Re: [RFC PATCH] asynchronous page fault.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Sun, 27 Dec 2009 18:47:25 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:
> > 
> > =
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > 
> > Asynchronous page fault.
> > 
> > This patch is for avoidng mmap_sem in usual page fault. At running highly
> > multi-threaded programs, mm->mmap_sem can use much CPU because of false
> > sharing when it causes page fault in parallel. (Run after fork() is a typical
> > case, I think.)
> > This patch uses a speculative vma lookup to reduce that cost.
> > 
> > Considering vma lookup, rb-tree lookup, the only operation we do is checking
> > node->rb_left,rb_right. And there are no complicated operation.
> > At page fault, there are no demands for accessing sorted-vma-list or access
> > prev or next in many case. Except for stack-expansion, we always need a vma
> > which contains page-fault address. Then, we can access vma's RB-tree in
> > speculative way.
> > Even if RB-tree rotation occurs while we walk tree for look-up, we just
> > miss vma without oops. In other words, we can _try_ to find vma in lockless
> > manner. If failed, retry is ok.... we take lock and access vma.
> > 
> > For lockess walking, this uses RCU and adds find_vma_speculative(). And
> > per-vma wait-queue and reference count. This refcnt+wait_queue guarantees that
> > there are no thread which access the vma when we call subsystem's unmap
> > functions.
> > 
> > Test result on my tiny test program on 8core/2socket machine is here.
> > This measures how many page fault can occur in 60sec in parallel.
> > 
> > [root@bluextal memory]# /root/bin/perf stat -e page-faults,cache-misses --repeat 5 ./multi-fault-all-split 8
> > 
> >  Performance counter stats for './multi-fault-all-split 8' (5 runs):
> > 
> >        17481387  page-faults                ( +-   0.409% )
> >       509914595  cache-misses               ( +-   0.239% )
> > 
> >    60.002277793  seconds time elapsed   ( +-   0.000% )
> > 
> > 
> > [root@bluextal memory]# /root/bin/perf stat -e page-faults,cache-misses --repeat 5 ./multi-fault-all-split 8
> > 
> > 
> >  Performance counter stats for './multi-fault-all-split 8' (5 runs):
> > 
> >        35949073  page-faults                ( +-   0.364% )
> >       473091100  cache-misses               ( +-   0.304% )
> > 
> >    60.005444117  seconds time elapsed   ( +-   0.004% )
> > 
> > 
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> <snip>
> 
> 
> > +/* called when vma is unlinked and wait for all racy access.*/
> > +static void invalidate_vma_before_free(struct vm_area_struct *vma)
> > +{
> > +	atomic_dec(&vma->refcnt);
> > +	wait_event(vma->wait_queue, !atomic_read(&vma->refcnt));
> > +}
> 
> I think we have to make sure atomicity of both (atomic_dec and wait_event).
> 
I still consider how to do this.

	atomic_sub(&vma->refcnt, 65536)
	wait_event(..., atomic_read(&vma->refcnt) != 65536)

etc.



> > +
> >  /*
> >   * Requires inode->i_mapping->i_mmap_lock
> >   */
> > @@ -238,7 +256,7 @@ static struct vm_area_struct *remove_vma
> >  			removed_exe_file_vma(vma->vm_mm);
> >  	}
> >  	mpol_put(vma_policy(vma));
> > -	kmem_cache_free(vm_area_cachep, vma);
> > +	free_vma_rcu(vma);
> >  	return next;
> >  }
> >  
> > @@ -404,6 +422,8 @@ __vma_link_list(struct mm_struct *mm, st
> >  void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
> >  		struct rb_node **rb_link, struct rb_node *rb_parent)
> >  {
> > +	atomic_set(&vma->refcnt, 1);
> > +	init_waitqueue_head(&vma->wait_queue);
> >  	rb_link_node(&vma->vm_rb, rb_parent, rb_link);
> >  	rb_insert_color(&vma->vm_rb, &mm->mm_rb);
> >  }
> > @@ -614,6 +634,7 @@ again:			remove_next = 1 + (end > next->
> >  		 * us to remove next before dropping the locks.
> >  		 */
> >  		__vma_unlink(mm, next, vma);
> > +		invalidate_vma_before_free(next);
> >  		if (file)
> >  			__remove_shared_vm_struct(next, file, mapping);
> >  		if (next->anon_vma)
> > @@ -640,7 +661,7 @@ again:			remove_next = 1 + (end > next->
> >  		}
> >  		mm->map_count--;
> >  		mpol_put(vma_policy(next));
> > -		kmem_cache_free(vm_area_cachep, next);
> > +		free_vma_rcu(next);
> >  		/*
> >  		 * In mprotect's case 6 (see comments on vma_merge),
> >  		 * we must remove another next too. It would clutter
> > @@ -1544,6 +1565,55 @@ out:
> >  }
> >  
> >  /*
> > + * Returns vma which contains given address. This scans rb-tree in speculative
> > + * way and increment a reference count if found. Even if vma exists in rb-tree,
> > + * this function may return NULL in racy case. So, this function cannot be used
> > + * for checking whether given address is valid or not.
> > + */
> > +struct vm_area_struct *
> > +find_vma_speculative(struct mm_struct *mm, unsigned long addr)
> > +{
> > +	struct vm_area_struct *vma = NULL;
> > +	struct vm_area_struct *vma_tmp;
> > +	struct rb_node *rb_node;
> > +
> > +	if (unlikely(!mm))
> > +		return NULL;;
> > +
> > +	rcu_read_lock();
> > +	rb_node = rcu_dereference(mm->mm_rb.rb_node);
> > +	vma = NULL;
> > +	while (rb_node) {
> > +		vma_tmp = rb_entry(rb_node, struct vm_area_struct, vm_rb);
> > +
> > +		if (vma_tmp->vm_end > addr) {
> > +			vma = vma_tmp;
> > +			if (vma_tmp->vm_start <= addr)
> > +				break;
> > +			rb_node = rcu_dereference(rb_node->rb_left);
> > +		} else
> > +			rb_node = rcu_dereference(rb_node->rb_right);
> > +	}
> > +	if (vma) {
> > +		if ((vma->vm_start <= addr) && (addr < vma->vm_end)) {
> > +			if (!atomic_inc_not_zero(&vma->refcnt))
> > +				vma = NULL;
> > +		} else
> > +			vma = NULL;
> > +	}
> > +	rcu_read_unlock();
> > +	return vma;
> > +}
> > +
> > +void vma_put(struct vm_area_struct *vma)
> > +{
> > +	if ((atomic_dec_return(&vma->refcnt) == 1) &&
> > +	    waitqueue_active(&vma->wait_queue))
> > +		wake_up(&vma->wait_queue);
> > +	return;
> > +}
> > +
> 
> Let's consider following case. 
> 
> CPU 0					CPU 1
> 
> find_vma_speculative(refcnt = 2)
> 					do_unmap 
> 					invaliate_vma_before_free(refcount = 1)
> 					wait_event
> vma_put
> refcnt = 0
> skip wakeup 
> 
> Hmm.. 

Nice catch. I'll change this logic. Maybe some easy trick can fix this.

Thanks,
-Kame

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