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

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Message-ID: <CAKohponEAivnev-fcWdjD0OcwQaXHN58tESCfqbZ_-W+_N+DvA@mail.gmail.com>
Date:	Mon, 30 Mar 2015 21:47:01 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <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>, Mel Gorman <mgorman@...e.de>,
	dave@...olabs.net, Konstantin Khlebnikov <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 30 March 2015 at 19:29, Peter Zijlstra <peterz@...radead.org> wrote:
> Yeah, so that _should_ not trigger (obviously), and while I agree with
> the sentiment of sanity checks, I'm not sure its worth keeping that
> variable around just for that.

I read it as I can remove it then ? :)

> Anyway, while I'm looking at struct tvec_base I notice the cpu member
> should be second after the lock, that'll save 8 bytes on the structure
> on 64bit machines.

Hmm, I tried it on my macbook-pro.

$ uname -a
Linux vireshk 3.13.0-46-generic #79-Ubuntu SMP Tue Mar 10 20:06:50 UTC
2015 x86_64 x86_64 x86_64 GNU/Linux

$ gcc --version
gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2

config: arch/x86/configs/x86_64_defconfig

And all I get it is 8256 bytes, with or without the change.

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2d3f5c504939..afc5d74678df 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -77,12 +77,12 @@ struct tvec_root {

 struct tvec_base {
        spinlock_t lock;
+       int cpu;
        struct timer_list *running_timer;
        unsigned long timer_jiffies;
        unsigned long next_timer;
        unsigned long active_timers;
        unsigned long all_timers;
-       int cpu;
        struct tvec_root tv1;
        struct tvec tv2;
        struct tvec tv3;
--
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