linux-kernel - Re: [PATCH 01/15] tick/nohz: Remove duplicate between tick_nohz_switch_to_nohz() and tick_setup_sched

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Date: Thu, 25 Jan 2024 15:35:46 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, Peng Liu <liupeng17@...ovo.com>,
	Ingo Molnar <mingo@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 01/15] tick/nohz: Remove duplicate between
 tick_nohz_switch_to_nohz() and tick_setup_sched_timer()

On Thu, Jan 25, 2024 at 02:34:06PM +0100, Thomas Gleixner wrote:
> On Thu, Jan 25 2024 at 12:57, Frederic Weisbecker wrote:
> > On Thu, Jan 25, 2024 at 10:12:20AM +0100, Thomas Gleixner wrote:
> >> Adding
> >> 
> >> #define tick_nohz_highres_handler NULL
> >> 
> >> for CONFIG_HIGH_RES_TIMERS=n somewhere allows you to write that as:
> >> 
> >> 	if (IS_ENABLED(CONFIG_HIGH_RES_TIMERS) && mode == NOHZ_MODE_HIGHRES)
> >> 		ts->sched_timer.function = tick_nohz_highres_handler;
> >
> > So the next patch does:
> >
> > if (mode == NOHZ_MODE_HIGHRES)
> >     ts->sched_timer.function = tick_nohz_highres_handler;
> >
> > Because that condition only happens in CONFIG_HIGH_RES_TIMERS anyway
> > and the function field exists in any case.
> 
> Right, but the function does not exist for HRES=n, so you need the
> #define ... NULL. The IS_ENABLED() allows the compiler to eliminate the
> code for HRES=n completely.

Ah but the next patch moves tick_nohz_highres_handler out of CONFIG_HRES
(to soon become tick_nohz_handler()). But anyway I can add the IS_ENABLED()
to remove dead code.

Thanks.

> 
> Thanks,
> 
>         tglx