linux-kernel - Re: [PATCH 1/8] tick/nohz: Evaluate the CPU expression after the static key

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Date:   Tue, 4 May 2021 14:31:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Yunfeng Ye <yeyunfeng@...wei.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 1/8] tick/nohz: Evaluate the CPU expression after the
 static key

On Thu, Apr 22, 2021 at 02:01:51PM +0200, Frederic Weisbecker wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> When tick_nohz_full_cpu() is called with smp_processor_id(), the latter
> is unconditionally evaluated whether the static key is on or off. It is
> not necessary in the off-case though, so make sure the cpu expression
> is executed at the last moment.
> 
> Illustrate with the following test function:
> 
> 	int tick_nohz_test(void)
> 	{
> 		return tick_nohz_full_cpu(smp_processor_id());
> 	}
> 
> The resulting code before was:
> 
> 	mov    %gs:0x7eea92d1(%rip),%eax   # smp_processor_id() fetch
> 	nopl   0x0(%rax,%rax,1)
> 	xor    %eax,%eax
> 	retq
> 	cmpb   $0x0,0x29d393a(%rip)        # <tick_nohz_full_running>
> 	je     tick_nohz_test+0x29         # jump to below eax clear
> 	mov    %eax,%eax
> 	bt     %rax,0x29d3936(%rip)        # <tick_nohz_full_mask>
> 	setb   %al
> 	movzbl %al,%eax
> 	retq
> 	xor    %eax,%eax
> 	retq
> 
> Now it becomes:
> 
> 	nopl   0x0(%rax,%rax,1)
> 	xor    %eax,%eax
> 	retq
> 	cmpb   $0x0,0x29d3871(%rip)        # <tick_nohz_full_running>
> 	je     tick_nohz_test+0x29         # jump to below eax clear
> 	mov    %gs:0x7eea91f0(%rip),%eax   # smp_processor_id() fetch, after static key
> 	mov    %eax,%eax
> 	bt     %rax,0x29d3866(%rip)        # <tick_nohz_full_mask>
> 	setb   %al
> 	movzbl %al,%eax
> 	retq
> 	xor    %eax,%eax
> 	retq
> 
> Not-Yet-Signed-off-by: Peter Zijlstra <peterz@...radead.org>

Signed-off-by: Peter Zijlstra <peterz@...radead.org>

Thanks for writing the Changelog.