Date:   Thu, 23 Dec 2021 22:12:14 +0800
From:   Jeff Xie <xiehuan09@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, mingo@...hat.com,
        Tom Zanussi <zanussi@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v6 1/5] trace: Add trace any kernel object

Hi Masami,

On Tue, Dec 21, 2021 at 6:29 PM Jeff Xie <xiehuan09@...il.com> wrote:
>
> Hi Masami,
>
> On Tue, Dec 21, 2021 at 3:36 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > Hi Jeff,
> >
> > On Sat, 18 Dec 2021 00:32:57 +0800
> > Jeff Xie <xiehuan09@...il.com> wrote:
> >
> > > > > +struct object_instance {
> > > > > +     void *object;
> > > > > +     struct freelist_node free_list;
> > > > > +     struct list_head active_list;
> > > > > +};
> > > > > +
> > > > > +struct obj_pool {
> > > > > +     struct freelist_head free_list;
> > > > > +     struct list_head active_list;
> > > > > +};
> > > > > +static struct obj_pool *obj_pool;
> > > > > +
> > > > > +static bool object_exist(void *obj)
> > > > > +{
> > > > > +     struct object_instance *inst;
> > > > > +     bool ret = false;
> > > > > +
> > > > > +     list_for_each_entry_rcu(inst, &obj_pool->active_list, active_list) {
> > > > > +             if (inst->object == obj) {
> > > > > +                     ret = true;
> > > > > +                     goto out;
> > > > > +             }
> > > > > +     }
> > > > > +out:
> > > > > +     return ret;
> > > > > +}
> > > > > +
> > > > > +static bool object_empty(void)
> > > > > +{
> > > > > +     return list_empty(&obj_pool->active_list);
> > > > > +}
> > > > > +
> > > > > +static void set_trace_object(void *obj)
> > > > > +{
> > > > > +     struct freelist_node *fn;
> > > > > +     struct object_instance *ins;
> > > > > +     unsigned long flags;
> > > > > +
> > > > > +     if (in_nmi())
> > > > > +             return;
> > > > > +
> > > > > +     if (!obj)
> > > > > +             return;
> > > > > +
> > > > > +     if (object_exist(obj))
> > > > > +             return;
> > > > > +
> > > > > +     fn = freelist_try_get(&obj_pool->free_list);
> > > > > +     if (!fn) {
> > > > > +             trace_printk("object_pool is full, can't trace object:0x%px\n", obj);
> > > > > +             return;
> > > > > +     }
> > > > > +
> > > > > +     ins = container_of(fn, struct object_instance, free_list);
> > > > > +     ins->object = obj;
> > > > > +
> > > > > +     raw_spin_lock_irqsave(&object_spin_lock, flags);
> > > > > +     list_add_rcu(&ins->active_list, &obj_pool->active_list);
> > > > > +     raw_spin_unlock_irqrestore(&object_spin_lock, flags);
> > > >
> > > > Please add a comment that why this spinlock is needed here and why
> > > > other operation doesn't need it.
> > >
> > > (Only this place has write operations, and it cannot be concurrent)
> > > I agree, I will add it.
> >
> > BTW, I have another better solution for object pool. If the
> > object pool size is fixed (of course to avoid performance overhead,
> > it should be small enough) and it can not avoid using spinlock,
> > it is better to use fixed-size array. That makes the implementation
> > much simpler.
>
> This looks really simpler, I will add it in the next version ;-)
>
> > static struct object_instance {
> >         void *obj;      /* trace object */
> >         // add offset in the next patch
> > } traced_obj[MAX_TRACE_OBJECT];
> >
> > static atomic_t num_traced_obj;
> > static DEFINE_RAW_SPINLOCK(trace_obj_lock);
> >
> > static void set_trace_object(void *obj)
> > {
> >         ...
> >
> >         raw_spin_lock_irqsave(&trace_obj_lock, flags);
> >         if (num_traced_obj == MAX_TRACED_OBJECT)
> >                 goto out;
> >
> >         traced_obj[num_traced_obj].obj = obj;
> >         smp_wmb();      // make sure the num_traced_obj update always appears after trace_obj update.
> >         num_traced_obj++;
>
> I would like to ask whether this place need to add another smp_wmb()
> to match the smp_rmb() in the object_exist().
> I learned that rcu has a publish and subscribe mechanism ;-)

There is no need to add another smp_wmb(). I understood.

Have a nice holiday;-) Thanks.

>
> > out:
> >         raw_spin_unlock_irqrestore(&trace_obj_lock, flags);
> > }
> >
> > static bool object_exist(void *obj)
> > {
> >         ...
> >         max = num_traced_obj;
> >         smp_rmb();      // then the num_traced_obj will cap the max.
> >         for (i = 0; i < max; i++) {
> >                 if (traced_obj[i].obj == obj)
> >                         return true;
> >         }
> >         return false;
> > }
> >
> > static inline void free_object_pool(void)
> > {
> >         num_traced_obj = 0;
> >         memset(traced_obj, 0, sizeof(traced_obj));
> > }
> >
> > Sorry if I confuse you but I think you shouldn't take a time on those unneeded
> > complexity. :)
>
> Thanks, at least I learned a different way to implement it ;-)
>
> >
> >
> > > > > +static void submit_trace_object(unsigned long ip, unsigned long parent_ip,
> > > > > +                              unsigned long object)
> > > > > +{
> > > > > +
> > > > > +     struct trace_buffer *buffer;
> > > > > +     struct ring_buffer_event *event;
> > > > > +     struct trace_object_entry *entry;
> > > > > +     int pc;
> > > > > +
> > > > > +     pc = preempt_count();
> > > > > +     event = trace_event_buffer_lock_reserve(&buffer, &event_trace_file,
> > > > > +                     TRACE_OBJECT, sizeof(*entry), pc);
> > > > > +     if (!event)
> > > > > +             return;
> > > > > +     entry   = ring_buffer_event_data(event);
> > > > > +     entry->ip                       = ip;
> > > > > +     entry->parent_ip                = parent_ip;
> > > > > +     entry->object                   = object;
> > > > > +
> > > > > +     event_trigger_unlock_commit(&event_trace_file, buffer, event,
> > > > > +             entry, pc);
> > > > > +}
> > > > > +
> > > > > +static void
> > > > > +trace_object_events_call(unsigned long ip, unsigned long parent_ip,
> > > > > +             struct ftrace_ops *op, struct ftrace_regs *fregs)
> > > > > +{
> > > > > +     struct pt_regs *pt_regs = ftrace_get_regs(fregs);
> > > > > +     unsigned long obj;
> > > > > +     long disabled;
> > > > > +     int cpu, n;
> > > > > +
> > > > > +     preempt_disable_notrace();
> > > > > +
> > > > > +     cpu = raw_smp_processor_id();
> > > > > +     disabled = atomic_inc_return(&per_cpu(trace_object_event_disable, cpu));
> > > >
> > > > So what is the purpose of this check? (are there any issue if the same
> > > > cpu reenter here?)
> > > >
> > >
> > > There may be an interrupt context that can preempt it. I am not yet
> > > sure whether the cpu reenter  will affect it.
> > > I will debug and test it. (Referred from function_test_events_call())
> >
> > Maybe you can use ftrace_test_recursion_trylock(), as kprobe_ftrace_handler()
> > does.
>
> I will use it ,thanks.
>
> >
> > Thank you,
> >
> > --
> > Masami Hiramatsu <mhiramat@...nel.org>
> ---
> JeffXie
---
JeffXie

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