linux-kernel - Re: [RFC PATCH] x86-64: software IRQ masking and handling

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Date:	Mon, 12 Jul 2010 08:55:04 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH] x86-64: software IRQ masking and handling

On Mon, 12 Jul 2010, Tejun Heo wrote:

> I'm not very convinced either.  Nehalems are said to be able to do
> cli-sti sequences every 13 cycles or so, which sounds pretty good and
> managing it asynchronously might not buy anything.  But what they said
> was cli-sti bandwidth, probably meaning that if you do cli-sti's in
> succession or tight loop, each iteration will take 13 cycles.  So,
> there still could be cost related to instruction scheduling.

Note that Andi has repeatedly pointed out that it is not the cli-sti
instructions that cause the most latencies but the pushf/popf etc stack
operations.

> It only took me a couple of days to get it working and the changes are
> pretty localized, so I think it's worthwhile to see whether it
> actually helps anything on x86.  I'm thinking about doing raw IOs on
> SSDs which isn't too unrealistic and heavy on both IRQ masking and IRQ
> handling although actual hardware access cost might just drown any
> difference and workloads which are heavy on memory allocations and
> such might be better fit.  If you have any better ideas on testing,
> please let me know.

If it is a win for local_irq_save/restore then it will help any slab
allocator because the alloc / free hotpath must disable interupts to be
usable from hardware interrupps.

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