netdev - Re: [PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and tailcall handling in JIT

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Message-ID: <20200717021624.do6mrxxr37vc7ajd@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 16 Jul 2020 19:16:24 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>, ast@...nel.org,
        bpf@...r.kernel.org, netdev@...r.kernel.org, bjorn.topel@...el.com,
        magnus.karlsson@...el.com
Subject: Re: [PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and tailcall
 handling in JIT

On Fri, Jul 17, 2020 at 01:06:07AM +0200, Daniel Borkmann wrote:
> > +				ret = bpf_arch_text_poke(poke->tailcall_bypass,
> > +							 BPF_MOD_JUMP,
> > +							 NULL, bypass_addr);
> > +				BUG_ON(ret < 0 && ret != -EINVAL);
> > +				/* let other CPUs finish the execution of program
> > +				 * so that it will not possible to expose them
> > +				 * to invalid nop, stack unwind, nop state
> > +				 */
> > +				synchronize_rcu();
> 
> Very heavyweight that we need to potentially call this /multiple/ times for just a
> /single/ map update under poke mutex even ... but agree it's needed here to avoid
> racing. :(

Yeah. I wasn't clear with my suggestion earlier.
I meant to say that synchronize_rcu() can be done between two loops.
list_for_each_entry(elem, &aux->poke_progs, list)
   for (i = 0; i < elem->aux->size_poke_tab; i++)
        bpf_arch_text_poke(poke->tailcall_bypass, ...
synchronize_rcu();
list_for_each_entry(elem, &aux->poke_progs, list)
   for (i = 0; i < elem->aux->size_poke_tab; i++)
        bpf_arch_text_poke(poke->poke->tailcall_target, ...

Not sure how much better it will be though.
text_poke is heavy.
I think it's heavier than synchronize_rcu().
Long term we can do batch of text_poke-s.

I'm actually fine with above approach of synchronize_rcu() without splitting the loop.
This kind of optimizations can be done later as a follow up.
I'd really like to land this stuff in this bpf-next cycle.
It's a big improvement to tail_calls and bpf2bpf calls.