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

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Date:	Mon, 12 Jul 2010 07:11:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.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>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH] x86-64: software IRQ masking and handling

Le dimanche 11 juillet 2010 à 18:18 -0700, Linus Torvalds a écrit :
> On Sun, Jul 11, 2010 at 3:03 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > I have seen some hits with cli-sti. I was considering swapping all
> > preempt_disable() with local_irq_save() in ftrace, but hackbench showed
> > a 30% performance degradation when I did that.
> 
> Yeah, but in that case you almost certainly keep the per-cpu cacheline
> hot in the D$ L1 cache, and the stack tracer is presumably also not
> taking any extra I$ L1 misses. So you're not seeing any of the
> downsides. The upside of plain cli/sti is that they're small, and have
> no D$ footprint.
> 
> And it's possible that the interrupt flag - at least if/when
> positioned right - wouldn't have any additional D$ footprint under
> normal load either. IOW, if there is an existing per-cpu cacheline
> that is effectively always already dirty and in the cache,
> But that's something that really needs macro-benchmarks - exactly
> because microbenchmarks don't show those effects since they are always
> basically hot-cache.
> 

Some kernel dev incorrectly assume they own cpu caches...

This discussion reminds me I noticed a performance problem with
placement of cpu_online_bits and cpu_online_mask on separate sections
(and thus separate cache lines) and a network load.

static DECLARE_BITMAP(cpu_online_bits, CONFIG_NR_CPUS) __read_mostly;
const struct cpumask *const cpu_online_mask = to_cpumask(cpu_online_bits);

Two changes are possible :

1) Get rid of the cpu_online_mask (its a const pointer to a known
target). I cant see a reason for its need it actually...

2) Dont use a the last const qualifier but __read_mostly to move
cpu_online_mask on same section.

Rusty, could you comment on one or other way before I submit a patch ?

(Of course, possible/present/active have same problem)


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