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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
	ast@kernel.org, bpf@vger.kernel.org, netdev@vger.kernel.org,
	bjorn.topel@intel.com, magnus.karlsson@intel.com
Subject: Re: [PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and tailcall handling in JIT
Date: Thu, 16 Jul 2020 19:16:24 -0700	[thread overview]
Message-ID: <20200717021624.do6mrxxr37vc7ajd@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <932141f5-7abb-1c01-111d-a64baf187a40@iogearbox.net>

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.

  reply	other threads:[~2020-07-17  2:16 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-15 23:36 [PATCH bpf-next 0/5] bpf: tailcalls in BPF subprograms Maciej Fijalkowski
2020-07-15 23:36 ` [PATCH bpf-next 1/5] bpf, x64: use %rcx instead of %rax for tail call retpolines Maciej Fijalkowski
2020-07-16 20:36   ` Daniel Borkmann
2020-07-17  9:29     ` Maciej Fijalkowski
2020-07-15 23:36 ` [PATCH bpf-next 2/5] bpf: allow for tailcalls in BPF subprograms Maciej Fijalkowski
2020-07-16 21:10   ` Daniel Borkmann
2020-07-16 21:29   ` Daniel Borkmann
2020-07-16 22:46     ` Daniel Borkmann
2020-07-17 11:39       ` Maciej Fijalkowski
2020-07-15 23:36 ` [PATCH bpf-next 3/5] bpf: propagate poke descriptors to subprograms Maciej Fijalkowski
2020-07-16 21:16   ` Daniel Borkmann
2020-07-17  9:36     ` Maciej Fijalkowski
2020-07-15 23:36 ` [PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and tailcall handling in JIT Maciej Fijalkowski
2020-07-16 23:06   ` Daniel Borkmann
2020-07-17  2:16     ` Alexei Starovoitov [this message]
2020-07-17 10:57       ` Maciej Fijalkowski
2020-07-17 16:16         ` Alexei Starovoitov
2020-07-17 10:52     ` Maciej Fijalkowski
2020-07-15 23:36 ` [PATCH bpf-next 5/5] selftests: bpf: add dummy prog for bpf2bpf with tailcall Maciej Fijalkowski

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