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Date: Fri, 27 Mar 2015 13:02:40 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@linux.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
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Subject: Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work
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On Fri, Mar 27, 2015 at 06:11:44AM -0500, Christoph Lameter wrote:
> On Fri, 27 Mar 2015, Peter Zijlstra wrote:
> 
> > > We could align the base on 8 bytes to gain an extra bit in the pointer
> > > and use that bit to indicate the running state. Then these sites can
> > > spin on that bit while we can change the actual base pointer.
> >
> > Even though tvec_base has ____cacheline_aligned stuck on, most are
> > allocated using kzalloc_node() which does not actually respect that but
> > already guarantees a minimum u64 alignment, so I think we can use that
> > third bit without too much magic.
> 
> Create a new slab cache for this purpose that does the proper aligning?

That is certainly a possibility, but we'll only ever allocate nr_cpus-1
entries from it, a whole new slab cache might be overkill.

What's not clear to me is why that thing is allocated at all, AFAICT
something like:

static DEFINE_PER_CPU(struct tvec_base, tvec_bases);

Should do the right thing and be much simpler.