Re: [Qemu-devel] [PATCH v2] Register Linux dyntick timer as per-thread signal

[Anthony Liguori]

On 06/17/2011 04:25 AM, Jan Kiszka wrote:
> Derived from kvm-tool patch
> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/74309
>
> Ingo Molnar pointed out that sending the timer signal to the whole
> process, just blocking it everywhere, is suboptimal with an increasing
> number of threads. QEMU is also using this pattern so far.
>
> Linux provides a (non-portable) way to restrict the signal to a single
> thread: We can use SIGEV_THREAD_ID unless we are forced to emulate
> signalfd via an additional thread. That case could theoretically be
> optimized as well, but it doesn't look worth bothering.
>
> Signed-off-by: Jan Kiszka<address@hidden>

Applied.  Thanks.

Regards,

Anthony Liguori

> ---
>
> Changes in v2:
>   - refactored dynticks_start_timer changes as suggested by Richard
>     Henderson
>   - added reference to original kvm-tool patch
>
>   compatfd.c   |   11 +++++++++++
>   compatfd.h   |    1 +
>   qemu-timer.c |    8 ++++++++
>   3 files changed, 20 insertions(+), 0 deletions(-)
>
> diff --git a/compatfd.c b/compatfd.c
> index 41586ce..31654c6 100644
> --- a/compatfd.c
> +++ b/compatfd.c
> @@ -115,3 +115,14 @@ int qemu_signalfd(const sigset_t *mask)
>
>       return qemu_signalfd_compat(mask);
>   }
> +
> +bool qemu_signalfd_available(void)
> +{
> +#ifdef CONFIG_SIGNALFD
> +    errno = 0;
> +    syscall(SYS_signalfd, -1, NULL, _NSIG / 8);
> +    return errno != ENOSYS;
> +#else
> +    return false;
> +#endif
> +}
> diff --git a/compatfd.h b/compatfd.h
> index fc37915..6b04877 100644
> --- a/compatfd.h
> +++ b/compatfd.h
> @@ -39,5 +39,6 @@ struct qemu_signalfd_siginfo {
>   };
>
>   int qemu_signalfd(const sigset_t *mask);
> +bool qemu_signalfd_available(void);
>
>   #endif
> diff --git a/qemu-timer.c b/qemu-timer.c
> index 72066c7..743cf96 100644
> --- a/qemu-timer.c
> +++ b/qemu-timer.c
> @@ -803,6 +803,8 @@ static int64_t qemu_next_alarm_deadline(void)
>
>   #if defined(__linux__)
>
> +#include "compatfd.h"
> +
>   static int dynticks_start_timer(struct qemu_alarm_timer *t)
>   {
>       struct sigevent ev;
> @@ -822,6 +824,12 @@ static int dynticks_start_timer(struct qemu_alarm_timer *t)
>       memset(&ev, 0, sizeof(ev));
>       ev.sigev_value.sival_int = 0;
>       ev.sigev_notify = SIGEV_SIGNAL;
> +#ifdef SIGEV_THREAD_ID
> +    if (qemu_signalfd_available()) {
> +        ev.sigev_notify = SIGEV_THREAD_ID;
> +        ev._sigev_un._tid = qemu_get_thread_id();
> +    }
> +#endif /* SIGEV_THREAD_ID */
>       ev.sigev_signo = SIGALRM;
>
>       if (timer_create(CLOCK_REALTIME,&ev,&host_timer)) {