Re: [PATCH] accel/tcg: Make round-robin kick period configurable

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Ethan,

On 5/6/25 08:18, Ethan Chen via wrote:
> This change introduces a configurable round-robin kick period, giving users the
> flexibility to balance SMP simulation accuracy and performance according to
> their specific needs.
> 
> The round-robin kick period is the time one vCPU can run before scheduler
> switches to another vCPU when using a single thread TCG. The default value of
> 0.1 seconds may allow one vCPU to run for too long before the scheduler
> switches to another. This behavior may not be suitable for workloads with
> strict timing requirements.
> 
> Reducing the period can improve the fidelity of SMP simulation by allowing
> more frequent vCPU switching, though it may negatively impact overall
> simulation performance.
> 
> Signed-off-by: Ethan Chen <ethan84@andestech.com>
> ---
>   accel/tcg/tcg-accel-ops-rr.c |  2 +-
>   accel/tcg/tcg-accel-ops-rr.h |  2 +-
>   accel/tcg/tcg-all.c          | 35 +++++++++++++++++++++++++++++++++++
>   qemu-options.hx              |  9 ++++++++-
>   4 files changed, 45 insertions(+), 3 deletions(-)


> diff --git a/accel/tcg/tcg-accel-ops-rr.h b/accel/tcg/tcg-accel-ops-rr.h
> index 2a76a29612..324bb772cb 100644
> --- a/accel/tcg/tcg-accel-ops-rr.h
> +++ b/accel/tcg/tcg-accel-ops-rr.h
> @@ -10,7 +10,7 @@
>   #ifndef TCG_ACCEL_OPS_RR_H
>   #define TCG_ACCEL_OPS_RR_H
>   
> -#define TCG_KICK_PERIOD (NANOSECONDS_PER_SECOND / 10)
> +extern uint64_t rr_kick_period;

No need for another extern, pass it as argument:

-- >8 --
diff --git a/accel/tcg/tcg-accel-ops-rr.c b/accel/tcg/tcg-accel-ops-rr.c
index f62cf24e1d4..551864b5509 100644
--- a/accel/tcg/tcg-accel-ops-rr.c
+++ b/accel/tcg/tcg-accel-ops-rr.c
@@ -62,9 +62,9 @@ void rr_kick_vcpu_thread(CPUState *unused)
  static QEMUTimer *rr_kick_vcpu_timer;
  static CPUState *rr_current_cpu;

-static inline int64_t rr_next_kick_time(void)
+static inline int64_t rr_next_kick_time(uint64_t kick_delay_ns)
  {
-    return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + TCG_KICK_PERIOD;
+    return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + kick_delay_ns;
  }

  /* Kick the currently round-robin scheduled vCPU to next */
@@ -83,15 +83,20 @@ static void rr_kick_next_cpu(void)

  static void rr_kick_thread(void *opaque)
  {
-    timer_mod(rr_kick_vcpu_timer, rr_next_kick_time());
+    CPUState *cpu = opaque;
+
+    // here use rr_next_kick_time(cpu->accel->rr_kick_delay_ns):
+    timer_mod(rr_kick_vcpu_timer, rr_next_kick_time(TCG_KICK_PERIOD));
      rr_kick_next_cpu();
  }

  static void rr_start_kick_timer(void)
  {
-    if (!rr_kick_vcpu_timer && CPU_NEXT(first_cpu)) {
+    CPUState *next_cpu = CPU_NEXT(first_cpu);
+
+    if (!rr_kick_vcpu_timer && next_cpu) {
          rr_kick_vcpu_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
-                                           rr_kick_thread, NULL);
+                                           rr_kick_thread, next_cpu);
      }
      if (rr_kick_vcpu_timer && !timer_pending(rr_kick_vcpu_timer)) {
          timer_mod(rr_kick_vcpu_timer, rr_next_kick_time());
---

> diff --git a/accel/tcg/tcg-all.c b/accel/tcg/tcg-all.c
> index 6e5dc333d5..69390020aa 100644
> --- a/accel/tcg/tcg-all.c
> +++ b/accel/tcg/tcg-all.c
> @@ -36,6 +36,7 @@
>   #include "qapi/qapi-builtin-visit.h"
>   #include "qemu/units.h"
>   #include "qemu/target-info.h"
> +#include "qemu/timer.h"
>   #ifndef CONFIG_USER_ONLY
>   #include "hw/boards.h"
>   #endif
> @@ -50,6 +51,7 @@ struct TCGState {
>       bool one_insn_per_tb;
>       int splitwx_enabled;
>       unsigned long tb_size;
> +    uint64_t rr_kick_period;

'rr_kick_delay_ns' seems more accurate.

>   };
>   typedef struct TCGState TCGState;
>   
> @@ -76,9 +78,11 @@ static void tcg_accel_instance_init(Object *obj)
>   #else
>       s->splitwx_enabled = 0;
>   #endif
> +    s->rr_kick_period = NANOSECONDS_PER_SECOND / 10;
>   }
>   
>   bool one_insn_per_tb;
> +uint64_t rr_kick_period;

(Drop)

>   
>   static int tcg_init_machine(MachineState *ms)
>   {
> @@ -125,6 +129,7 @@ static int tcg_init_machine(MachineState *ms)
>   #endif
>   
>       tcg_allowed = true;
> +    rr_kick_period = s->rr_kick_period;

(Drop)

>   
>       page_init();
>       tb_htable_init();
> @@ -234,6 +239,30 @@ static int tcg_gdbstub_supported_sstep_flags(void)
>       }
>   }

Patch LGTM otherwise.

Regards,

Phil.