Re: Stolen and degraded time and schedulers

Dan Hecht
2007-03-14T20:47:07Z

On 03/14/2007 01:31 PM, Jeremy Fitzhardinge wrote:
> Dan Hecht wrote:
>> Sounds good. I don't see this in your patchset you sent yesterday
>> though; did you add it after sending out those patches?
>
> Yes.
>
>> if so, could you forward the new patch? does it explicitly prevent
>> stolen time from getting accounted as user/system time or does it
>> just rely on NO_HZ mode sort of happening to work that way (since the
>> one shot timer is skipped ahead for missed ticks)?
>
> Hm, not sure. It doesn't care how often it gets called; it just
> accumulates results up to that point, but I'm not sure if the time would
> get double accounted. Perhaps it doesn't matter when using
> xen_sched_clock().
>

I think you might be double counting time in some cases. sched_clock()
isn't really relevant to stolen time accounting (i.e. cpustat->steal).

I think what you want is to make sure that the sum of the cputime passed
to all of:

account_user_time
account_system_time
account_steal_time

adds up to the total amount of time that has passed. I think it is sort
of working for you (i.e. doesn't always double count stolen ticks) since
in NO_HZ mode, update_process_time (which calls account_user_time &
account_system_time) happens to be skipped during periods of stolen time
due to the hrtimer_forward()'ing of the one shot expiry.

> Did the get_scheduled_time -> sched_clock make sense to you?
>

The get_scheduled_time change should work fine for vmi.

> J
>
>
> ------------------------------------------------------------------------
>
> Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
> Cc: john stultz <johnstul@us.ibm.com>
>
> ---
> arch/i386/xen/time.c | 101 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 101 insertions(+)
>
> ===================================================================
> --- a/arch/i386/xen/time.c
> +++ b/arch/i386/xen/time.c
> @@ -2,6 +2,7 @@
> #include <linux/interrupt.h>
> #include <linux/clocksource.h>
> #include <linux/clockchips.h>
> +#include <linux/kernel_stat.h>
>
> #include <asm/xen/hypervisor.h>
> #include <asm/xen/hypercall.h>
> @@ -14,6 +15,7 @@
>
> #define XEN_SHIFT 22
> #define TIMER_SLOP 100000 /* Xen may fire a timer up to this many ns early */
> +#define NS_PER_TICK (1000000000ll / HZ)
>
> static DEFINE_PER_CPU(struct clock_event_device, xen_clock_events);
>
> @@ -28,6 +30,99 @@ struct shadow_time_info {
>
> static DEFINE_PER_CPU(struct shadow_time_info, shadow_time);
>
> +/* runstate info updated by Xen */
> +static DEFINE_PER_CPU(struct vcpu_runstate_info, runstate);
> +
> +/* snapshots of runstate info */
> +static DEFINE_PER_CPU(struct vcpu_runstate_info, runstate_snapshot);
> +
> +/* unused ns of stolen and blocked time */
> +static DEFINE_PER_CPU(u64, residual_stolen);
> +static DEFINE_PER_CPU(u64, residual_blocked);
> +
> +/*
> + Runstate accounting
> + */
> +static void get_runstate_snapshot(struct vcpu_runstate_info *res)
> +{
> + u64 state_time;
> + struct vcpu_runstate_info *state;
> +
> + preempt_disable();
> +
> + state = &__get_cpu_var(runstate);
> +
> + do {
> + state_time = state->state_entry_time;
> + barrier();
> + *res = *state;
> + barrier();
> + } while(state->state_entry_time != state_time);
> +
> + preempt_enable();
> +}
> +
> +static void setup_runstate_info(void)
> +{
> + struct vcpu_register_runstate_memory_area area;
> +
> + area.addr.v = &__get_cpu_var(runstate);
> +
> + if (HYPERVISOR_vcpu_op(VCPUOP_register_runstate_memory_area,
> + smp_processor_id(), &area))
> + BUG();
> +
> + get_runstate_snapshot(&__get_cpu_var(runstate_snapshot));
> +}
> +
> +static void do_stolen_accounting(void)
> +{
> + struct vcpu_runstate_info state;
> + struct vcpu_runstate_info *snap;
> + u64 blocked, runnable, offline, stolen;
> + cputime_t ticks;
> +
> + get_runstate_snapshot(&state);
> +
> + WARN_ON(state.state != RUNSTATE_running);
> +
> + snap = &__get_cpu_var(runstate_snapshot);
> +
> + /* work out how much time the VCPU has not been runn*ing* */
> + blocked = state.time[RUNSTATE_blocked] - snap->time[RUNSTATE_blocked];
> + runnable = state.time[RUNSTATE_runnable] - snap->time[RUNSTATE_runnable];
> + offline = state.time[RUNSTATE_offline] - snap->time[RUNSTATE_offline];
> +
> + *snap = state;
> +
> + /* Add the appropriate number of ticks of stolen time,
> + including any left-overs from last time. Passing NULL to
> + account_steal_time accounts the time as stolen. */
> + stolen = runnable + offline + __get_cpu_var(residual_stolen);
> + ticks = 0;
> + while(stolen >= NS_PER_TICK) {
> + ticks++;
> + stolen -= NS_PER_TICK;
> + }
> + __get_cpu_var(residual_stolen) = stolen;
> + account_steal_time(NULL, ticks);
> +
> + /* Add the appropriate number of ticks of blocked time,
> + including any left-overs from last time. Passing idle to
> + account_steal_time accounts the time as idle/wait. */
> + blocked += __get_cpu_var(residual_blocked);
> + ticks = 0;
> + while(blocked >= NS_PER_TICK) {
> + ticks++;
> + blocked -= NS_PER_TICK;
> + }
> + __get_cpu_var(residual_blocked) = blocked;
> + account_steal_time(idle_task(smp_processor_id()), ticks);
> +}
> +
> +
> +
> +/* Get the CPU speed from Xen */
> unsigned long xen_cpu_khz(void)
> {
> u64 cpu_khz = 1000000ULL << 32;
> @@ -264,6 +359,8 @@ static irqreturn_t xen_timerop_timer_int
> ret = IRQ_HANDLED;
> }
>
> + do_stolen_accounting();
> +
> return ret;
> }
>
> @@ -338,6 +435,8 @@ static irqreturn_t xen_vcpuop_timer_inte
> ret = IRQ_HANDLED;
> }
>
> + do_stolen_accounting();
> +
> return ret;
> }
>
> @@ -380,6 +479,8 @@ static void xen_setup_timer(int cpu)
> evt->cpumask = cpumask_of_cpu(cpu);
> evt->irq = irq;
> clockevents_register_device(evt);
> +
> + setup_runstate_info();
>
> put_cpu_var(xen_clock_events);
> }
>
>
> ------------------------------------------------------------------------
>
> Subject: Implement xen_sched_clock
>
> Implement xen_sched_clock, which returns the number of ns the current
> vcpu has been actually in the running state (vs blocked,
> runnable-but-not-running, or offline) since boot.
>
> Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
> Cc: john stultz <johnstul@us.ibm.com>
>
> ---
> arch/i386/xen/enlighten.c | 2 +-
> arch/i386/xen/time.c | 14 ++++++++++++++
> arch/i386/xen/xen-ops.h | 1 +
> 3 files changed, 16 insertions(+), 1 deletion(-)
>
> ===================================================================
> --- a/arch/i386/xen/enlighten.c
> +++ b/arch/i386/xen/enlighten.c
> @@ -664,7 +664,7 @@ static const struct paravirt_ops xen_par
> .set_wallclock = xen_set_wallclock,
> .get_wallclock = xen_get_wallclock,
> .get_cpu_khz = xen_cpu_khz,
> - .get_scheduled_cycles = native_read_tsc,
> + .sched_clock = xen_sched_clock,
>
> #ifdef CONFIG_X86_LOCAL_APIC
> .apic_write = paravirt_nop,
> ===================================================================
> --- a/arch/i386/xen/time.c
> +++ b/arch/i386/xen/time.c
> @@ -16,6 +16,8 @@
> #define XEN_SHIFT 22
> #define TIMER_SLOP 100000 /* Xen may fire a timer up to this many ns early */
> #define NS_PER_TICK (1000000000ll / HZ)
> +
> +static cycle_t xen_clocksource_read(void);
>
> static DEFINE_PER_CPU(struct clock_event_device, xen_clock_events);
>
> @@ -120,6 +122,18 @@ static void do_stolen_accounting(void)
> account_steal_time(idle_task(smp_processor_id()), ticks);
> }
>
> +/* Xen sched_clock implementation. Returns the number of RUNNING ns */
> +unsigned long long xen_sched_clock(void)
> +{
> + struct vcpu_runstate_info state;
> + cycle_t now = xen_clocksource_read();
> +
> + get_runstate_snapshot(&state);
> +
> + WARN_ON(state.state != RUNSTATE_running);
> +
> + return state.time[RUNSTATE_running] + (now - state.state_entry_time);
> +}
>
>
> /* Get the CPU speed from Xen */
> ===================================================================
> --- a/arch/i386/xen/xen-ops.h
> +++ b/arch/i386/xen/xen-ops.h
> @@ -14,6 +14,7 @@ void __init xen_time_init(void);
> void __init xen_time_init(void);
> unsigned long xen_get_wallclock(void);
> int xen_set_wallclock(unsigned long time);
> +unsigned long long xen_sched_clock(void);
>
> void xen_mark_init_mm_pinned(void);
>