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From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
	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: Fri, 17 Jul 2020 12:57:44 +0200	[thread overview]
Message-ID: <20200717105744.GB11239@ranger.igk.intel.com> (raw)
In-Reply-To: <20200717021624.do6mrxxr37vc7ajd@ast-mbp.dhcp.thefacebook.com>

On Thu, Jul 16, 2020 at 07:16:24PM -0700, Alexei Starovoitov wrote:
> 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.

Yeah since we introduce another poke target we could come up with
preparing the vector of pokes as you're saying?

> 
> 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 11:02 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
2020-07-17 10:57       ` Maciej Fijalkowski [this message]
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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