linux-kernel - Re: [PATCH] timer: Use static_branch_likely() for timers_nohz

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Date:   Wed, 23 Jun 2021 11:17:01 +0800
From:   Jisheng Zhang <Jisheng.Zhang@...aptics.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] timer: Use static_branch_likely() for
 timers_nohz_active

Hi Thomas,

On Tue, 22 Jun 2021 17:19:20 +0200
Thomas Gleixner <tglx@...utronix.de> wrote:


> 
> On Thu, May 13 2021 at 14:33, Jisheng Zhang wrote:
> > NOHZ is likely to be enabled, so use static_branch_likely() to  
> 
> Why is it likely to be enabled? Did you make a survey of the wider
> distro universe or what?

No, I didn't make any survey. I only checked debian and ubuntu kernel
image config file, both have CONFIG_NO_HZ_COMMON=y
So IMHO, if CONFIG_NO_HZ_COMMON=y, the static key timers_nohz_active is
true in most cases.

> 
> > reflect this fact. This could improve the finally generated code  
> 
> could improve? Either it does or it does not.

Per include/linux/jump_label.h, if the timers_nohz_active is true,
it can save two jmp instructions.


 * type\branch| likely (1)            | unlikely (0)
 * -----------+-----------------------+------------------
 *            |                       |
 *  true (1)  |    ...                |    ...
 *            |    NOP                |    JMP L
 *            |    <br-stmts>         | 1: ...
 *            | L: ...                |
 *            |                       |
 *            |                       | L: <br-stmts>
 *            |                       |    jmp 1b


Thanks