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SubjectRe: [PATCH] sched: Fix the irqtime code to deal with u64 wraps
On Fri, 10 Dec 2010 21:38:26 +0100, Peter Zijlstra <peterz@infradead.org> wrote:
> OK, so here's the latest version, using fancy __this_cpu thingies.
>
> I started a new thread since the old one was quite unwieldy.
>
> Now, admittedly this patch is a tad large, esp for -rc5. So either we
> need lots of Reviewed-by and such or I need to shrink this patch
> somehow.
>
>
> ---
> Subject: sched: Fix the irqtime code to deal with u64 wraps
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Thu Dec 09 14:15:34 CET 2010
>
> ARM systems have a 32bit sched_clock() [ which needs to be fixed ],
> but this exposed a bug in the irq_time code as well, it doesn't deal
> with wraps at all.
>
> Fix the irq_time code to deal with u64 wraps by re-writing the code to
> only use delta increments, which avoids the whole issue.
>
> Furthermore, solve the problem of 32bit arches reading partial updates
> of the u64 time values.
>
> Cc: Venkatesh Pallipadi <venki@google.com>
> Reported-by: Mikael Pettersson <mikpe@it.uu.se>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Tested 2.6.37-rc5 + this patch on ARM/mach-ixp4xx and ARM/mach-iop32x,
and it did solve the previously reported interactivity problems. Thanks.

Tested-by: Mikael Pettersson <mikpe@it.uu.se>

I did however have to apply the following to avoid a compilation error:

--- linux-2.6.37-rc5/kernel/sched.c.~1~ 2010-12-12 13:03:07.000000000 +0100
+++ linux-2.6.37-rc5/kernel/sched.c 2010-12-12 13:05:40.000000000 +0100
@@ -636,6 +636,8 @@ static inline struct task_group *task_gr

#endif /* CONFIG_CGROUP_SCHED */

+static void update_rq_clock_task(struct rq *rq, s64 delta);
+
static void update_rq_clock(struct rq *rq)
{
s64 delta;
> ---
> kernel/sched.c | 176 +++++++++++++++++++++++++++++++++++++++------------------
> 1 file changed, 121 insertions(+), 55 deletions(-)
>
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -636,22 +636,16 @@ static inline struct task_group *task_gr
>
> #endif /* CONFIG_CGROUP_SCHED */
>
> -static u64 irq_time_cpu(int cpu);
> -static void sched_irq_time_avg_update(struct rq *rq, u64 irq_time);
> -
> -inline void update_rq_clock(struct rq *rq)
> +static void update_rq_clock(struct rq *rq)
> {
> - if (!rq->skip_clock_update) {
> - int cpu = cpu_of(rq);
> - u64 irq_time;
> + s64 delta;
>
> - rq->clock = sched_clock_cpu(cpu);
> - irq_time = irq_time_cpu(cpu);
> - if (rq->clock - irq_time > rq->clock_task)
> - rq->clock_task = rq->clock - irq_time;
> + if (rq->skip_clock_update)
> + return;
>
> - sched_irq_time_avg_update(rq, irq_time);
> - }
> + delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
> + rq->clock += delta;
> + update_rq_clock_task(rq, delta);
> }
>
> /*
> @@ -1918,90 +1912,162 @@ static void deactivate_task(struct rq *r
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
>
> /*
> - * There are no locks covering percpu hardirq/softirq time.
> - * They are only modified in account_system_vtime, on corresponding CPU
> - * with interrupts disabled. So, writes are safe.
> + * There are no locks covering percpu hardirq/softirq time. They are only
> + * modified in account_system_vtime, on corresponding CPU with interrupts
> + * disabled. So, writes are safe.
> + *
> * They are read and saved off onto struct rq in update_rq_clock().
> - * This may result in other CPU reading this CPU's irq time and can
> - * race with irq/account_system_vtime on this CPU. We would either get old
> - * or new value (or semi updated value on 32 bit) with a side effect of
> - * accounting a slice of irq time to wrong task when irq is in progress
> - * while we read rq->clock. That is a worthy compromise in place of having
> - * locks on each irq in account_system_time.
> + *
> + * This may result in other CPU reading this CPU's irq time and can race with
> + * irq/account_system_vtime on this CPU. We would either get old or new value
> + * with a side effect of accounting a slice of irq time to wrong task when irq
> + * is in progress while we read rq->clock. That is a worthy compromise in place
> + * of having locks on each irq in account_system_time.
> */
> static DEFINE_PER_CPU(u64, cpu_hardirq_time);
> static DEFINE_PER_CPU(u64, cpu_softirq_time);
> -
> static DEFINE_PER_CPU(u64, irq_start_time);
> -static int sched_clock_irqtime;
>
> -void enable_sched_clock_irqtime(void)
> +#ifndef CONFIG_64BIT
> +static DEFINE_PER_CPU(seqcount_t, irq_time_seq);
> +
> +static inline void irq_time_write_begin(void)
> {
> - sched_clock_irqtime = 1;
> + __this_cpu_inc(irq_time_seq.sequence);
> + smp_wmb();
> }
>
> -void disable_sched_clock_irqtime(void)
> +static inline void irq_time_write_end(void)
> {
> - sched_clock_irqtime = 0;
> + smp_wmb();
> + __this_cpu_inc(irq_time_seq.sequence);
> }
>
> -static u64 irq_time_cpu(int cpu)
> +static inline u64 irq_time_read(int cpu)
> {
> - if (!sched_clock_irqtime)
> - return 0;
> + u64 irq_time;
> + unsigned seq;
> +
> + do {
> + seq = read_seqcount_begin(&per_cpu(irq_time_seq, cpu));
> + irq_time = per_cpu(cpu_softirq_time, cpu) +
> + per_cpu(cpu_hardirq_time, cpu);
> + } while (read_seqcount_retry(&per_cpu(irq_time_seq, cpu), seq));
> +
> + return irq_time;
> +}
> +#else /* CONFIG_64BIT */
> +static inline void irq_time_write_begin(void)
> +{
> +}
>
> +static inline void irq_time_write_end(void)
> +{
> +}
> +
> +static inline u64 irq_time_read(int cpu)
> +{
> return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
> }
> +#endif /* CONFIG_64BIT */
> +
> +static int sched_clock_irqtime;
> +
> +void enable_sched_clock_irqtime(void)
> +{
> + sched_clock_irqtime = 1;
> +}
> +
> +void disable_sched_clock_irqtime(void)
> +{
> + sched_clock_irqtime = 0;
> +}
>
> +/*
> + * Called before incrementing preempt_count on {soft,}irq_enter
> + * and before decrementing preempt_count on {soft,}irq_exit.
> + */
> void account_system_vtime(struct task_struct *curr)
> {
> unsigned long flags;
> + s64 delta;
> int cpu;
> - u64 now, delta;
>
> if (!sched_clock_irqtime)
> return;
>
> local_irq_save(flags);
> -
> cpu = smp_processor_id();
> - now = sched_clock_cpu(cpu);
> - delta = now - per_cpu(irq_start_time, cpu);
> - per_cpu(irq_start_time, cpu) = now;
> + delta = sched_clock_cpu(cpu) - __this_cpu_read(irq_start_time);
> + __this_cpu_add(irq_start_time, delta);
> +
> + irq_time_write_begin();
> +
> + if (hardirq_count())
> + __this_cpu_add(cpu_hardirq_time, delta);
> /*
> - * We do not account for softirq time from ksoftirqd here.
> - * We want to continue accounting softirq time to ksoftirqd thread
> - * in that case, so as not to confuse scheduler with a special task
> - * that do not consume any time, but still wants to run.
> + * We do not account for softirq time from ksoftirqd here. We want to
> + * continue accounting softirq time to ksoftirqd thread in that case,
> + * so as not to confuse scheduler with a special task that do not
> + * consume any time, but still wants to run.
> */
> - if (hardirq_count())
> - per_cpu(cpu_hardirq_time, cpu) += delta;
> else if (in_serving_softirq() && !(curr->flags & PF_KSOFTIRQD))
> - per_cpu(cpu_softirq_time, cpu) += delta;
> + __this_cpu_add(cpu_softirq_time, delta);
>
> + irq_time_write_end();
> local_irq_restore(flags);
> }
> EXPORT_SYMBOL_GPL(account_system_vtime);
>
> -static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time)
> +static u64 irq_time_cpu(struct rq *rq)
> {
> - if (sched_clock_irqtime && sched_feat(NONIRQ_POWER)) {
> - u64 delta_irq = curr_irq_time - rq->prev_irq_time;
> - rq->prev_irq_time = curr_irq_time;
> - sched_rt_avg_update(rq, delta_irq);
> - }
> + /*
> + * See the comment in update_rq_clock_task(), ideally we'd update
> + * the *irq_time values using rq->clock here.
> + */
> + return irq_time_read(cpu_of(rq));
> }
>
> -#else
> -
> -static u64 irq_time_cpu(int cpu)
> +static void update_rq_clock_task(struct rq *rq, s64 delta)
> {
> - return 0;
> + s64 irq_delta;
> +
> + irq_delta = irq_time_cpu(rq) - rq->prev_irq_time;
> +
> + /*
> + * Since irq_time is only updated on {soft,}irq_exit, we might run into
> + * this case when a previous update_rq_clock() happened inside a
> + * {soft,}irq region.
> + *
> + * When this happens, we stop ->clock_task and only update the
> + * prev_irq_time stamp to account for the part that fit, so that a next
> + * update will consume the rest. This ensures ->clock_task is
> + * monotonic.
> + *
> + * It does however cause some slight miss-attribution of {soft,}irq
> + * time, a more accurate solution would be to update the irq_time using
> + * the current rq->clock timestamp, except that would require using
> + * atomic ops.
> + */
> + if (irq_delta > delta)
> + irq_delta = delta;
> +
> + rq->prev_irq_time += irq_delta;
> + delta -= irq_delta;
> + rq->clock_task += delta;
> +
> + if (irq_delta && sched_feat(NONIRQ_POWER))
> + sched_rt_avg_update(rq, irq_delta);
> }
>
> -static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time) { }
> +#else /* CONFIG_IRQ_TIME_ACCOUNTING */
>
> -#endif
> +static void update_rq_clock_task(struct rq *rq, s64 delta)
> +{
> + rq->clock_task += delta;
> +}
> +
> +#endif /* CONFIG_IRQ_TIME_ACCOUNTING */
>
> #include "sched_idletask.c"
> #include "sched_fair.c"


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