linux-kernel - Re: [PATCH V2 2/2] hrtimer: Iterate only over active clock-bases

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Date:	Thu, 9 Apr 2015 08:12:55 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH V2 2/2] hrtimer: Iterate only over active clock-bases

On 9 April 2015 at 01:41, Thomas Gleixner <tglx@...utronix.de> wrote:
> I'm really not too excited about this incomprehensible macro mess and
> especially not about the code it generates.
>
>                 x86_64  i386    ARM     power
>
> Mainline        7668    6942    8077    10253
>
> + Patch         8068    7294    8313    10861
>
>                 +400    +352    +236     +608
>
> That's insane.

After Peter's mail yesterday, I did check it on x86_64 and it surely
looked a lot bigger.

> What's wrong with just adding
>
>         if (!(cpu_base->active_bases & (1 << i)))
>                 continue;
>
> to the iterating sites?
>
> Index: linux/kernel/time/hrtimer.c
> ===================================================================
> --- linux.orig/kernel/time/hrtimer.c
> +++ linux/kernel/time/hrtimer.c
> @@ -451,6 +451,9 @@ static ktime_t __hrtimer_get_next_event(
>                 struct timerqueue_node *next;
>                 struct hrtimer *timer;
>
> +               if (!(cpu_base->active_bases & (1 << i)))
> +                       continue;
> +
>                 next = timerqueue_getnext(&base->active);
>                 if (!next)
>                         continue;

Isn't the check we already have here lightweight enough for this ?
timerqueue_getnext() returns head->next..

What benefit are we getting with this extra check ?

Maybe we can drop 'active_bases' from struct hrtimer_cpu_base ?

'active_bases' can be used effectively, if we can quit early from this
loop, i.e. by checking for !active_bases on every iteration.

But that generates a lot more code and is probably not that helpful
for small loop size that we have here.

--
viresh
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