Re: [Qemu-devel] [RFC PATCH] Implement qemu_thread_yield for posix, use it in mttcg to handle EXCP_YIELD

["Alex Bennée" <alex.bennee@linaro.org>]

Nicholas Piggin <npiggin@gmail.com> writes:

> This is a bit of proof of concept in case mttcg becomes more important
> yield could be handled like this. You can have by accident or deliberately
> force vCPUs onto the same physical CPU and cause inversion issues when the
> lock holder was preempted by the waiter. This is lightly tested but not
> to the point of measuring performance difference.

Sorry I'm so late replying.

Really this comes down to what EXCP_YIELD semantics are meant to mean.
For ARM it's a hint operation because we also have WFE which should halt
until there is some sort of change of state. In those cases exiting the
main-loop and sitting in wait_for_io should be the correct response. If
a vCPU is suspended waiting on the halt condition doesn't it have the
same effect?

>
> I really consider the previous confer/prod patches more important just to
> provide a more complete guest environment and better test coverage, than
> performance, but maybe someone wants to persue this.
>
> Thanks,
> Nick
> ---
>  cpus.c                   |  6 ++++++
>  hw/ppc/spapr_hcall.c     | 14 +++++++-------
>  include/qemu/thread.h    |  1 +
>  util/qemu-thread-posix.c |  5 +++++
>  util/qemu-thread-win32.c |  4 ++++
>  5 files changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index 927a00aa90..f036e062d9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1760,6 +1760,12 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>                  qemu_mutex_unlock_iothread();
>                  cpu_exec_step_atomic(cpu);
>                  qemu_mutex_lock_iothread();
> +                break;
> +            case EXCP_YIELD:
> +                qemu_mutex_unlock_iothread();
> +                qemu_thread_yield();
> +                qemu_mutex_lock_iothread();
> +                break;
>              default:
>                  /* Ignore everything else? */
>                  break;
> diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> index 57c1ee0fe1..9c24a64dfe 100644
> --- a/hw/ppc/spapr_hcall.c
> +++ b/hw/ppc/spapr_hcall.c
> @@ -1162,13 +1162,13 @@ static target_ulong h_confer(PowerPCCPU *cpu, SpaprMachineState *spapr,
>              return H_SUCCESS;
>          }
>  
> -        /*
> -         * The targeted confer does not do anything special beyond yielding
> -         * the current vCPU, but even this should be better than nothing.
> -         * At least for single-threaded tcg, it gives the target a chance to
> -         * run before we run again. Multi-threaded tcg does not really do
> -         * anything with EXCP_YIELD yet.
> -         */
> +       /*
> +        * The targeted confer does not do anything special beyond yielding
> +        * the current vCPU, but even this should be better than nothing.
> +        * For single-threaded tcg, it gives the target a chance to run
> +        * before we run again, multi-threaded tcg will yield the CPU to
> +        * another vCPU.
> +        */
>      }
>  
>      cs->exception_index = EXCP_YIELD;
> diff --git a/include/qemu/thread.h b/include/qemu/thread.h
> index 55d83a907c..8525b0a70a 100644
> --- a/include/qemu/thread.h
> +++ b/include/qemu/thread.h
> @@ -160,6 +160,7 @@ void qemu_thread_get_self(QemuThread *thread);
>  bool qemu_thread_is_self(QemuThread *thread);
>  void qemu_thread_exit(void *retval);
>  void qemu_thread_naming(bool enable);
> +void qemu_thread_yield(void);
>  
>  struct Notifier;
>  /**
> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
> index 1bf5e65dea..91b12a1082 100644
> --- a/util/qemu-thread-posix.c
> +++ b/util/qemu-thread-posix.c
> @@ -573,3 +573,8 @@ void *qemu_thread_join(QemuThread *thread)
>      }
>      return ret;
>  }
> +
> +void qemu_thread_yield(void)
> +{
> +    pthread_yield();
> +}
> diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
> index 572f88535d..72fe406bef 100644
> --- a/util/qemu-thread-win32.c
> +++ b/util/qemu-thread-win32.c
> @@ -442,3 +442,7 @@ bool qemu_thread_is_self(QemuThread *thread)
>  {
>      return GetCurrentThreadId() == thread->tid;
>  }
> +
> +void qemu_thread_yield(void)
> +{
> +}


-- 
Alex Bennée