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Date:   Mon, 8 Jul 2019 18:02:01 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Song Liu <songliubraving@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Kairui Song <kasong@redhat.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/urgent] bpf: Fix ORC unwinding in non-JIT BPF code
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On Mon, Jul 08, 2019 at 05:53:59PM -0500, Josh Poimboeuf wrote:
> On Mon, Jul 08, 2019 at 03:49:33PM -0700, Alexei Starovoitov wrote:
> > > > Sorry for delay. I'm mostly offgrid until next week.
> > > > As far as -fno-gcse.. I don't mind as long as it doesn't hurt performance.
> > > > Which I suspect it will :(
> > > > All these indirect gotos are there for performance.
> > > > Single indirect goto and a bunch of jmp select_insn
> > > > are way slower, since there is only one instruction
> > > > for cpu branch predictor to work with.
> > > > When every insn is followed by "jmp *jumptable"
> > > > there is more room for cpu to speculate.
> > > > It's been long time, but when I wrote it the difference
> > > > between all indirect goto vs single indirect goto was almost 2x.
> > >
> > > Just to clarify, -fno-gcse doesn't get rid of any of the indirect jumps.
> > > It still has 166 indirect jumps.  It just gets rid of the second
> > > optimization, where the jumptable address is placed in a register.
> > 
> > what about other functions in core.c ?
> > May be it's easier to teach objtool to recognize that pattern?
> 
> The GCC man page actually recommends using -fno-gcse for computed goto
> code, for better performance.  So if that's actually true, then it would
> be win-win because objtool wouldn't need a change for it.
> 
> Otherwise I can teach objtool to recognize the new pattern.
> 
> > > If you have a benchmark which is relatively easy to use, I could try to
> > > run some tests.
> > 
> > modprobe test_bpf
> > selftests/bpf/test_progs
> > both print runtime.
> > Some of test_progs have high run-to-run variations though.
> 
> Thanks, I'll give it a shot.

I modprobed test_bpf with JIT disabled.

Before: 2.493018s
After:  2.523572s

So it looks like it's either no change, or slightly slower.

I'll just teach objtool to recognize the optimization.

-- 
Josh