Commit 753e9c5c authored by Eric Dumazet's avatar Eric Dumazet Committed by Linus Torvalds
Browse files

Optimize timespec_trunc() 



The first thing done by timespec_trunc() is :

  if (gran <= jiffies_to_usecs(1) * 1000)

This should really be a test against a constant known at compile time.

Alas, it isnt. jiffies_to_usec() was unilined so C compiler emits a function
call and a multiply to compute : a CONSTANT.

mov    $0x1,%edi
mov    %rbx,0xffffffffffffffe8(%rbp)
mov    %r12,0xfffffffffffffff0(%rbp)
mov    %edx,%ebx
mov    %rsi,0xffffffffffffffc8(%rbp)
mov    %rsi,%r12
callq  ffffffff80232010 <jiffies_to_usecs>
imul   $0x3e8,%eax,%eax
cmp    %ebx,%eax

This patch reorders kernel/time.c a bit so that jiffies_to_usecs() is defined
before timespec_trunc() so that compiler now generates :

cmp    $0x3d0900,%edx  (HZ=250 on my machine)

This gives a better code (timespec_trunc() becoming a leaf function), and
shorter kernel size as well.

Signed-off-by: default avatarEric Dumazet <dada1@cosmosbay.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2e17c550

kernel/time.c

+30 −30
Original line number Diff line number Diff line
@@ -247,6 +247,36 @@ struct timespec current_fs_time(struct super_block *sb) 
}

EXPORT_SYMBOL(current_fs_time);



/*

 * Convert jiffies to milliseconds and back.

 *

 * Avoid unnecessary multiplications/divisions in the

 * two most common HZ cases:

 */

unsigned int inline jiffies_to_msecs(const unsigned long j)

{

#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)

	return (MSEC_PER_SEC / HZ) * j;

#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)

	return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);

#else

	return (j * MSEC_PER_SEC) / HZ;

#endif

}

EXPORT_SYMBOL(jiffies_to_msecs);



unsigned int inline jiffies_to_usecs(const unsigned long j)

{

#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)

	return (USEC_PER_SEC / HZ) * j;

#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)

	return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);

#else

	return (j * USEC_PER_SEC) / HZ;

#endif

}

EXPORT_SYMBOL(jiffies_to_usecs);



/**

 * timespec_trunc - Truncate timespec to a granularity

 * @t: Timespec
@@ -472,36 +502,6 @@ struct timeval ns_to_timeval(const s64 nsec) 
}

EXPORT_SYMBOL(ns_to_timeval);



/*

 * Convert jiffies to milliseconds and back.

 *

 * Avoid unnecessary multiplications/divisions in the

 * two most common HZ cases:

 */

unsigned int jiffies_to_msecs(const unsigned long j)

{

#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)

	return (MSEC_PER_SEC / HZ) * j;

#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)

	return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);

#else

	return (j * MSEC_PER_SEC) / HZ;

#endif

}

EXPORT_SYMBOL(jiffies_to_msecs);



unsigned int jiffies_to_usecs(const unsigned long j)

{

#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)

	return (USEC_PER_SEC / HZ) * j;

#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)

	return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);

#else

	return (j * USEC_PER_SEC) / HZ;

#endif

}

EXPORT_SYMBOL(jiffies_to_usecs);



/*

 * When we convert to jiffies then we interpret incoming values

 * the following way:

CRA Git | Maintained and supported by SUSTech CRA and CCSE