linux-kernel - Re: [PATCH -v2] use per cpu data for single cpu ipi calls

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Date:	Thu, 29 Jan 2009 10:08:23 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>, npiggin@...e.de,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>,
	jens.axboe@...cle.com
Subject: Re: [PATCH -v2] use per cpu data for single cpu ipi calls

On Thu, 29 Jan 2009, Steven Rostedt wrote:
> 
> Actually, we are locking against the destination CPU.

Oh.

THAT'S JUST INCOMPETENT.

What the *fuck* is the point of having per-CPU data, and then using it for 
the wrong CPU?

Stop doing that idiocy. Put the per-cpu data on the senders side, and stop 
the idiocy. You are going to get cross-CPU cacheline bouncing anyway, 
there's no way to avoid it, but as long as you do it on the wrong CPU's 
local data, you're missing the whole POINT of having per-cpu data in the 
first place.

But yeah, that explains the locking. One stupid design mistake leads to 
another.

And are you really sure it cannot be called from within interrupts? I'm 
finding a lot of callers of smp_call_function_single(), and while I 
couldn't find any that look like interrupts, I also couldn't find any 
indication that it never happens.

		Linus
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