Re: [PATCH] uprobes: reduce contention on uprobes_tree access

[Jonthan Haslam]

Hi Masami,

> > This change has been tested against production workloads that exhibit
> > significant contention on the spinlock and an almost order of magnitude
> > reduction for mean uprobe execution time is observed (28 -> 3.5 microsecs).
> 
> Looks good to me.
> 
> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> BTW, how did you measure the overhead? I think spinlock overhead
> will depend on how much lock contention happens.

Absolutely. I have the original production workload to test this with and
a derived one that mimics this test case. The production case has ~24
threads running on a 192 core system which access 14 USDTs around 1.5
million times per second in total (across all USDTs). My test case is
similar but can drive a higher rate of USDT access across more threads and
therefore generate higher contention.

All measurements are done using bpftrace scripts around relevant parts of
code in uprobes.c and application code.

Jon.

> 
> Thank you,
> 
> > 
> > [0] https://docs.kernel.org/locking/spinlocks.html
> > 
> > Signed-off-by: Jonathan Haslam <jonathan.haslam@xxxxxxxxx>
> > ---
> >  kernel/events/uprobes.c | 22 +++++++++++-----------
> >  1 file changed, 11 insertions(+), 11 deletions(-)
> > 
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 929e98c62965..42bf9b6e8bc0 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -39,7 +39,7 @@ static struct rb_root uprobes_tree = RB_ROOT;
> >   */
> >  #define no_uprobe_events()	RB_EMPTY_ROOT(&uprobes_tree)
> >  
> > -static DEFINE_SPINLOCK(uprobes_treelock);	/* serialize rbtree access */
> > +static DEFINE_RWLOCK(uprobes_treelock);	/* serialize rbtree access */
> >  
> >  #define UPROBES_HASH_SZ	13
> >  /* serialize uprobe->pending_list */
> > @@ -669,9 +669,9 @@ static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
> >  {
> >  	struct uprobe *uprobe;
> >  
> > -	spin_lock(&uprobes_treelock);
> > +	read_lock(&uprobes_treelock);
> >  	uprobe = __find_uprobe(inode, offset);
> > -	spin_unlock(&uprobes_treelock);
> > +	read_unlock(&uprobes_treelock);
> >  
> >  	return uprobe;
> >  }
> > @@ -701,9 +701,9 @@ static struct uprobe *insert_uprobe(struct uprobe *uprobe)
> >  {
> >  	struct uprobe *u;
> >  
> > -	spin_lock(&uprobes_treelock);
> > +	write_lock(&uprobes_treelock);
> >  	u = __insert_uprobe(uprobe);
> > -	spin_unlock(&uprobes_treelock);
> > +	write_unlock(&uprobes_treelock);
> >  
> >  	return u;
> >  }
> > @@ -935,9 +935,9 @@ static void delete_uprobe(struct uprobe *uprobe)
> >  	if (WARN_ON(!uprobe_is_active(uprobe)))
> >  		return;
> >  
> > -	spin_lock(&uprobes_treelock);
> > +	write_lock(&uprobes_treelock);
> >  	rb_erase(&uprobe->rb_node, &uprobes_tree);
> > -	spin_unlock(&uprobes_treelock);
> > +	write_unlock(&uprobes_treelock);
> >  	RB_CLEAR_NODE(&uprobe->rb_node); /* for uprobe_is_active() */
> >  	put_uprobe(uprobe);
> >  }
> > @@ -1298,7 +1298,7 @@ static void build_probe_list(struct inode *inode,
> >  	min = vaddr_to_offset(vma, start);
> >  	max = min + (end - start) - 1;
> >  
> > -	spin_lock(&uprobes_treelock);
> > +	read_lock(&uprobes_treelock);
> >  	n = find_node_in_range(inode, min, max);
> >  	if (n) {
> >  		for (t = n; t; t = rb_prev(t)) {
> > @@ -1316,7 +1316,7 @@ static void build_probe_list(struct inode *inode,
> >  			get_uprobe(u);
> >  		}
> >  	}
> > -	spin_unlock(&uprobes_treelock);
> > +	read_unlock(&uprobes_treelock);
> >  }
> >  
> >  /* @vma contains reference counter, not the probed instruction. */
> > @@ -1407,9 +1407,9 @@ vma_has_uprobes(struct vm_area_struct *vma, unsigned long start, unsigned long e
> >  	min = vaddr_to_offset(vma, start);
> >  	max = min + (end - start) - 1;
> >  
> > -	spin_lock(&uprobes_treelock);
> > +	read_lock(&uprobes_treelock);
> >  	n = find_node_in_range(inode, min, max);
> > -	spin_unlock(&uprobes_treelock);
> > +	read_unlock(&uprobes_treelock);
> >  
> >  	return !!n;
> >  }
> > -- 
> > 2.43.0
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>