Re: [PATCH v2 1/2] tracing: fprobe: optimization for entry only case

[Menglong Dong <menglong.dong@linux.dev>]

On 2025/10/12 12:07, Masami Hiramatsu wrote:
> Hi Menglong,
> 
> On Fri, 10 Oct 2025 11:38:46 +0800
> Menglong Dong <menglong8.dong@gmail.com> wrote:
> 
> > For now, fgraph is used for the fprobe, even if we need trace the entry
> > only. However, the performance of ftrace is better than fgraph, and we
> > can use ftrace_ops for this case.
> > 
> > Then performance of kprobe-multi increases from 54M to 69M. Before this
> > commit:
> > 
> >   $ ./benchs/run_bench_trigger.sh kprobe-multi
> >   kprobe-multi   :   54.663 ± 0.493M/s
> > 
> > After this commit:
> > 
> >   $ ./benchs/run_bench_trigger.sh kprobe-multi
> >   kprobe-multi   :   69.447 ± 0.143M/s
> > 
> > Mitigation is disable during the bench testing above.
> > 
> 
> Thanks for updating!
> 
> This looks good to me. Just a nit comment below;
> 
> [...]
> > @@ -379,11 +380,82 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
> >  NOKPROBE_SYMBOL(fprobe_return);
> >  
> >  static struct fgraph_ops fprobe_graph_ops = {
> > -	.entryfunc	= fprobe_entry,
> > +	.entryfunc	= fprobe_fgraph_entry,
> >  	.retfunc	= fprobe_return,
> >  };
> >  static int fprobe_graph_active;
> >  
> > +/* ftrace_ops callback, this processes fprobes which have only entry_handler. */
> > +static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
> > +	struct ftrace_ops *ops, struct ftrace_regs *fregs)
> > +{
> > +	struct fprobe_hlist_node *node;
> > +	struct rhlist_head *head, *pos;
> > +	struct fprobe *fp;
> > +	int bit;
> > +
> > +	bit = ftrace_test_recursion_trylock(ip, parent_ip);
> > +	if (bit < 0)
> > +		return;
> > +
> 
> nit: We'd better to explain why we need this here too;
> 
> 	/*
> 	 * ftrace_test_recursion_trylock() disables preemption, but
> 	 * rhltable_lookup() checks whether rcu_read_lcok is held.
> 	 * So we take rcu_read_lock() here.
> 	 */

It's very nice! I'll send a V3 now.

Thanks!
Menglong Dong

> 
> > +	rcu_read_lock();
> > +	head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params);
> > +
> > +	rhl_for_each_entry_rcu(node, pos, head, hlist) {
> > +		if (node->addr != ip)
> > +			break;
> > +		fp = READ_ONCE(node->fp);
> > +		if (unlikely(!fp || fprobe_disabled(fp) || fp->exit_handler))
> > +			continue;
> > +
> > +		if (fprobe_shared_with_kprobes(fp))
> > +			__fprobe_kprobe_handler(ip, parent_ip, fp, fregs, NULL);
> > +		else
> > +			__fprobe_handler(ip, parent_ip, fp, fregs, NULL);
> > +	}
> > +	rcu_read_unlock();
> > +	ftrace_test_recursion_unlock(bit);
> > +}
> > +NOKPROBE_SYMBOL(fprobe_ftrace_entry);
> 
> Thank you,
> 
> > +
> > +static struct ftrace_ops fprobe_ftrace_ops = {
> > +	.func	= fprobe_ftrace_entry,
> > +	.flags	= FTRACE_OPS_FL_SAVE_REGS,
> > +};
> > +static int fprobe_ftrace_active;
> > +
> > +static int fprobe_ftrace_add_ips(unsigned long *addrs, int num)
> > +{
> > +	int ret;
> > +
> > +	lockdep_assert_held(&fprobe_mutex);
> > +
> > +	ret = ftrace_set_filter_ips(&fprobe_ftrace_ops, addrs, num, 0, 0);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (!fprobe_ftrace_active) {
> > +		ret = register_ftrace_function(&fprobe_ftrace_ops);
> > +		if (ret) {
> > +			ftrace_free_filter(&fprobe_ftrace_ops);
> > +			return ret;
> > +		}
> > +	}
> > +	fprobe_ftrace_active++;
> > +	return 0;
> > +}
> > +
> > +static void fprobe_ftrace_remove_ips(unsigned long *addrs, int num)
> > +{
> > +	lockdep_assert_held(&fprobe_mutex);
> > +
> > +	fprobe_ftrace_active--;
> > +	if (!fprobe_ftrace_active)
> > +		unregister_ftrace_function(&fprobe_ftrace_ops);
> > +	if (num)
> > +		ftrace_set_filter_ips(&fprobe_ftrace_ops, addrs, num, 1, 0);
> > +}
> > +
> >  /* Add @addrs to the ftrace filter and register fgraph if needed. */
> >  static int fprobe_graph_add_ips(unsigned long *addrs, int num)
> >  {
> > @@ -498,9 +570,12 @@ static int fprobe_module_callback(struct notifier_block *nb,
> >  	} while (node == ERR_PTR(-EAGAIN));
> >  	rhashtable_walk_exit(&iter);
> >  
> > -	if (alist.index > 0)
> > +	if (alist.index > 0) {
> >  		ftrace_set_filter_ips(&fprobe_graph_ops.ops,
> >  				      alist.addrs, alist.index, 1, 0);
> > +		ftrace_set_filter_ips(&fprobe_ftrace_ops,
> > +				      alist.addrs, alist.index, 1, 0);
> > +	}
> >  	mutex_unlock(&fprobe_mutex);
> >  
> >  	kfree(alist.addrs);
> > @@ -733,7 +808,11 @@ int register_fprobe_ips(struct fprobe *fp, unsigned long *addrs, int num)
> >  	mutex_lock(&fprobe_mutex);
> >  
> >  	hlist_array = fp->hlist_array;
> > -	ret = fprobe_graph_add_ips(addrs, num);
> > +	if (fp->exit_handler)
> > +		ret = fprobe_graph_add_ips(addrs, num);
> > +	else
> > +		ret = fprobe_ftrace_add_ips(addrs, num);
> > +
> >  	if (!ret) {
> >  		add_fprobe_hash(fp);
> >  		for (i = 0; i < hlist_array->size; i++) {
> > @@ -829,7 +908,10 @@ int unregister_fprobe(struct fprobe *fp)
> >  	}
> >  	del_fprobe_hash(fp);
> >  
> > -	fprobe_graph_remove_ips(addrs, count);
> > +	if (fp->exit_handler)
> > +		fprobe_graph_remove_ips(addrs, count);
> > +	else
> > +		fprobe_ftrace_remove_ips(addrs, count);
> >  
> >  	kfree_rcu(hlist_array, rcu);
> >  	fp->hlist_array = NULL;
> 
> 
>