linux-kernel - Re: [PATCH] cpuidle: avoid using smp_processor_id() in preemptible code (nr_iowait

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Date:	Thu, 17 Jun 2010 10:34:09 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Jiri Slaby <jslaby@...e.cz>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuidle: avoid using smp_processor_id() in preemptible
 code (nr_iowait_cpu) v4

On (06/16/10 23:59), Andrew Morton wrote:
> [..] if someone runs
> update_ts_time_stats() before the initcalls then conceivably the
> `swapper' process's accounting will go a little bit wrong, but I doubt
> it.
> 

That was the sing that scared me - update_ts_time_stats call before init. 
Having ".cpu = cpu" in tick_get_tick_sched guarantees correct .cpu and... 
and it sucks.

> Still, it'd be better to do it earlier, I guess.  tick_init() is called
> super-early and that would be a good place.  tick_init() is presently a
> no-op if !CONFIG_GENERIC_CLOCKEVENTS, but all this code depends on
> CONFIG_GENERIC_CLOCKEVENTS anwyay.
> 
> So how does this look?  If "OK" then would you be able to test it please?
> 
>
I'll test it in 2 hours. Thanks.

 
> [ Sigh.  The field tick_sched.cpu shouldn't even exist on
>   uniprocessor builds.  Ifdeffing it away is trivial and a bit messy,
>   but it's still only a partial solution.  Passing the `cpu' argument
>   to nr_iowait_cpu() will generate additional code, and it's unneeded
>   on uniprocessor builds.]
> 
> 
You're right.


	Sergey

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