linux-kernel - Re: [PATCH v7 08/14] x86/ftrace: Use text_poke

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Date:   Thu, 6 Dec 2018 17:28:57 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Damian Tometzki <linux_dti@...oud.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v7 08/14] x86/ftrace: Use text_poke_*() infrastructure


* Nadav Amit <namit@...are.com> wrote:

> > On Dec 4, 2018, at 5:34 PM, Nadav Amit <namit@...are.com> wrote:
> > 
> > A following patch is going to make module allocated memory
> > non-executable. This requires to modify ftrace and make the memory
> > executable again after it is configured.
> > 
> > In addition, this patch makes ftrace use the general text poking
> > infrastructure instead ftrace's homegrown text patching. This provides
> > the advantages of having slightly "safer" code patching and avoiding
> > races with module removal or other mechanisms that patch the kernel
> > code.
> > 
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Nadav Amit <namit@...are.com>
> > ---
> > arch/x86/kernel/ftrace.c | 74 +++++++++++++---------------------------
> > 1 file changed, 23 insertions(+), 51 deletions(-)
> 
> Steven Rostedt pointed that using text_poke() instead of
> probe_kernel_write() would introduce considerable overheads. Running:
> 
>   # time { echo function > current_tracer; } 
> 
> takes 0.24s without this patch and 0.7s with. I don’t know whether to
> consider it “so bad”. Obviously we can introduce a batching mechanism and/or
> do some micro-optimization (the latter will not buy us much though).

This should definitely not regress, so can we try the batching approach?

Thanks,

	Ingo