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  • Anton Anton anjo
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  1. Apr 06, 2022
  3. Jun 16, 2021
  5. Sep 23, 2020
    • Stefan Hajnoczi's avatar
      qemu/atomic.h: rename atomic_ to qatomic_ · d73415a3
      Stefan Hajnoczi authored 
      
clang's C11 atomic_fetch_*() functions only take a C11 atomic type
pointer argument. QEMU uses direct types (int, etc) and this causes a
compiler error when a QEMU code calls these functions in a source file
that also included <stdatomic.h> via a system header file:

  $ CC=clang CXX=clang++ ./configure ... && make
  ../util/async.c:79:17: error: address argument to atomic operation must be a pointer to _Atomic type ('unsigned int *' invalid)

Avoid using atomic_*() names in QEMU's atomic.h since that namespace is
used by <stdatomic.h>. Prefix QEMU's APIs with 'q' so that atomic.h
and <stdatomic.h> can co-exist. I checked /usr/include on my machine and
searched GitHub for existing "qatomic_" users but there seem to be none.

This patch was generated using:

  $ git grep -h -o '\<atomic\(64\)\?_[a-z0-9_]\+' include/qemu/atomic.h | \
    sort -u >/tmp/changed_identifiers
  $ for identifier in $(</tmp/changed_identifiers); do
        sed -i "s%\<$identifier\>%q$identifier%g" \
            $(git grep -I -l "\<$identifier\>")
    done

I manually fixed line-wrap issues and misaligned rST tables.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarPhilippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200923105646.47864-1-stefanha@redhat.com>
      d73415a3
  7. Sep 17, 2020
  9. Sep 16, 2019
  11. Jan 11, 2019
  13. Aug 23, 2018
    • Emilio G. Cota's avatar
      qsp: QEMU's Synchronization Profiler · fe9959a2
      Emilio G. Cota authored 
      
The goal of this module is to profile synchronization primitives (i.e.
mutexes, recursive mutexes and condition variables) so that scalability
issues can be quickly diagnosed.

Sync primitives are profiled by QSP based on the vaddr of the object accessed
as well as the call site (file:line_nr). That means the same object called
from two different call sites will be tracked in separate entries, which
might be reported together or separately (see subsequent commit on
call site coalescing).

Some perf numbers:

Host: Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
Command: taskset -c 0 tests/atomic_add-bench -d 5 -m

- Before: 54.80 Mops/s
- After:  54.75 Mops/s

That is, a negligible slowdown due to the now indirect call to
qemu_mutex_lock. Note that using a branch instead of an indirect
call introduces a more severe slowdown (53.65 Mops/s, i.e. 2% slowdown).

Enabling the profiler (with -p, added in this series) is more interesting:

- No profiling: 54.75 Mops/s
- W/ profiling: 12.53 Mops/s

That is, a 4.36X slowdown.

We can break down this slowdown by removing the get_clock calls or
the entry lookup:

- No profiling:     54.75 Mops/s
- W/o get_clock:    25.37 Mops/s
- W/o entry lookup: 19.30 Mops/s
- W/ profiling:     12.53 Mops/s

Signed-off-by: default avatarEmilio G. Cota <cota@braap.org>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      fe9959a2
  15. Jun 28, 2018
    • Peter Xu's avatar
      qemu-thread: introduce qemu-thread-common.h · f1aff7aa
      Peter Xu authored 
      
Introduce some hooks for the shared part of qemu thread between POSIX
and Windows implementations.  Note that in qemu_mutex_unlock_impl() we
moved the call before unlock operation which should make more sense.
And we don't need qemu_mutex_post_unlock() hook.

Put all these shared hooks into the header files.  It should be internal
to qemu-thread but not for qemu-thread users, hence put into util/
directory.

Reviewed-by: default avatarEmilio G. Cota <cota@braap.org>
Signed-off-by: default avatarPeter Xu <peterx@redhat.com>
Message-Id: <20180425025459.5258-3-peterx@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      f1aff7aa
  17. Jan 16, 2018
  19. Jul 04, 2017
  21. May 05, 2017
    • Jose Ricardo Ziviani's avatar
      trace: add qemu mutex lock and unlock trace events · 31f5a726
      Jose Ricardo Ziviani authored 
      
These trace events were very useful to help me to understand and find a
reordering issue in vfio, for example:

qemu_mutex_lock locked mutex 0x10905ad8
  vfio_region_write  (0001:03:00.0:region1+0xc0, 0x2020c, 4)
qemu_mutex_unlock unlocked mutex 0x10905ad8
qemu_mutex_lock locked mutex 0x10905ad8
  vfio_region_write  (0001:03:00.0:region1+0xc4, 0xa0000, 4)
qemu_mutex_unlock unlocked mutex 0x10905ad8

that also helped me to see the desired result after the fix:

qemu_mutex_lock locked mutex 0x10905ad8
  vfio_region_write  (0001:03:00.0:region1+0xc0, 0x2000c, 4)
  vfio_region_write  (0001:03:00.0:region1+0xc4, 0xb0000, 4)
qemu_mutex_unlock unlocked mutex 0x10905ad8

So it could be a good idea to have these traces implemented. It's worth
mentioning that they should be surgically enabled during the debugging,
otherwise it can flood the trace logs with lock/unlock messages.

How to use it:
trace-event qemu_mutex_lock on|off
trace-event qemu_mutex_unlock on|off
or
trace-event qemu_mutex* on|off

Signed-off-by: default avatarJose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Message-Id: <1493054398-26013-1-git-send-email-joserz@linux.vnet.ibm.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
[Also handle trylock, cond_wait and win32; trace "unlocked" while still
 in the critical section, so that "unlocked" always comes before the
 next "locked" tracepoint. - Paolo]
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      31f5a726
  23. Mar 27, 2017
    • Andrey Shedel's avatar
      win32: replace custom mutex and condition variable with native primitives · 12f8def0
      Andrey Shedel authored 
      
The multithreaded TCG implementation exposed deadlocks in the win32
condition variables: as implemented, qemu_cond_broadcast waited on
receivers, whereas the pthreads API it was intended to emulate does
not. This was causing a deadlock because broadcast was called while
holding the IO lock, as well as all possible waiters blocked on the
same lock.

This patch replaces all the custom synchronisation code for mutexes
and condition variables with native Windows primitives (SRWlocks and
condition variables) with the same semantics as their POSIX
equivalents. To enable that, it requires a Windows Vista or newer host
OS.

Signed-off-by: default avatarAndrey Shedel <ashedel@microsoft.com>
[AB: edited commit message]
Signed-off-by: default avatarAndrew Baumann <Andrew.Baumann@microsoft.com>
Message-Id: <20170324220141.10104-1-Andrew.Baumann@microsoft.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      12f8def0
  25. Jan 19, 2017
  27. Jan 16, 2017
  29. Oct 28, 2016
  31. Oct 24, 2016
    • Paolo Bonzini's avatar
      qemu-thread: use acquire/release to clarify semantics of QemuEvent · 374293ca
      Paolo Bonzini authored 
      
Do not use the somewhat mysterious atomic_mb_read/atomic_mb_set,
instead make sure that the operations on QemuEvent are annotated
with the desired acquire and release semantics.

In particular, qemu_event_set wakes up the waiting thread, so it must
be a release from the POV of the waker (compare with qemu_mutex_unlock).
And it actually needs a full barrier, because that's the only thing that
provides something like a "load-release".

Use smp_mb_acquire until we have atomic_load_acquire and
atomic_store_release in atomic.h.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      374293ca
  33. Feb 04, 2016
  35. Sep 24, 2015
    • Paolo Bonzini's avatar
      qemu-thread: add a fast path to the Win32 QemuEvent · 7c9b2bf6
      Paolo Bonzini authored 
      
QemuEvents are used heavily by call_rcu.  We do not want them to be slow,
but the current implementation does a kernel call on every invocation
of qemu_event_* and won't cut it.

So, wrap a Win32 manual-reset event with a fast userspace path.  The
states and transitions are the same as for the futex and mutex/condvar
implementations, but the slow path is different of course.  The idea
is to reset the Win32 event lazily, as part of a test-reset-test-wait
sequence.  Such a sequence is, indeed, how QemuEvents are used by
RCU and other subsystems!

The patch includes a formal model of the algorithm.

Tested-by: default avatarStefan Weil <sw@weilnetz.de>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarStefan Weil <sw@weilnetz.de>
      7c9b2bf6
  37. Jan 13, 2015
    • Paolo Bonzini's avatar
      qemu-thread: add per-thread atexit functions · ef57137f
      Paolo Bonzini authored 
      
Destructors are the main additional feature of pthread TLS compared
to __thread.  If we were using C++ (hint, hint!) we could have used
thread-local objects with a destructor.  Since we are not, instead,
we add a simple Notifier-based API.

Note that the notifier must be per-thread as well.  We can add a
global list as well later, perhaps.

The Win32 implementation has some complications because a) detached
threads used not to have a QemuThreadData; b) the main thread does
not go through win32_start_routine, so we have to use atexit too.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Message-id: 1417518350-6167-3-git-send-email-pbonzini@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      ef57137f
  39. Mar 27, 2014
  41. Mar 09, 2014
  43. Oct 17, 2013
    • Paolo Bonzini's avatar
      qemu-thread: add QemuEvent · c7c4d063
      Paolo Bonzini authored 
      
This emulates Win32 manual-reset events using futexes or conditional
variables.  Typical ways to use them are with multi-producer,
single-consumer data structures, to test for a complex condition whose
elements come from different threads:

    for (;;) {
        qemu_event_reset(ev);
        ... test complex condition ...
        if (condition is true) {
            break;
        }
        qemu_event_wait(ev);
    }

Or more efficiently (but with some duplication):

    ... evaluate condition ...
    while (!condition) {
        qemu_event_reset(ev);
        ... evaluate condition ...
        if (!condition) {
            qemu_event_wait(ev);
            ... evaluate condition ...
        }
    }

QemuEvent provides a very fast userspace path in the common case when
no other thread is waiting, or the event is not changing state.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
      c7c4d063
  45. Jan 12, 2013
  47. Dec 23, 2012
  49. Dec 19, 2012
  51. Oct 31, 2012
  53. Aug 02, 2012
  55. Feb 07, 2012
  57. Dec 15, 2011
  59. Dec 12, 2011
  61. Sep 21, 2011
  63. Aug 21, 2011
  65. Mar 19, 2011
  67. Mar 13, 2011