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Re: [PATCH] replay: implement fair mutex
From: |
Alex Bennée |
Subject: |
Re: [PATCH] replay: implement fair mutex |
Date: |
Mon, 18 May 2020 17:07:29 +0100 |
User-agent: |
mu4e 1.4.6; emacs 28.0.50 |
Pavel Dovgalyuk <address@hidden> writes:
> In record/replay icount mode main loop thread and vCPU thread
> do not perform simultaneously. They take replay mutex to synchronize
> the actions. Sometimes vCPU thread waits for locking the mutex for
> very long time, because main loop releases the mutex and takes it
> back again.
Where in the main loop do we keep bouncing the mutex like this? Surely
that is the problem we should fix?
> Standard qemu mutex do not provide the ordering
> capabilities.
>
> This patch adds a "queue" for replay mutex. Therefore thread ordering
> becomes more "fair". Threads are executed in the same order as
> they are trying to take the mutex.
>
> Signed-off-by: Pavel Dovgalyuk <address@hidden>
> ---
> replay/replay-internal.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/replay/replay-internal.c b/replay/replay-internal.c
> index eba8246aae..2e8a3e947a 100644
> --- a/replay/replay-internal.c
> +++ b/replay/replay-internal.c
> @@ -22,6 +22,9 @@
> It also protects replay events queue which stores events to be
> written or read to the log. */
> static QemuMutex lock;
> +/* Condition and queue for fair ordering of mutex lock requests. */
> +static QemuCond mutex_cond;
> +static unsigned long mutex_head, mutex_tail;
>
> /* File for replay writing */
> static bool write_error;
> @@ -197,9 +200,10 @@ static __thread bool replay_locked;
> void replay_mutex_init(void)
> {
> qemu_mutex_init(&lock);
> + qemu_cond_init(&mutex_cond);
> /* Hold the mutex while we start-up */
> - qemu_mutex_lock(&lock);
> replay_locked = true;
> + ++mutex_tail;
> }
>
> bool replay_mutex_locked(void)
> @@ -211,10 +215,16 @@ bool replay_mutex_locked(void)
> void replay_mutex_lock(void)
> {
> if (replay_mode != REPLAY_MODE_NONE) {
> + unsigned long id;
> g_assert(!qemu_mutex_iothread_locked());
> g_assert(!replay_mutex_locked());
> qemu_mutex_lock(&lock);
> + id = mutex_tail++;
> + while (id != mutex_head) {
> + qemu_cond_wait(&mutex_cond, &lock);
> + }
> replay_locked = true;
> + qemu_mutex_unlock(&lock);
> }
> }
>
> @@ -222,7 +232,10 @@ void replay_mutex_unlock(void)
> {
> if (replay_mode != REPLAY_MODE_NONE) {
> g_assert(replay_mutex_locked());
> + qemu_mutex_lock(&lock);
> + ++mutex_head;
> replay_locked = false;
> + qemu_cond_broadcast(&mutex_cond);
> qemu_mutex_unlock(&lock);
> }
> }
--
Alex Bennée
- Re: [PATCH] replay: implement fair mutex,
Alex Bennée <=