Re: [Qemu-devel] [PATCH v2] block: avoid SIGUSR2

[Kevin Wolf <kwolf@redhat.com>]

Am 19.09.2011 17:02, schrieb Paolo Bonzini:
> On 09/19/2011 04:37 PM, Frediano Ziglio wrote:
>> Now that iothread is always compiled sending a signal seems only an
>> additional step. This patch also avoid writing to two pipe (one from signal
>> and one in qemu_service_io).
>>
>> Work with kvm enabled or disabled. strace output is more readable (less syscalls).
>>
>> Signed-off-by: Frediano Ziglio<freddy77@gmail.com>
>> ---
>>   cpus.c             |    5 -----
>>   posix-aio-compat.c |   29 +++++++++--------------------
>>   2 files changed, 9 insertions(+), 25 deletions(-)
>>
>> diff --git a/cpus.c b/cpus.c
>> index 54c188c..d0cfe91 100644
>> --- a/cpus.c
>> +++ b/cpus.c
>> @@ -380,11 +380,6 @@ static int qemu_signal_init(void)
>>       int sigfd;
>>       sigset_t set;
>>
>> -    /* SIGUSR2 used by posix-aio-compat.c */
>> -    sigemptyset(&set);
>> -    sigaddset(&set, SIGUSR2);
>> -    pthread_sigmask(SIG_UNBLOCK,&set, NULL);
>> -
>>       /*
>>        * SIG_IPI must be blocked in the main thread and must not be caught
>>        * by sigwait() in the signal thread. Otherwise, the cpu thread will
>> diff --git a/posix-aio-compat.c b/posix-aio-compat.c
>> index 3193dbf..185d5b2 100644
>> --- a/posix-aio-compat.c
>> +++ b/posix-aio-compat.c
>> @@ -42,7 +42,6 @@ struct qemu_paiocb {
>>       int aio_niov;
>>       size_t aio_nbytes;
>>   #define aio_ioctl_cmd   aio_nbytes /* for QEMU_AIO_IOCTL */
>> -    int ev_signo;
>>       off_t aio_offset;
>>
>>       QTAILQ_ENTRY(qemu_paiocb) node;
>> @@ -309,6 +308,8 @@ static ssize_t handle_aiocb_rw(struct qemu_paiocb *aiocb)
>>       return nbytes;
>>   }
>>
>> +static void posix_aio_notify_event(void);
>> +
>>   static void *aio_thread(void *unused)
>>   {
>>       pid_t pid;
>> @@ -381,7 +382,7 @@ static void *aio_thread(void *unused)
>>           aiocb->ret = ret;
>>           mutex_unlock(&lock);
>>
>> -        if (kill(pid, aiocb->ev_signo)) die("kill failed");
>> +        posix_aio_notify_event();
>>       }
>>
>>       cur_threads--;
>> @@ -548,18 +549,14 @@ static int posix_aio_flush(void *opaque)
>>
>>   static PosixAioState *posix_aio_state;
>>
>> -static void aio_signal_handler(int signum)
>> +static void posix_aio_notify_event(void)
>>   {
>> -    if (posix_aio_state) {
>> -        char byte = 0;
>> -        ssize_t ret;
>> -
>> -        ret = write(posix_aio_state->wfd,&byte, sizeof(byte));
>> -        if (ret<  0&&  errno != EAGAIN)
>> -            die("write()");
>> -    }
>> +    char byte = 0;
>> +    ssize_t ret;
>>
>> -    qemu_service_io();
>> +    ret = write(posix_aio_state->wfd,&byte, sizeof(byte));
>> +    if (ret<  0&&  errno != EAGAIN)
>> +        die("write()");
>>   }
>>
>>   static void paio_remove(struct qemu_paiocb *acb)
>> @@ -623,7 +620,6 @@ BlockDriverAIOCB *paio_submit(BlockDriverState *bs, int fd,
>>           return NULL;
>>       acb->aio_type = type;
>>       acb->aio_fildes = fd;
>> -    acb->ev_signo = SIGUSR2;
>>
>>       if (qiov) {
>>           acb->aio_iov = qiov->iov;
>> @@ -651,7 +647,6 @@ BlockDriverAIOCB *paio_ioctl(BlockDriverState *bs, int fd,
>>           return NULL;
>>       acb->aio_type = QEMU_AIO_IOCTL;
>>       acb->aio_fildes = fd;
>> -    acb->ev_signo = SIGUSR2;
>>       acb->aio_offset = 0;
>>       acb->aio_ioctl_buf = buf;
>>       acb->aio_ioctl_cmd = req;
>> @@ -665,7 +660,6 @@ BlockDriverAIOCB *paio_ioctl(BlockDriverState *bs, int fd,
>>
>>   int paio_init(void)
>>   {
>> -    struct sigaction act;
>>       PosixAioState *s;
>>       int fds[2];
>>       int ret;
>> @@ -675,11 +669,6 @@ int paio_init(void)
>>
>>       s = g_malloc(sizeof(PosixAioState));
>>
>> -    sigfillset(&act.sa_mask);
>> -    act.sa_flags = 0; /* do not restart syscalls to interrupt select() */
>> -    act.sa_handler = aio_signal_handler;
>> -    sigaction(SIGUSR2,&act, NULL);
>> -
>>       s->first_aio = NULL;
>>       if (qemu_pipe(fds) == -1) {
>>           fprintf(stderr, "failed to create pipe\n");
> 
> I think it is possible to go a step further, turn 
> posix_aio_process_queue into a bottom half and get rid of the pipe 
> altogether.  This in turn would remove the only real user of 
> io_process_queue in qemu_aio_set_fd_handler.  However, this is already a 
> nice improvement.

But without the fd, wouldn't the I/O thread possibly wait for much
longer until its select() times out and it starts processing BHs?

Kevin