linux-kernel - Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

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Message-ID: <20150327120240.GC23123@twins.programming.kicks-ass.net>
Date:	Fri, 27 Mar 2015 13:02:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>, hannes@...xchg.org,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	vinmenon@...eaurora.org, shashim@...eaurora.org,
	Michal Hocko <mhocko@...e.cz>, mgorman@...e.de,
	dave@...olabs.net, koct9i@...il.com,
	Linux Memory Management List <linux-mm@...ck.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

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.


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