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Re: [Qemu-devel] [PATCH 2/3] Make paio subsystem use threadlets


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 2/3] Make paio subsystem use threadlets
Date: Thu, 21 Oct 2010 10:17:42 +0100

On Thu, Oct 21, 2010 at 9:40 AM, Arun R Bharadwaj
<address@hidden> wrote:
> * Stefan Hajnoczi <address@hidden> [2010-10-20 10:30:38]:
>
>> On Tue, Oct 19, 2010 at 6:43 PM, Arun R Bharadwaj
>> <address@hidden> wrote:
>> > From: Gautham R Shenoy <address@hidden>
>> >
>> > This patch makes the paio subsystem use the threadlet framework thereby
>> > decoupling asynchronous threading framework portion out of
>> > posix-aio-compat.c
>> >
>> > The patch has been tested with fstress.
>> >
>> > Signed-off-by: Gautham R Shenoy <address@hidden>
>> > Signed-off-by: Sripathi Kodi <address@hidden>
>> > ---
>> >  posix-aio-compat.c |  166 
>> > +++++++++-------------------------------------------
>> >  1 files changed, 30 insertions(+), 136 deletions(-)
>> >
>> > diff --git a/posix-aio-compat.c b/posix-aio-compat.c
>> > index 7b862b5..6977c18 100644
>> > --- a/posix-aio-compat.c
>> > +++ b/posix-aio-compat.c
>> > @@ -29,6 +29,7 @@
>> >  #include "block_int.h"
>> >
>> >  #include "block/raw-posix-aio.h"
>> > +#include "qemu-threadlets.h"
>> >
>> >
>> >  struct qemu_paiocb {
>> > @@ -51,6 +52,7 @@ struct qemu_paiocb {
>> >     struct qemu_paiocb *next;
>> >
>> >     int async_context_id;
>> > +    ThreadletWork work;
>>
>> The QTAILQ_ENTRY(qemu_paiocb) node field is no longer used, please remove it.
>>
>> >  };
>> >
>> >  typedef struct PosixAioState {
>> > @@ -59,15 +61,6 @@ typedef struct PosixAioState {
>> >  } PosixAioState;
>> >
>> >
>> > -static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
>> > -static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
>> > -static pthread_t thread_id;
>> > -static pthread_attr_t attr;
>> > -static int max_threads = 64;
>> > -static int cur_threads = 0;
>> > -static int idle_threads = 0;
>> > -static QTAILQ_HEAD(, qemu_paiocb) request_list;
>> > -
>> >  #ifdef CONFIG_PREADV
>> >  static int preadv_present = 1;
>> >  #else
>> > @@ -85,39 +78,6 @@ static void die(const char *what)
>> >     die2(errno, what);
>> >  }
>> >
>> > -static void mutex_lock(pthread_mutex_t *mutex)
>> > -{
>> > -    int ret = pthread_mutex_lock(mutex);
>> > -    if (ret) die2(ret, "pthread_mutex_lock");
>> > -}
>> > -
>> > -static void mutex_unlock(pthread_mutex_t *mutex)
>> > -{
>> > -    int ret = pthread_mutex_unlock(mutex);
>> > -    if (ret) die2(ret, "pthread_mutex_unlock");
>> > -}
>> > -
>> > -static int cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex,
>> > -                           struct timespec *ts)
>> > -{
>> > -    int ret = pthread_cond_timedwait(cond, mutex, ts);
>> > -    if (ret && ret != ETIMEDOUT) die2(ret, "pthread_cond_timedwait");
>> > -    return ret;
>> > -}
>> > -
>> > -static void cond_signal(pthread_cond_t *cond)
>> > -{
>> > -    int ret = pthread_cond_signal(cond);
>> > -    if (ret) die2(ret, "pthread_cond_signal");
>> > -}
>> > -
>> > -static void thread_create(pthread_t *thread, pthread_attr_t *attr,
>> > -                          void *(*start_routine)(void*), void *arg)
>> > -{
>> > -    int ret = pthread_create(thread, attr, start_routine, arg);
>> > -    if (ret) die2(ret, "pthread_create");
>> > -}
>> > -
>> >  static ssize_t handle_aiocb_ioctl(struct qemu_paiocb *aiocb)
>> >  {
>> >     int ret;
>> > @@ -301,106 +261,51 @@ static ssize_t handle_aiocb_rw(struct qemu_paiocb 
>> > *aiocb)
>> >     return nbytes;
>> >  }
>> >
>> > -static void *aio_thread(void *unused)
>> > +static void aio_thread(ThreadletWork *work)
>> >  {
>> >     pid_t pid;
>> > +    struct qemu_paiocb *aiocb = container_of(work, struct qemu_paiocb, 
>> > work);
>> > +    ssize_t ret = 0;
>> >
>> >     pid = getpid();
>> > +    aiocb->active = 1;
>> >
>> > -    while (1) {
>> > -        struct qemu_paiocb *aiocb;
>> > -        ssize_t ret = 0;
>> > -        qemu_timeval tv;
>> > -        struct timespec ts;
>> > -
>> > -        qemu_gettimeofday(&tv);
>> > -        ts.tv_sec = tv.tv_sec + 10;
>> > -        ts.tv_nsec = 0;
>> > -
>> > -        mutex_lock(&lock);
>> > -
>> > -        while (QTAILQ_EMPTY(&request_list) &&
>> > -               !(ret == ETIMEDOUT)) {
>> > -            ret = cond_timedwait(&cond, &lock, &ts);
>> > -        }
>> > -
>> > -        if (QTAILQ_EMPTY(&request_list))
>> > -            break;
>> > -
>> > -        aiocb = QTAILQ_FIRST(&request_list);
>> > -        QTAILQ_REMOVE(&request_list, aiocb, node);
>> > -        aiocb->active = 1;
>> > -        idle_threads--;
>> > -        mutex_unlock(&lock);
>> > -
>> > -        switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
>> > -        case QEMU_AIO_READ:
>> > -        case QEMU_AIO_WRITE:
>> > -            ret = handle_aiocb_rw(aiocb);
>> > -            break;
>> > -        case QEMU_AIO_FLUSH:
>> > -            ret = handle_aiocb_flush(aiocb);
>> > -            break;
>> > -        case QEMU_AIO_IOCTL:
>> > -            ret = handle_aiocb_ioctl(aiocb);
>> > -            break;
>> > -        default:
>> > -            fprintf(stderr, "invalid aio request (0x%x)\n", 
>> > aiocb->aio_type);
>> > -            ret = -EINVAL;
>> > -            break;
>> > -        }
>> > -
>> > -        mutex_lock(&lock);
>> > -        aiocb->ret = ret;
>> > -        idle_threads++;
>> > -        mutex_unlock(&lock);
>> > -
>> > -        if (kill(pid, aiocb->ev_signo)) die("kill failed");
>> > +    switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
>> > +    case QEMU_AIO_READ:
>> > +    case QEMU_AIO_WRITE:
>> > +        ret = handle_aiocb_rw(aiocb);
>> > +        break;
>> > +    case QEMU_AIO_FLUSH:
>> > +        ret = handle_aiocb_flush(aiocb);
>> > +        break;
>> > +    case QEMU_AIO_IOCTL:
>> > +        ret = handle_aiocb_ioctl(aiocb);
>> > +        break;
>> > +    default:
>> > +        fprintf(stderr, "invalid aio request (0x%x)\n", aiocb->aio_type);
>> > +        ret = -EINVAL;
>> > +        break;
>> >     }
>> >
>> > -    idle_threads--;
>> > -    cur_threads--;
>> > -    mutex_unlock(&lock);
>> > +    aiocb->ret = ret;
>> >
>> > -    return NULL;
>> > -}
>> > -
>> > -static void spawn_thread(void)
>> > -{
>> > -    sigset_t set, oldset;
>> > -
>> > -    cur_threads++;
>> > -    idle_threads++;
>> > -
>> > -    /* block all signals */
>> > -    if (sigfillset(&set)) die("sigfillset");
>> > -    if (sigprocmask(SIG_SETMASK, &set, &oldset)) die("sigprocmask");
>> > -
>> > -    thread_create(&thread_id, &attr, aio_thread, NULL);
>> > -
>> > -    if (sigprocmask(SIG_SETMASK, &oldset, NULL)) die("sigprocmask 
>> > restore");
>> > +    if (kill(pid, aiocb->ev_signo)) die("kill failed");
>> >  }
>> >
>> >  static void qemu_paio_submit(struct qemu_paiocb *aiocb)
>> >  {
>> >     aiocb->ret = -EINPROGRESS;
>> >     aiocb->active = 0;
>> > -    mutex_lock(&lock);
>> > -    if (idle_threads == 0 && cur_threads < max_threads)
>> > -        spawn_thread();
>> > -    QTAILQ_INSERT_TAIL(&request_list, aiocb, node);
>> > -    mutex_unlock(&lock);
>> > -    cond_signal(&cond);
>> > +
>> > +    aiocb->work.func = aio_thread;
>> > +    submit_threadletwork(&aiocb->work);
>> >  }
>> >
>> >  static ssize_t qemu_paio_return(struct qemu_paiocb *aiocb)
>> >  {
>> >     ssize_t ret;
>> >
>> > -    mutex_lock(&lock);
>> >     ret = aiocb->ret;
>> > -    mutex_unlock(&lock);
>> > -
>> >     return ret;
>> >  }
>> >
>> > @@ -536,14 +441,14 @@ static void paio_cancel(BlockDriverAIOCB *blockacb)
>> >     struct qemu_paiocb *acb = (struct qemu_paiocb *)blockacb;
>> >     int active = 0;
>> >
>> > -    mutex_lock(&lock);
>> >     if (!acb->active) {
>>
>> I'm not sure the active field serves any purpose.  No memory barriers
>> are used so the value of active is 0 before the work is executed and 0
>> *or* 1 while the work is executed.
>>
>> The cancel_threadletwork() function already indicates whether
>> cancellation succeeded.  Why not just try to cancel instead of using
>> the active field?
>>
>
> This series does not touch the active field anywhere. So I feel we can
> implement this as a separate patch instead of clubbing it with this.

I'd prefer for this to be addressed in this patch because the active
field served a function before but no longer works with threadlets.
You're right that the patch doesn't touch it, and QEMU still compiles
fine with it, but it's still broken as I described in the previous
email.

In other words, the patch breaks the active field, please fix it.

Stefan



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