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

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Date:	Sat, 28 Mar 2015 09:48:28 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, hannes@...xchg.org,
	Christoph Lameter <cl@...ux.com>,
	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 3:00 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Mar 27, 2015 at 10:16:13AM +0100, Peter Zijlstra wrote:

>> So the issue seems to be that we need base->running_timer in order to
>> tell if a callback is running, right?
>>
>> 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.

Right. So what I tried earlier was very much similar to you are suggesting.
The only difference was that I haven't made much attempts towards
saving memory.

But Thomas didn't like it for sure and I believe that Rant will hold true for
what you are suggesting as well, isn't it ?

http://lists.linaro.org/pipermail/linaro-kernel/2013-November/008866.html
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