linux-kernel - Re: [PATCH 08/15] tracing: Introduce names for ring buffers

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Message-ID: <eb567d99-e03b-4718-9cc9-e27c5e408ea2@amazon.com>
Date:   Wed, 13 Dec 2023 01:35:16 +0100
From:   Alexander Graf <graf@...zon.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kexec@...ts.infradead.org>, <linux-doc@...r.kernel.org>,
        <x86@...nel.org>, Eric Biederman <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rob Herring <robh+dt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ashish Kalra <ashish.kalra@....com>,
        James Gowans <jgowans@...zon.com>,
        "Stanislav Kinsburskii" <skinsburskii@...ux.microsoft.com>,
        <arnd@...db.de>, <pbonzini@...hat.com>,
        <madvenka@...ux.microsoft.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        Usama Arif <usama.arif@...edance.com>,
        "David Woodhouse" <dwmw@...zon.co.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 08/15] tracing: Introduce names for ring buffers

Hi Steve,

On 13.12.23 01:15, Steven Rostedt wrote:
>
> On Wed, 13 Dec 2023 00:04:45 +0000
> Alexander Graf <graf@...zon.com> wrote:
>
>> With KHO (Kexec HandOver), we want to preserve trace buffers across
>> kexec. To carry over their state between kernels, the kernel needs a
>> common handle for them that exists on both sides. As handle we introduce
>> names for ring buffers. In a follow-up patch, the kernel can then use
>> these names to recover buffer contents for specific ring buffers.
>>
> Is there a way to use the trace_array name instead?
>
> The trace_array is the structure that represents each tracing instance. And
> it already has a name field. And if you can get the associated ring buffer
> from that too.
>
> struct trace_array *tr;
>
>          tr->array_buffer.buffer
>
>          tr->name
>
> When you do: mkdir /sys/kernel/tracing/instance/foo
>
> You create a new trace_array instance where tr->name = "foo" and allocates
> the buffer for it as well.

The name in the ring buffer is pretty much just a copy of the trace 
array name. I use it to reconstruct which buffer we're actually 
referring to inside __ring_buffer_alloc().

I'm all ears for alternative suggestions. I suppose we could pass tr as 
argument to ring_buffer_alloc() instead of the name?


Alex




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