linux-kernel - Re: [PATCH v2 1/3] tracing: Replace TRACE_FLAG_IRQS

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Message-ID: <20241022095241.RFY4Iiu_@linutronix.de>
Date: Tue, 22 Oct 2024 11:52:41 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
	mingo@...nel.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, ankur.a.arora@...cle.com, efault@....de,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v2 1/3] tracing: Replace TRACE_FLAG_IRQS_NOSUPPORT with
 its config option.

On 2024-10-22 03:14:18 [-0400], Steven Rostedt wrote:
> On Mon, 21 Oct 2024 17:08:40 +0200
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > From: Thomas Gleixner <tglx@...utronix.de>
> > 
> > The TRACE_FLAG_IRQS_NOSUPPORT flag is used by tracing_gen_ctx.*() to
> > signal that CONFIG_TRACE_IRQFLAGS_SUPPORT is not enabled and tracing IRQ
> > flags is not supported.
> > 
> > This could be replaced by using the 0 as flags and then deducting that
> > there is no IRQFLAGS_SUPPORT based on the config option. The downside is
> > that without CONFIG_TRACE_IRQFLAGS_SUPPORT we can not distinguish
> > between no-IRQ passed flags and callers which passed 0. On the upside we
> > have room for one additional flags which could be used for LAZY_PREEMPTION.
> > 
> > Remove TRACE_FLAG_IRQS_NOSUPPORT and set it flags are 0 and
> > CONFIG_TRACE_IRQFLAGS_SUPPORT is not set.
> 
> We could also add that we have:
> 
> #
> # Minimum requirements an architecture has to meet for us to
> # be able to offer generic tracing facilities:
> #
> config TRACING_SUPPORT
>         bool
>         depends on TRACE_IRQFLAGS_SUPPORT
>         depends on STACKTRACE_SUPPORT
>         default y
> 
> So this can't even be built without TRACE_IRQFLAGS_SUPPORT!

Good point. So we could TRACE_FLAG_IRQS_NOSUPPORT since it can't be
used. This is since commit 0ea5ee035133a ("tracing: Remove PPC32 wart
from config TRACING_SUPPORT").

> > +++ b/include/linux/trace_events.h
> > @@ -184,8 +184,7 @@ unsigned int tracing_gen_ctx_irq_test(un
> >  
> >  enum trace_flag_type {
> >  	TRACE_FLAG_IRQS_OFF		= 0x01,
> > -	TRACE_FLAG_IRQS_NOSUPPORT	= 0x02,
> > -	TRACE_FLAG_NEED_RESCHED		= 0x04,
> > +	TRACE_FLAG_NEED_RESCHED		= 0x02,
> 
> These flags are user visible (I probably should move them into uapi).
> They are parsed by libtraceevent.
> 
> Please just remove NOSUPPORT and do not touch NEED_RESCHED.

Then I put the lazy bit where we have not NOSUPPORT.

> >  	TRACE_FLAG_HARDIRQ		= 0x08,
> >  	TRACE_FLAG_SOFTIRQ		= 0x10,
> >  	TRACE_FLAG_PREEMPT_RESCHED	= 0x20,
> > --- a/kernel/trace/trace_output.c
> > +++ b/kernel/trace/trace_output.c
> > @@ -460,7 +460,7 @@ int trace_print_lat_fmt(struct trace_seq
> >  		(entry->flags & TRACE_FLAG_IRQS_OFF && bh_off) ? 'D' :
> >  		(entry->flags & TRACE_FLAG_IRQS_OFF) ? 'd' :
> >  		bh_off ? 'b' :
> > -		(entry->flags & TRACE_FLAG_IRQS_NOSUPPORT) ? 'X' :
> > +		!IS_ENABLED(CONFIG_TRACE_IRQFLAGS_SUPPORT) ? 'X' :
> 
> Probably can even remove this check.

Yes.

> -- Steve

Sebastian