netdev - Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num

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Message-ID: <20180328153819.654c1600@gandalf.local.home>
Date:   Wed, 28 Mar 2018 15:38:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Alexei Starovoitov <ast@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num_args at build
 time

On Wed, 28 Mar 2018 15:32:20 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> -#define __DO_TRACE(tp, proto, args, cond, rcucheck)			\
> +#define __DO_TRACE(name, proto, args, cond, rcucheck)			\
>  	do {								\
>  		struct tracepoint_func *it_func_ptr;			\
>  		void *it_func;						\
> @@ -140,7 +140,7 @@ extern void syscall_unregfunc(void);
>  		if (rcucheck)						\
>  			rcu_irq_enter_irqson();				\
>  		rcu_read_lock_sched_notrace();				\
> -		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\
> +		it_func_ptr = rcu_dereference_sched(__trace_##name##_funcs); \

What we lose in data size, we may make up for in text (which is even
more important). This will remove a dereference in the hot path.

I'll make a few builds and run size on the vmlinux images to see how
this pans out.

-- Steve


>  		if (it_func_ptr) {					\
>  			do {						\