1. 06 11月, 2015 1 次提交
  2. 09 10月, 2015 1 次提交
  3. 12 9月, 2015 1 次提交
    • M
      sys_membarrier(): system-wide memory barrier (generic, x86) · 5b25b13a
      Mathieu Desnoyers 提交于
      Here is an implementation of a new system call, sys_membarrier(), which
      executes a memory barrier on all threads running on the system.  It is
      implemented by calling synchronize_sched().  It can be used to
      distribute the cost of user-space memory barriers asymmetrically by
      transforming pairs of memory barriers into pairs consisting of
      sys_membarrier() and a compiler barrier.  For synchronization primitives
      that distinguish between read-side and write-side (e.g.  userspace RCU
      [1], rwlocks), the read-side can be accelerated significantly by moving
      the bulk of the memory barrier overhead to the write-side.
      
      The existing applications of which I am aware that would be improved by
      this system call are as follows:
      
      * Through Userspace RCU library (http://urcu.so)
        - DNS server (Knot DNS) https://www.knot-dns.cz/
        - Network sniffer (http://netsniff-ng.org/)
        - Distributed object storage (https://sheepdog.github.io/sheepdog/)
        - User-space tracing (http://lttng.org)
        - Network storage system (https://www.gluster.org/)
        - Virtual routers (https://events.linuxfoundation.org/sites/events/files/slides/DPDK_RCU_0MQ.pdf)
        - Financial software (https://lkml.org/lkml/2015/3/23/189)
      
      Those projects use RCU in userspace to increase read-side speed and
      scalability compared to locking.  Especially in the case of RCU used by
      libraries, sys_membarrier can speed up the read-side by moving the bulk of
      the memory barrier cost to synchronize_rcu().
      
      * Direct users of sys_membarrier
        - core dotnet garbage collector (https://github.com/dotnet/coreclr/issues/198)
      
      Microsoft core dotnet GC developers are planning to use the mprotect()
      side-effect of issuing memory barriers through IPIs as a way to implement
      Windows FlushProcessWriteBuffers() on Linux.  They are referring to
      sys_membarrier in their github thread, specifically stating that
      sys_membarrier() is what they are looking for.
      
      To explain the benefit of this scheme, let's introduce two example threads:
      
      Thread A (non-frequent, e.g. executing liburcu synchronize_rcu())
      Thread B (frequent, e.g. executing liburcu
      rcu_read_lock()/rcu_read_unlock())
      
      In a scheme where all smp_mb() in thread A are ordering memory accesses
      with respect to smp_mb() present in Thread B, we can change each
      smp_mb() within Thread A into calls to sys_membarrier() and each
      smp_mb() within Thread B into compiler barriers "barrier()".
      
      Before the change, we had, for each smp_mb() pairs:
      
      Thread A                    Thread B
      previous mem accesses       previous mem accesses
      smp_mb()                    smp_mb()
      following mem accesses      following mem accesses
      
      After the change, these pairs become:
      
      Thread A                    Thread B
      prev mem accesses           prev mem accesses
      sys_membarrier()            barrier()
      follow mem accesses         follow mem accesses
      
      As we can see, there are two possible scenarios: either Thread B memory
      accesses do not happen concurrently with Thread A accesses (1), or they
      do (2).
      
      1) Non-concurrent Thread A vs Thread B accesses:
      
      Thread A                    Thread B
      prev mem accesses
      sys_membarrier()
      follow mem accesses
                                  prev mem accesses
                                  barrier()
                                  follow mem accesses
      
      In this case, thread B accesses will be weakly ordered. This is OK,
      because at that point, thread A is not particularly interested in
      ordering them with respect to its own accesses.
      
      2) Concurrent Thread A vs Thread B accesses
      
      Thread A                    Thread B
      prev mem accesses           prev mem accesses
      sys_membarrier()            barrier()
      follow mem accesses         follow mem accesses
      
      In this case, thread B accesses, which are ensured to be in program
      order thanks to the compiler barrier, will be "upgraded" to full
      smp_mb() by synchronize_sched().
      
      * Benchmarks
      
      On Intel Xeon E5405 (8 cores)
      (one thread is calling sys_membarrier, the other 7 threads are busy
      looping)
      
      1000 non-expedited sys_membarrier calls in 33s =3D 33 milliseconds/call.
      
      * User-space user of this system call: Userspace RCU library
      
      Both the signal-based and the sys_membarrier userspace RCU schemes
      permit us to remove the memory barrier from the userspace RCU
      rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
      accelerating them. These memory barriers are replaced by compiler
      barriers on the read-side, and all matching memory barriers on the
      write-side are turned into an invocation of a memory barrier on all
      active threads in the process. By letting the kernel perform this
      synchronization rather than dumbly sending a signal to every process
      threads (as we currently do), we diminish the number of unnecessary wake
      ups and only issue the memory barriers on active threads. Non-running
      threads do not need to execute such barrier anyway, because these are
      implied by the scheduler context switches.
      
      Results in liburcu:
      
      Operations in 10s, 6 readers, 2 writers:
      
      memory barriers in reader:    1701557485 reads, 2202847 writes
      signal-based scheme:          9830061167 reads,    6700 writes
      sys_membarrier:               9952759104 reads,     425 writes
      sys_membarrier (dyn. check):  7970328887 reads,     425 writes
      
      The dynamic sys_membarrier availability check adds some overhead to
      the read-side compared to the signal-based scheme, but besides that,
      sys_membarrier slightly outperforms the signal-based scheme. However,
      this non-expedited sys_membarrier implementation has a much slower grace
      period than signal and memory barrier schemes.
      
      Besides diminishing the number of wake-ups, one major advantage of the
      membarrier system call over the signal-based scheme is that it does not
      need to reserve a signal. This plays much more nicely with libraries,
      and with processes injected into for tracing purposes, for which we
      cannot expect that signals will be unused by the application.
      
      An expedited version of this system call can be added later on to speed
      up the grace period. Its implementation will likely depend on reading
      the cpu_curr()->mm without holding each CPU's rq lock.
      
      This patch adds the system call to x86 and to asm-generic.
      
      [1] http://urcu.so
      
      membarrier(2) man page:
      
      MEMBARRIER(2)              Linux Programmer's Manual             MEMBARRIER(2)
      
      NAME
             membarrier - issue memory barriers on a set of threads
      
      SYNOPSIS
             #include <linux/membarrier.h>
      
             int membarrier(int cmd, int flags);
      
      DESCRIPTION
             The cmd argument is one of the following:
      
             MEMBARRIER_CMD_QUERY
                    Query  the  set  of  supported commands. It returns a bitmask of
                    supported commands.
      
             MEMBARRIER_CMD_SHARED
                    Execute a memory barrier on all threads running on  the  system.
                    Upon  return from system call, the caller thread is ensured that
                    all running threads have passed through a state where all memory
                    accesses  to  user-space  addresses  match program order between
                    entry to and return from the system  call  (non-running  threads
                    are de facto in such a state). This covers threads from all pro=E2=80=90
                    cesses running on the system.  This command returns 0.
      
             The flags argument needs to be 0. For future extensions.
      
             All memory accesses performed  in  program  order  from  each  targeted
             thread is guaranteed to be ordered with respect to sys_membarrier(). If
             we use the semantic "barrier()" to represent a compiler barrier forcing
             memory  accesses  to  be performed in program order across the barrier,
             and smp_mb() to represent explicit memory barriers forcing full  memory
             ordering  across  the barrier, we have the following ordering table for
             each pair of barrier(), sys_membarrier() and smp_mb():
      
             The pair ordering is detailed as (O: ordered, X: not ordered):
      
                                    barrier()   smp_mb() sys_membarrier()
                    barrier()          X           X            O
                    smp_mb()           X           O            O
                    sys_membarrier()   O           O            O
      
      RETURN VALUE
             On success, these system calls return zero.  On error, -1 is  returned,
             and errno is set appropriately. For a given command, with flags
             argument set to 0, this system call is guaranteed to always return the
             same value until reboot.
      
      ERRORS
             ENOSYS System call is not implemented.
      
             EINVAL Invalid arguments.
      
      Linux                             2015-04-15                     MEMBARRIER(2)
      Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
      Reviewed-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
      Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
      Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
      Cc: Steven Rostedt <rostedt@goodmis.org>
      Cc: Nicholas Miell <nmiell@comcast.net>
      Cc: Ingo Molnar <mingo@redhat.com>
      Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
      Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
      Cc: Stephen Hemminger <stephen@networkplumber.org>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: David Howells <dhowells@redhat.com>
      Cc: Pranith Kumar <bobby.prani@gmail.com>
      Cc: Michael Kerrisk <mtk.manpages@gmail.com>
      Cc: Shuah Khan <shuahkh@osg.samsung.com>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      5b25b13a
  4. 05 9月, 2015 1 次提交
  5. 21 7月, 2015 1 次提交
    • A
      x86/entry/syscalls: Wire up 32-bit direct socket calls · 9dea5dc9
      Andy Lutomirski 提交于
      On x86_64, there's no socketcall syscall; instead all of the
      socket calls are real syscalls.  For 32-bit programs, we're
      stuck offering the socketcall syscall, but it would be nice to
      expose the direct calls as well.  This will enable seccomp to
      filter socket calls (for new userspace only, but that's fine for
      some applications) and it will provide a tiny performance boost.
      Signed-off-by: NAndy Lutomirski <luto@kernel.org>
      Cc: Alexander Larsson <alexl@redhat.com>
      Cc: Andy Lutomirski <luto@amacapital.net>
      Cc: Cosimo Cecchi <cosimo@endlessm.com>
      Cc: Dan Nicholson <nicholson@endlessm.com>
      Cc: Linus Torvalds <torvalds@linux-foundation.org>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
      Cc: libc-alpha <libc-alpha@sourceware.org>
      Link: http://lkml.kernel.org/r/cb5138299d37d5800e2d135b01a7667fa6115854.1436912629.git.luto@kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>
      9dea5dc9
  6. 04 6月, 2015 1 次提交
    • I
      x86/asm/entry: Move the arch/x86/syscalls/ definitions to arch/x86/entry/syscalls/ · 1f57d5d8
      Ingo Molnar 提交于
      The build time generated syscall definitions are entry code related, move
      them into the arch/x86/entry/ directory.
      
      Cc: Borislav Petkov <bp@alien8.de>
      Cc: H. Peter Anvin <hpa@zytor.com>
      Cc: Linus Torvalds <torvalds@linux-foundation.org>
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: Andy Lutomirski <luto@amacapital.net>
      Cc: Denys Vlasenko <dvlasenk@redhat.com>
      Cc: Brian Gerst <brgerst@gmail.com>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: linux-kernel@vger.kernel.org
      Signed-off-by: NIngo Molnar <mingo@kernel.org>
      1f57d5d8