From mboxrd@z Thu Jan  1 00:00:00 1970
From: marc.zyngier@arm.com (Marc Zyngier)
Date: Tue, 9 Aug 2016 11:08:49 +0100
Subject: [PATCH 1/1] irqchip: irq-gic: forward SGI to itself for cortex-a7
 single core
In-Reply-To: <20160809093942.GA10803@shlinux2>
References: <20160808130754.GB12649@leverpostej>
 <20160808132847.GB17680@shlinux2> <20160808134842.GE12649@leverpostej>
 <20160808145916.0924e868@arm.com> <20160809034613.GB31105@shlinux2>
 <20160809063401.3117dc94@arm.com> <20160809055701.GC31105@shlinux2>
 <20160809075930.529f98ca@arm.com> <20160809071821.GD31105@shlinux2>
 <57A99A4A.6010301@arm.com> <20160809093942.GA10803@shlinux2>
Message-ID: <57A9ABB1.6060109@arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 09/08/16 10:39, Peter Chen wrote:
> 
> Your patch can work for my case. Below is objdump for gic_raise_softirq,
> the code with your patch seems have more instructions.

Well, I've added code to the function, so that's hardly surprising. The
important thing is that the hot path is kept relatively fast.

> 
> - With your patch:
> 00000c44 <gic_raise_softirq>:
>      c44:	e1a0c00d 	mov	ip, sp
>      c48:	e92ddbf0 	push	{r4, r5, r6, r7, r8, r9, fp, ip, lr, pc}
>      c4c:	e24cb004 	sub	fp, ip, #4
>      c50:	e59f908c 	ldr	r9, [pc, #140]	; ce4 <gic_raise_softirq+0xa0>
>      c54:	e1a06000 	mov	r6, r0
>      c58:	e1a07001 	mov	r7, r1
>      c5c:	e5993000 	ldr	r3, [r9]
>      c60:	e3530001 	cmp	r3, #1
>      c64:	0a000019 	beq	cd0 <gic_raise_softirq+0x8c>

So the overhead is two loads, one comparison and a branch. Can you
actually measure the difference? If you can, I'd like to see some cycle
numbers, not just instruction counts. If you can consistently measure
something that is not just noise, we can then turn this into a static key.

I bet you won't see anything.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...