linux-kernel - Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

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Date:   Mon, 7 Jan 2019 17:32:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Edward Cree <ecree@...arflare.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>, X86 ML <x86@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>, adrian.hunter@...el.com,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

On Thu, Jan 03, 2019 at 02:18:15PM -0800, Andi Kleen wrote:
> Nadav Amit <namit@...are.com> writes:
> >
> > - Do we use periodic learning or not? Josh suggested to reconfigure the
> >   branches whenever a new target is found. However, I do not know at
> >   this time how to do learning efficiently, without making learning much
> >   more expensive.
> 
> FWIW frequent patching will likely completely break perf Processor Trace
> decoding, which needs a somewhat stable kernel text image to decode the
> traces generated by the CPU. Right now it relies on kcore dumped after
> the trace usually being stable because jumplabel changes happen only
> infrequently. But if you start patching frequently this assumption will
> break.
> 
> You would either need a way to turn this off, or provide
> updates for every change to the trace, so that the decoder can
> keep track.

I'm thining it would be entirely possible to create and feed text_poke
events into the regular (!aux) buffer which can be timestamp correlated
to the PT data.