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Subject: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20190108172721.GN6118@tassilo.jf.intel.com>
Date:   Tue, 8 Jan 2019 10:28:02 -0800
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Edward Cree <ecree@solarflare.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        songliubraving@fb.com
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References: <20181231072112.21051-1-namit@vmware.com>
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 <20190107163227.GH14122@hirez.programming.kicks-ass.net>
 <c4e836e8-1949-27f5-f6a0-ed860f590ec5@intel.com>
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 <20190108172721.GN6118@tassilo.jf.intel.com>
To:     Andi Kleen <ak@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 8, 2019, at 9:27 AM, Andi Kleen <ak@linux.intel.com> wrote:
>=20
> On Tue, Jan 08, 2019 at 11:10:58AM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 08, 2019 at 12:01:11PM +0200, Adrian Hunter wrote:
>>> The problem is that the jitted code gets freed from memory, which is =
why I
>>> suggested the ability to pin it for a while.
>>=20
>> Then what do you tell the guy that keeps PT running for a day and =
runs
>> out of memory because he likes to JIT a lot?
>=20
> It only would need to be saved until the next kcore dump, so they =
would
> need to do regular kcore dumps, after each of which the JIT code could =
be freed.
>=20
> In a sense it would be like RCU for code.
>=20
> You would somehow need to tell the kernel when that happens though
> so it can schedule the frees.
>=20
> It doesn't work when the code is modified in place, like the
> patch in the $SUBJECT.

Excuse my ignorance - can you be more concrete what will break where?

I am looking at perf-with-kcore, and intuitively the main thing that is
required is to take text_mutex while kcore is copied, to get a =
point-in-time
snapshot.

Is it really that important for debugging to get the instructions at the
time of execution? Wouldn=E2=80=99t it be easier to annotate the =
instructions that
might change? After all, it is not as if any instruction can change to =
any
other instruction.