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

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Date:   Tue, 14 Jul 2020 22:50:35 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [RFC PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and
 tailcall handling in JIT

On Mon, Jul 13, 2020 at 08:36:30PM -0700, Alexei Starovoitov wrote:
> On Tue, Jul 14, 2020 at 03:00:45AM +0200, Maciej Fijalkowski wrote:
> > On Fri, Jul 10, 2020 at 08:25:20PM -0700, Alexei Starovoitov wrote:
> > > On Fri, Jul 10, 2020 at 8:20 PM Alexei Starovoitov
> > > <alexei.starovoitov@...il.com> wrote:
> > > >
> > > > Of course you are right.
> > > > pop+nop+push is incorrect.
> > > >
> > > > How about the following instead:
> > > > - during JIT:
> > > > emit_jump(to_skip_below)  <- poke->tailcall_bypass
> > 
> > That's the jump to the instruction right after the poke->tailcall_target.
> 
> right. Mainly looking for better names than ip and ip_aux.
> 
> > > > pop_callee_regs
> > > > emit_jump(to_tailcall_target) <- poke->tailcall_target
> > 
> > During JIT there's no tailcall_target so this will be nop5, right?
> 
> I thought it will be always jmp, but with new info I agree that
> it will start with nop.
> 
> > 
> > > >
> > > > - Transition from one target to another:
> > > > text_poke(poke->tailcall_target, MOD_JMP, old_jmp, new_jmp)
> > > > if (new_jmp != NULL)
> > > >   text_poke(poke->tailcall_bypass, MOD jmp into nop);
> > > > else
> > > >   text_poke(poke->tailcall_bypass, MOD nop into jmp);
> > > 
> > > One more correction. I meant:
> > > 
> > > if (new_jmp != NULL) {
> > >   text_poke(poke->tailcall_target, MOD_JMP, old_jmp, new_jmp)
> > 
> > Problem with having the old_jmp here is that you could have the
> > tailcall_target removed followed by the new program being inserted. So for
> > that case old_jmp is NULL but we decided to not poke the
> > poke->tailcall_target when removing the program, only the tailcall_bypass
> > is poked back to jmp from nop. IOW old_jmp is not equal to what
> > poke->tailcall_target currently stores. This means that
> > bpf_arch_text_poke() would not be successful for this update and that is
> > the reason of faking it in this patch.
> 
> got it.
> I think it can be solved two ways:
> 1. add synchronize_rcu() after poking of tailcall_bypass into jmp
> and then update tailcall_target into nop.
> so the race you've described in cover letter won't happen.
> In the future with sleepable progs we'd need to call sync_rcu_tasks_trace too.
> Which will make poke_run even slower.
> 
> 2. add a flag to bpf_arch_text_poke() to ignore 5 bytes in there
> and update tailcall_target to new jmp.
> The speed of poke_run will be faster,
> but considering the speed of text_poke_bp() it's starting to feel like
> premature optimization.
> 
> I think approach 1 is cleaner.
> Then the pseudo code will be:
> if (new_jmp != NULL) {
>    text_poke(poke->tailcall_target, MOD_JMP, old ? old_jmp : NULL, new_jmp);
>    if (!old)
>      text_poke(poke->tailcall_bypass, MOD_JMP, bypass_addr, NULL /* into nop */);
> } else {
>    text_poke(poke->tailcall_bypass, MOD_JMP, NULL /* from nop */, bypass_addr);
>    sync_rcu(); /* let progs finish */
>    text_poke(poke->tailcall_target, MOD_JMP, old_jmp, NULL /* into nop */)
> }

Seems like this does the job :) clever stuff with sync_rcu.
I tried this approach and one last thing that needs to be covered
separately is the case of nop->nop update. We should simply avoid poking
in this case. With this in place everything is functional.

I will update the patch and descriptions and send the non-RFC revision, if
you don't mind of course.

> 
> > 
> > >   text_poke(poke->tailcall_bypass, MOD jmp into nop);
> > > } else {
> > >   text_poke(poke->tailcall_bypass, MOD nop into jmp);
> > > }
> > 
> > I think that's what we currently (mostly) have. map_poke_run() is skipping
> > the poke of poke->tailcall_target if new bpf_prog is NULL, just like
> > you're proposing above. Of course I can rename the members in poke
> > descriptor to names you're suggesting. I also assume that by text_poke you
> > meant the bpf_arch_text_poke?
> 
> yep.
> 
> > 
> > I've been able to hide the nop5 detection within the bpf_arch_text_poke so
> > map_poke_run() is arch-independent in that approach. My feeling is that
> > we don't need the old bpf_prog at all.
> > 
> > Some bits might change here due to the jump target alignment that I'm
> > trying to introduce.
> 
> > Can you explain under what circumstances bpf_jit_binary_alloc() would not
> > use get_random_int() ? Out of curiosity as from a quick look I can't tell
> > when.
> 
> I meant when you're doing benchmarking get rid of that randomization
> from bpf_jit_binary_alloc in your test kernel.
> 
> > I'm hitting the following check in do_jit():
> 
> I think aligning bypass_addr is a bit too much. Let it all be linear for now.
> Since iTLB is sporadic it could be due to randomization and nothing to do
> with additional jmp and unwind that this set is introducing.