
> >  		 * then any change of cpu_slab will cause the cmpxchg to fail
> >  		 * since the freelist pointers are unique per slab.
> >  		 */
> > @@ -1801,9 +1837,15 @@ static __always_inline void slab_free(st
> >  			__slab_free(s, page, x, addr, c->offset);
> >  			break;
> >  		}
> > -		object[c->offset] = freelist;
> > +		object[c->offset] =
> > +			(void **)((unsigned long)page_address(c->page) |
> > +					(freeoffset & c->off_mask));
> >  		stat(c, FREE_FASTPATH);
> > -	} while (cmpxchg_local(&c->freelist, freelist, object) != freelist);
> > +		newoffset = freeoffset + PAGE_SIZE;
> > +		newoffset &= ~c->off_mask;
> > +		newoffset |= (unsigned long)object & c->off_mask;
> > +	} while (cmpxchg_local(&c->freeoffset, freeoffset, newoffset)
> > +			!= freeoffset);
> 
> Hmmm.. The cmpxchg_local was intended to check for a change of slab too 
> (addresses from different slabs are disjunct). 
> That is no longer possible since the upper bits are used by the 
> versioning? (Same issue in slab_alloc).
> 

Here is the trick : if, on top of the cmpxchg_local, we have an
interrupt that calls slab_alloc or slab_free which causes a change of
slab, this interrupt will have to go through the slow path.

These slow paths use set_freelist_ptr() to update the c->freeoffset,
which also increments the sequence number. When we return from
interrupt, the cmpxchg_local loop will fail because the sequence number
has changed.

In short, the we also use the versioning to check for change of slab.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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