linux-kernel - Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

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Date:	Thu, 5 May 2011 21:13:53 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	werner <w.landgraf@...ru>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

* Christoph Lameter <cl@...ux.com> wrote:

> On Thu, 5 May 2011, Tejun Heo wrote:
> 
> > Anyways, that's what I've been thinking.  I'll get to it in the next
> > devel cycle or the one after that.  What do you guys think about soft
> > irq masking idea?
> 
> Great idea. Would make the whole irq on/off business much cheaper.

The tradeoffs are *not at all* clear and the result (on x86) is not
'much cheaper', at all ...

In particular the irq-enable path gets complicated by the need to check the 
flag and call a hardirq handling function in that case - a far cry from the 
single-byte POPF instruction. It will be somewhat cheaper cycle-wise - but the 
code gets bloated, so the instruction cache impact has to be measured 
carefully. (See my other mail for details.)

There's also the fact that PUSHF+CLI+POPF sequence has been getting cheaper all 
the time with newer hardware generations. CLI+STI is even cheaper, 10 cycles 
both on Intel and AMD CPUs. So it's an optimization that might get narrower and 
narrower with every CPU generation.

Thanks,

	Ingo
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