linux-kernel - Re: [PATCH 1/6] Free up pf flag PF

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Date:	Thu, 21 Oct 2010 07:23:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
	Shaun Ruffell <sruffell@...ium.com>,
	Yong Zhang <yong.zhang0@...il.com>
Subject: Re: [PATCH 1/6] Free up pf flag PF_KSOFTIRQD

Le mercredi 20 octobre 2010 à 15:48 -0700, Venkatesh Pallipadi a écrit :
> +int is_ksoftirqd_context(void)
> +{
> +       return (current == __get_cpu_var(ksoftirqd));
> +} 

"return (X == Y);" should be "return X == Y;"

I believe this function should be inlined, and use this_cpu_read()
You probably can pass 'current' as a pointer.

static inline bool is_ksoftirq(struct task_struct *p)
{
	return p == this_cpu_read(ksoftirqd);
}

Your version is a bit expensive :

<is_ksoftirqd_context>:
	55                   	push   %rbp
	48 89 e5             	mov    %rsp,%rbp
	48 83 ec 10          	sub    $0x10,%rsp
	48 89 1c 24          	mov    %rbx,(%rsp)
	4c 89 64 24 08       	mov    %r12,0x8(%rsp)
	48 c7 c3 40 03 01 00 	mov    $0x10340,%rbx
	e8 93 e5 24 00       	callq  ffffffff812aeab0 <debug_smp_processor_id>
	89 c0                	mov    %eax,%eax
	48 8b 04 c5 a0 b8 b5 	mov    -0x7e4a4760(,%rax,8),%rax
	81 
	65 4c 8b 24 25 00 cc 	mov    %gs:0xcc00,%r12
	00 00 
	4c 39 24 18          	cmp    %r12,(%rax,%rbx,1)
	48 8b 1c 24          	mov    (%rsp),%rbx
	4c 8b 64 24 08       	mov    0x8(%rsp),%r12
	c9                   	leaveq 
	0f 94 c0             	sete   %al
	0f b6 c0             	movzbl %al,%eax
	c3                   	retq   

While alternate version :

				cmp    %gs:0x10340,%rdi

(So it should be as fast as previous flag based test)

Thanks


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