• Manfred Spraul's avatar
    ipc/sem.c: optimize single sops when semval is zero · d987f8b2
    Manfred Spraul authored
    If multiple simple decrements on the same semaphore are pending, then the
    current code scans all decrement operations, even if the semaphore value
    is already 0.
    
    The patch optimizes that: if the semaphore value is 0, then there is no
    need to scan the q->alter entries.
    
    Note that this is a common case: It happens if 100 decrements by one are
    pending and now an increment by one increases the semaphore value from 0
    to 1.  Without this patch, all 100 entries are scanned.  With the patch,
    only one entry is scanned, then woken up.  Then the new rule triggers and
    the scanning is aborted, without looking at the remaining 99 tasks.
    
    With this patch, single sop increment/decrement by 1 are now O(1).
    (same as with Nick's patch)
    Signed-off-by: default avatarManfred Spraul <manfred@colorfullife.com>
    Cc: Nick Piggin <npiggin@suse.de>
    Cc: Pierre Peiffer <peifferp@gmail.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    d987f8b2
sem.c 35.8 KB