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Message-ID: <5014869.GXAFRqVoOG@7940hx>
Date: Wed, 24 Sep 2025 09:33:16 +0800
From: menglong.dong@...ux.dev
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH 2/2] tracing: fprobe: optimization for entry only case
On 2025/9/24 08:23 Masami Hiramatsu <mhiramat@...nel.org> write:
> Hi Menglong,
>
> Please add a cover letter if you make a series of patches.
>
> On Tue, 23 Sep 2025 17:20:01 +0800
> Menglong Dong <menglong8.dong@...il.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.
>
> Hmm, indeed. If it is used only for entry, it can use ftrace.
>
> Also, please merge [1/2] and [2/2]. [1/2] is meaningless (and do
> nothing) without this change. Moreover, it changes the same file.
>
> You can split the patch if "that cleanup is meaningful independently"
> or "that changes different subsystem/component (thus you need an Ack
> from another maintainer)".
OK, I see now :)
>
> But basically looks good to me. Just have some nits.
>
> >
> > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > ---
> > kernel/trace/fprobe.c | 88 +++++++++++++++++++++++++++++++++++++++----
> > 1 file changed, 81 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 1785fba367c9..de4ae075548d 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -292,7 +292,7 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
> > if (node->addr != func)
> > continue;
> > fp = READ_ONCE(node->fp);
> > - if (fp && !fprobe_disabled(fp))
> > + if (fp && !fprobe_disabled(fp) && fp->exit_handler)
> > fp->nmissed++;
> > }
> > return 0;
> > @@ -312,11 +312,11 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
> > if (node->addr != func)
> > continue;
> > fp = READ_ONCE(node->fp);
> > - if (!fp || fprobe_disabled(fp))
> > + if (unlikely(!fp || fprobe_disabled(fp) || !fp->exit_handler))
> > continue;
> >
> > data_size = fp->entry_data_size;
> > - if (data_size && fp->exit_handler)
> > + if (data_size)
> > data = fgraph_data + used + FPROBE_HEADER_SIZE_IN_LONG;
> > else
> > data = NULL;
> > @@ -327,7 +327,7 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
> > ret = __fprobe_handler(func, ret_ip, fp, fregs, data);
> >
> > /* If entry_handler returns !0, nmissed is not counted but skips exit_handler. */
> > - if (!ret && fp->exit_handler) {
> > + if (!ret) {
> > int size_words = SIZE_IN_LONG(data_size);
> >
> > if (write_fprobe_header(&fgraph_data[used], fp, size_words))
> > @@ -384,6 +384,70 @@ static struct fgraph_ops fprobe_graph_ops = {
> > };
> > static int fprobe_graph_active;
> >
>
> > +/* ftrace_ops backend (entry-only) */
> ^ callback ?
ACK
>
> Also, add similar comments on top of fprobe_fgraph_entry.
>
> /* fgraph_ops callback, this processes fprobes which have exit_handler. */
ACK
>
> > +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;
> > +
> > + guard(rcu)();
> > + 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;
> > + /* entry-only path: no exit_handler nor per-call data */
> > + if (fprobe_shared_with_kprobes(fp))
> > + __fprobe_kprobe_handler(ip, parent_ip, fp, fregs, NULL);
> > + else
> > + __fprobe_handler(ip, parent_ip, fp, fregs, NULL);
> > + }
> > +}
> > +NOKPROBE_SYMBOL(fprobe_ftrace_entry);
>
> OK.
>
> > +
> > +static struct ftrace_ops fprobe_ftrace_ops = {
> > + .func = fprobe_ftrace_entry,
> > + .flags = FTRACE_OPS_FL_SAVE_REGS,
>
> [OT] I just wonder we can have FTRACE_OPS_FL_SAVE_FTRACE_REGS instead.
I'll give it a try.
Thanks!
Menglong Dong
>
> > +};
> > +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)
> > {
> > @@ -500,9 +564,12 @@ static int fprobe_module_callback(struct notifier_block *nb,
> > } while (node == ERR_PTR(-EAGAIN));
> > rhashtable_walk_exit(&iter);
> >
> > - if (alist.index < alist.size && alist.index > 0)
> > + if (alist.index < alist.size && alist.index > 0) {
>
> Oops, here is my bug. Let me fix it.
>
> Thank you,
>
> > 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);
> > @@ -735,7 +802,11 @@ 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++) {
> > @@ -831,7 +902,10 @@ 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;
> > --
> > 2.51.0
> >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
>
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