• Eric Dumazet's avatar
    Optimize timespec_trunc() · 753e9c5c
    Eric Dumazet authored
    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>
    753e9c5c
time.c 18.8 KB