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

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Message-Id: <201007121732.26092.rusty@rustcorp.com.au>
Date:	Mon, 12 Jul 2010 17:32:24 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>, 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

On Mon, 12 Jul 2010 02:41:33 pm Eric Dumazet wrote:
> 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...

There was a reason, but I'm trying to remember it.

ISTR, it was to catch direct frobbing of the masks.  That was important:
we were converting code everywhere to hand around cpumasks by ptr
rather than by copy.  But that semantic change meant that a function which
previously harmlessly frobbed a copy would now frob (say) cpu_online_mask.

However, ((const struct cpumask *)cpu_online_bits)) would work for that
too.  (Well, renaming cpu_online_bits to __cpu_online_bits would be better
since it's not non-static).

Ideally, those masks too would be dynamically allocated.  But the boot
changes required for that are best left until someone really needs > 64k
CPUs.

> 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)

Yep.  Might want to do a patch to get rid of the remaining 100 references
to cpu_online_map (etc) as well if you're feeling enthusiastic :)

Thanks!
Rusty.
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