linux-kernel - Re: [PATCH] system timer: fix crash in <100Hz system timer

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Message-Id: <20080108231507.ff34862f.akpm@linux-foundation.org>
Date:	Tue, 8 Jan 2008 23:15:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Fries <david@...es.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] system timer: fix crash in <100Hz system timer

On Sat, 5 Jan 2008 16:16:55 -0600 David Fries <david@...es.net> wrote:

> --- a/kernel/time.c
> +++ b/kernel/time.c
> @@ -565,7 +565,11 @@ EXPORT_SYMBOL(jiffies_to_timeval);
>  clock_t jiffies_to_clock_t(long x)
>  {
>  #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
> +	#if HZ < USER_HZ
> +	return x * (USER_HZ / HZ);
> +	#else
>  	return x / (HZ / USER_HZ);
> +	#endif
>  #else
>  	u64 tmp = (u64)x * TICK_NSEC;
>  	do_div(tmp, (NSEC_PER_SEC / USER_HZ));
> @@ -598,7 +602,12 @@ EXPORT_SYMBOL(clock_t_to_jiffies);
>  u64 jiffies_64_to_clock_t(u64 x)
>  {
>  #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
> -	do_div(x, HZ / USER_HZ);
> +	#if HZ < USER_HZ
> +		x *= USER_HZ;
> +		do_div(x, HZ);
> +	#else
> +		do_div(x, HZ / USER_HZ);
> +	#endif
>  #else

Somwhat off-topic:

I guess HZ=USER_HZ is a not-uncommon case, and it's pretty silly calling
do_div(x, 1) all the time.  How about we optimise that case?

Perhaps there are other places...


--- a/kernel/time.c~speed-up-jiffies-conversion-functions-if-hz==user_hz
+++ a/kernel/time.c
@@ -618,8 +618,10 @@ u64 jiffies_64_to_clock_t(u64 x)
 # if HZ < USER_HZ
 	x *= USER_HZ;
 	do_div(x, HZ);
-# else
+# elif HZ > USER_HZ
 	do_div(x, HZ / USER_HZ);
+# else
+	/* Nothing to do */
 # endif
 #else
 	/*
_

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