linux-kernel - Re: [RFC] x86_64: A real proposal for iret-less return to kernel

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Message-ID: <CA+55aFyOS_MYH6aD1Xkbc9kvgmxmM2mgE4ji-USEWQLhA1oHiA@mail.gmail.com>
Date:	Thu, 22 May 2014 08:34:50 +0900
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Thu, May 22, 2014 at 8:23 AM, Andi Kleen <andi@...stfloor.org> wrote:
>> Hardware-interrupts during kernel are actually fairly common under
>> network-intensive loads, even outside of idle (but idle is admittedly
>> likely *the* most common one). Many network loads are fairly
>> kernel-intensive.
>
> For network workloads we can arbitarily coalesce interrupts or just use NAPI
> to lower the costs.  No need to optimize network interrupts too much.

BS. Lots of network loads are latency-criticial, to the point that
people sometimes actually turn off coalescing. But even with
coalescing, it doesn't do crap for ping-pong kinds of loads that are
not "interrupt storm from tons and tons of separate packets", but
"lots of individual packets that are data-dependent", so you don't
have new ones coming in while processing old ones.

Ask Andy L. He had numbers. Interrupt overhead was quite big for him.

And you ignored the real issue: special-casing idle is *stupid*. It's
more complicated, and gives fewer cases where it helps. It's simply
fundamentally stupid and wrong.

         Linus
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