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

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Date:   Tue, 26 Oct 2021 23:23:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "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 Tue, Oct 26, 2021 at 04:43:43PM -0400, Steven Rostedt wrote:
> On Thu, 7 Oct 2021 09:23:58 -0400
> Steven Rostedt <rostedt@...dmis.org> 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 ;-)
> 
> Peter,
> 
> Do you have any real issue if I just take this patch set through my tree?

No real objections; just weary, huge events like that are fairly sucky
for performance.