Hi Philippe,
Thank you for your response.
Since TCGState is currently defined only in tcg-all.c, we’re unable to access
rr_kick_delay_ns from other .c files through the struct.
One potential approach would be to introduce a tcg-all.h header and move the
TCGState definition there. This would allow us to access the struct in other
components by including the header and using TCG_STATE(current_accel()).
Would you be open to this direction, or do you have any alternative
suggestions?
Thanks,
Ethan
On Thu, Jun 05, 2025 at 09:21:55AM +0200, Philippe Mathieu-Daudé wrote:
> 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 <etha...@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.
