linux-kernel - Re: [PATCH 1/1] cpumask: smp_call_function

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Message-Id: <200811201757.07726.nickpiggin@yahoo.com.au>
Date:	Thu, 20 Nov 2008 17:57:07 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	linux-kernel@...r.kernel.org, Mike Travis <travis@....com>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com, npiggin@...e.de,
	axboe@...nel.dk
Subject: Re: [PATCH 1/1] cpumask: smp_call_function_many()

On Thursday 20 November 2008 15:44, Rusty Russell wrote:
> On Thursday 20 November 2008 13:51:49 Nick Piggin wrote:
> > I don't like changing of this whole smp_call_function_many scheme
> > with no justification or even hint of it in the changelog.
>
> Hi Nick,
>
> Hmm, it said "if allocation fails we fallback to smp_call_function_single
> rather than using the baroque quiescing code."
>
> More words would have been far less polite :)

Hmm, OK I missed that.


> > Of course it is obvious that smp_call_function_mask can be implemented
> > with multiple call singles. But some architectures that can do
> > broadcast IPIs (or otherwise a little extra work eg. in programming
> > the interrupt controller) will lose here.
> >
> > Also the locking and cacheline behaviour is probably actually worse.
>
> Dude, we've failed kmalloc.  To paraphrase Monty Python, the parrot is
> fucked. By this stage the disks are churning, the keyboard isn't responding
> and the OOM killer is killing the mission-critical database and other vital
> apps. Everything else is failing on random syscalls like unlink().  Admins
> wondering how long it'll take to fsck if they just hit the big red switch
> now.

Oh no it happens. It's a GFP_ATOMIC allocation isn't it? But yeah it's not
performance critical.


> OK, maybe it's not that bad, but worrying about cacheline behaviour?  I'd
> worry about how recently that failure path has been tested.
>
> I can prepare a separate patch which just changes this over, rather than
> doing it as part of the smp_call_function_many() conversion, but I couldn't
> stomach touching that quiescing code :(

What's wrong with it? It's well commented and I would have thought pretty
simple. A bit ugly, but straightforward. I still don't really see why it
needs changing.
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