linux-kernel - Re: [PATCH] USB: improve ehci_watchdog's side effect in CPU power management

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Date:	Thu, 2 Oct 2008 16:56:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	yi.y.yang@...el.com
Cc:	linux-kernel@...r.kernel.org, dbrownell@...rs.sourceforge.net,
	gregkh@...e.de, linux-usb@...r.kernel.org
Subject: Re: [PATCH] USB: improve ehci_watchdog's side effect in CPU power
 management

On Thu, 25 Sep 2008 17:25:44 +0800
Yi Yang <yi.y.yang@...el.com> wrote:

> ehci_watchdog will wake up CPU very frequently so that CPU
> stays at C3 very short, average residence time is about 50
> ms on Aspire One, but we expect it should be about 1 second
> or more, so this kind of periodic timer is very bad for power
> saving.
> 
> We can't remove this timer because of some bad USB controller
> chipset, but at least we should reduce its side effect to as
> possible as low.
> 
> This patch can make CPU stay at C3 longer, average residence time
> is about twice as long as original. 
> 
> Please consider to apply it, thanks
> 
> Signed-off-by: Yi Yang <yi.y.yang@...el.com>
> ---
>  ehci.h |   12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/usb/host/ehci.h b/drivers/usb/host/ehci.h
> index 5799298..9d530d9 100644
> --- a/drivers/usb/host/ehci.h
> +++ b/drivers/usb/host/ehci.h
> @@ -181,14 +181,16 @@ timer_action (struct ehci_hcd *ehci, enum ehci_timer_action action)
>  	 * the async ring; just the I/O watchdog.  Note that if a
>  	 * SHRINK were pending, OFF would never be requested.
>  	 */
> -	if (timer_pending(&ehci->watchdog)
> -			&& ((BIT(TIMER_ASYNC_SHRINK) | BIT(TIMER_ASYNC_OFF))
> -				& ehci->actions))
> -		return;
> +	enum ehci_timer_action oldactions = ehci->actions;
>  
>  	if (!test_and_set_bit (action, &ehci->actions)) {
>  		unsigned long t;
>  
> +		if (timer_pending(&ehci->watchdog)
> +			&& ((BIT(TIMER_ASYNC_SHRINK) | BIT(TIMER_ASYNC_OFF))
> +				& oldactions))
> +			return;
> +
>  		switch (action) {
>  		case TIMER_IO_WATCHDOG:
>  			t = EHCI_IO_JIFFIES;
> @@ -204,7 +206,7 @@ timer_action (struct ehci_hcd *ehci, enum ehci_timer_action action)
>  			t = DIV_ROUND_UP(EHCI_SHRINK_FRAMES * HZ, 1000) + 1;
>  			break;
>  		}
> -		mod_timer(&ehci->watchdog, t + jiffies);
> +		mod_timer(&ehci->watchdog, round_jiffies(t + jiffies));
>  	}
>  }
>  

<looks>

<regrets it>


Why does this:

	t = DIV_ROUND_UP(EHCI_SHRINK_FRAMES * HZ, 1000) + 1;

add "1000" to a jiffies value when it doesn't know what HZ is?  It'll
be adding anywhere from one second up to ten seconds to the timeout
interval depending upon compile-time options.

I suspect s/1000/HZ/ would improve things here.  Or just delete it -
doesn't the subsequent round_jiffies() do the same thing, only better? 
This code needs help, I suspect.


Also, do we really need to inline this large function into at least
five callsites?

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