linux-kernel - Re: [PATCH 1/2] tracing: show size of requested buffer

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Message-ID: <robbat2-20211007T172058-955036195Z@orbis-terrarum.net>
Date:   Thu, 7 Oct 2021 17:26:04 +0000
From:   "Robin H. Johnson" <robbat2@...too.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Robin H. Johnson" <robbat2@...too.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        rjohnson@...italocean.com,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 1/2] tracing: show size of requested buffer

On Thu, Oct 07, 2021 at 09:23:58AM -0400, Steven Rostedt wrote:
> On Thu, 7 Oct 2021 09:11:51 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > > +++ b/kernel/trace/trace_event_perf.c
> > > > @@ -400,7 +400,8 @@ void *perf_trace_buf_alloc(int size, struct pt_regs **regs, int *rctxp)
> > > >  	BUILD_BUG_ON(PERF_MAX_TRACE_SIZE % sizeof(unsigned long));
> > > >  
> > > >  	if (WARN_ONCE(size > PERF_MAX_TRACE_SIZE,
> > > > -		      "perf buffer not large enough"))
> > > > +		      "perf buffer not large enough, wanted %d, have %d",
> > > > +		      size, PERF_MAX_TRACE_SIZE))  
> > 
> > Priting a constant seems daft.. why is any of this important in any way?
> 
> I see your point, but it can be useful if you changed it, and want to know
> if you are running the kernel with the change or not.
> 
> I've done daft things were I changed a const and was running a kernel
> without the change and couldn't understand why it wasn't working ;-)
Yes, my initial internal versions of this series only bumped the
constant, and then I was running a few different kernels, and being able
to compare which value was in use became a problem.

I was trying to think further what would make sense for the constant.
- What are the negative impacts of a too-large value?
- Is there demand for more reconfigurability? 
- Should PERF_MAX_TRACE_SIZE be a knob in Kconfig?

-- 
Robin Hugh Johnson
Gentoo Linux: Dev, Infra Lead, Foundation Treasurer
E-Mail   : robbat2@...too.org
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