1. 22 9月, 2005 1 次提交
    • P
      [PATCH] Remove unused var from asm/futex.h · 676067cf
      Paolo 'Blaisorblade' Giarrusso 提交于
      As recently done by Russell King for ARM, commit
      4732efbe introduces a generic asm/futex.h copied
      along most arches, which includes a "-ENOSYS support" to be changed if needed.
      However, it includes an unused var (taken from the "real" version) which GCC
      warns about.
      
      Remove it from all arches having that file version (i.e. same GIT id).
      $ git-diff-tree -r HEAD
      and
      $ git-ls-tree  -r HEAD include/|grep 9feff4ce1424bc390608326240be369eb13aa648
      
      may be more interesting than looking at the patch itself, to make sure I've
      just copied the arm header to all other archs having the original dummy version
      of this file.
      
      Cc: Jakub Jelinek <jakub@redhat.com>
      Signed-off-by: NPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
      Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
      676067cf
  2. 08 9月, 2005 1 次提交
    • J
      [PATCH] FUTEX_WAKE_OP: pthread_cond_signal() speedup · 4732efbe
      Jakub Jelinek 提交于
      ATM pthread_cond_signal is unnecessarily slow, because it wakes one waiter
      (which at least on UP usually means an immediate context switch to one of
      the waiter threads).  This waiter wakes up and after a few instructions it
      attempts to acquire the cv internal lock, but that lock is still held by
      the thread calling pthread_cond_signal.  So it goes to sleep and eventually
      the signalling thread is scheduled in, unlocks the internal lock and wakes
      the waiter again.
      
      Now, before 2003-09-21 NPTL was using FUTEX_REQUEUE in pthread_cond_signal
      to avoid this performance issue, but it was removed when locks were
      redesigned to the 3 state scheme (unlocked, locked uncontended, locked
      contended).
      
      Following scenario shows why simply using FUTEX_REQUEUE in
      pthread_cond_signal together with using lll_mutex_unlock_force in place of
      lll_mutex_unlock is not enough and probably why it has been disabled at
      that time:
      
      The number is value in cv->__data.__lock.
              thr1            thr2            thr3
      0       pthread_cond_wait
      1       lll_mutex_lock (cv->__data.__lock)
      0       lll_mutex_unlock (cv->__data.__lock)
      0       lll_futex_wait (&cv->__data.__futex, futexval)
      0                       pthread_cond_signal
      1                       lll_mutex_lock (cv->__data.__lock)
      1                                       pthread_cond_signal
      2                                       lll_mutex_lock (cv->__data.__lock)
      2                                         lll_futex_wait (&cv->__data.__lock, 2)
      2                       lll_futex_requeue (&cv->__data.__futex, 0, 1, &cv->__data.__lock)
                                # FUTEX_REQUEUE, not FUTEX_CMP_REQUEUE
      2                       lll_mutex_unlock_force (cv->__data.__lock)
      0                         cv->__data.__lock = 0
      0                         lll_futex_wake (&cv->__data.__lock, 1)
      1       lll_mutex_lock (cv->__data.__lock)
      0       lll_mutex_unlock (cv->__data.__lock)
                # Here, lll_mutex_unlock doesn't know there are threads waiting
                # on the internal cv's lock
      
      Now, I believe it is possible to use FUTEX_REQUEUE in pthread_cond_signal,
      but it will cost us not one, but 2 extra syscalls and, what's worse, one of
      these extra syscalls will be done for every single waiting loop in
      pthread_cond_*wait.
      
      We would need to use lll_mutex_unlock_force in pthread_cond_signal after
      requeue and lll_mutex_cond_lock in pthread_cond_*wait after lll_futex_wait.
      
      Another alternative is to do the unlocking pthread_cond_signal needs to do
      (the lock can't be unlocked before lll_futex_wake, as that is racy) in the
      kernel.
      
      I have implemented both variants, futex-requeue-glibc.patch is the first
      one and futex-wake_op{,-glibc}.patch is the unlocking inside of the kernel.
       The kernel interface allows userland to specify how exactly an unlocking
      operation should look like (some atomic arithmetic operation with optional
      constant argument and comparison of the previous futex value with another
      constant).
      
      It has been implemented just for ppc*, x86_64 and i?86, for other
      architectures I'm including just a stub header which can be used as a
      starting point by maintainers to write support for their arches and ATM
      will just return -ENOSYS for FUTEX_WAKE_OP.  The requeue patch has been
      (lightly) tested just on x86_64, the wake_op patch on ppc64 kernel running
      32-bit and 64-bit NPTL and x86_64 kernel running 32-bit and 64-bit NPTL.
      
      With the following benchmark on UP x86-64 I get:
      
      for i in nptl-orig nptl-requeue nptl-wake_op; do echo time elf/ld.so --library-path .:$i /tmp/bench; \
      for j in 1 2; do echo ( time elf/ld.so --library-path .:$i /tmp/bench ) 2>&1; done; done
      time elf/ld.so --library-path .:nptl-orig /tmp/bench
      real 0m0.655s user 0m0.253s sys 0m0.403s
      real 0m0.657s user 0m0.269s sys 0m0.388s
      time elf/ld.so --library-path .:nptl-requeue /tmp/bench
      real 0m0.496s user 0m0.225s sys 0m0.271s
      real 0m0.531s user 0m0.242s sys 0m0.288s
      time elf/ld.so --library-path .:nptl-wake_op /tmp/bench
      real 0m0.380s user 0m0.176s sys 0m0.204s
      real 0m0.382s user 0m0.175s sys 0m0.207s
      
      The benchmark is at:
      http://sourceware.org/ml/libc-alpha/2005-03/txt00001.txt
      Older futex-requeue-glibc.patch version is at:
      http://sourceware.org/ml/libc-alpha/2005-03/txt00002.txt
      Older futex-wake_op-glibc.patch version is at:
      http://sourceware.org/ml/libc-alpha/2005-03/txt00003.txt
      Will post a new version (just x86-64 fixes so that the patch
      applies against pthread_cond_signal.S) to libc-hacker ml soon.
      
      Attached is the kernel FUTEX_WAKE_OP patch as well as a simple-minded
      testcase that will not test the atomicity of the operation, but at least
      check if the threads that should have been woken up are woken up and
      whether the arithmetic operation in the kernel gave the expected results.
      Acked-by: NIngo Molnar <mingo@redhat.com>
      Cc: Ulrich Drepper <drepper@redhat.com>
      Cc: Jamie Lokier <jamie@shareable.org>
      Cc: Rusty Russell <rusty@rustcorp.com.au>
      Signed-off-by: NYoichi Yuasa <yuasa@hh.iij4u.or.jp>
      Signed-off-by: NAndrew Morton <akpm@osdl.org>
      Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
      4732efbe