[PATCH 2/2] sched,time: call __acct_update_integrals once a jiffy

[riel]

From: Rik van Riel <r...@redhat.com>

Because __acct_update_integrals does nothing unless the time
interval in question exceeds a jiffy, there is no real reason
to call it more than once a jiffy from the syscall, irq, and
guest entry & exit paths.

If tasks get rescheduled frequently, the scheduler will still
update their time statistics normally. This patch only impacts
longer running tasks.

This speeds up

Signed-off-by: Rik van Riel <r...@redhat.com>
---
 include/linux/sched.h  |  1 +
 kernel/sched/cputime.c | 35 +++++++++++++++++++++++++++++------
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a10494a94cc3..019c3af98503 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1532,6 +1532,7 @@ struct task_struct {
        struct prev_cputime prev_cputime;
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
        seqcount_t vtime_seqcount;
+       unsigned long vtime_jiffies;
        unsigned long long vtime_snap;
        enum {
                /* Task is sleeping or running in a CPU with VTIME inactive */
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index b2ab2ffb1adc..923c110319b1 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -668,6 +668,15 @@ void thread_group_cputime_adjusted(struct task_struct *p, 
cputime_t *ut, cputime
 #endif /* !CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
+static bool vtime_jiffies_changed(struct task_struct *tsk, unsigned long now)
+{
+       if (tsk->vtime_jiffies == jiffies)
+               return false;
+
+       tsk->vtime_jiffies = jiffies;
+       return true;
+}
+
 static unsigned long long vtime_delta(struct task_struct *tsk)
 {
        unsigned long long clock;
@@ -699,6 +708,9 @@ static void __vtime_account_system(struct task_struct *tsk)
 
 void vtime_account_system(struct task_struct *tsk)
 {
+       if (!vtime_jiffies_changed(tsk, jiffies))
+               return;
+
        write_seqcount_begin(&tsk->vtime_seqcount);
        __vtime_account_system(tsk);
        write_seqcount_end(&tsk->vtime_seqcount);
@@ -707,7 +719,8 @@ void vtime_account_system(struct task_struct *tsk)
 void vtime_gen_account_irq_exit(struct task_struct *tsk)
 {
        write_seqcount_begin(&tsk->vtime_seqcount);
-       __vtime_account_system(tsk);
+       if (vtime_jiffies_changed(tsk, jiffies))
+               __vtime_account_system(tsk);
        if (context_tracking_in_user())
                tsk->vtime_snap_whence = VTIME_USER;
        write_seqcount_end(&tsk->vtime_seqcount);
@@ -718,16 +731,19 @@ void vtime_account_user(struct task_struct *tsk)
        cputime_t delta_cpu;
 
        write_seqcount_begin(&tsk->vtime_seqcount);
-       delta_cpu = get_vtime_delta(tsk);
        tsk->vtime_snap_whence = VTIME_SYS;
-       account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
+       if (vtime_jiffies_changed(tsk, jiffies)) {
+               delta_cpu = get_vtime_delta(tsk);
+               account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
+       }
        write_seqcount_end(&tsk->vtime_seqcount);
 }
 
 void vtime_user_enter(struct task_struct *tsk)
 {
        write_seqcount_begin(&tsk->vtime_seqcount);
-       __vtime_account_system(tsk);
+       if (vtime_jiffies_changed(tsk, jiffies))
+               __vtime_account_system(tsk);
        tsk->vtime_snap_whence = VTIME_USER;
        write_seqcount_end(&tsk->vtime_seqcount);
 }
@@ -742,7 +758,8 @@ void vtime_guest_enter(struct task_struct *tsk)
         * that can thus safely catch up with a tickless delta.
         */
        write_seqcount_begin(&tsk->vtime_seqcount);
-       __vtime_account_system(tsk);
+       if (vtime_jiffies_changed(tsk, jiffies))
+               __vtime_account_system(tsk);
        current->flags |= PF_VCPU;
        write_seqcount_end(&tsk->vtime_seqcount);
 }
@@ -759,8 +776,12 @@ EXPORT_SYMBOL_GPL(vtime_guest_exit);
 
 void vtime_account_idle(struct task_struct *tsk)
 {
-       cputime_t delta_cpu = get_vtime_delta(tsk);
+       cputime_t delta_cpu;
+
+       if (!vtime_jiffies_changed(tsk, jiffies))
+               return;
 
+       delta_cpu = get_vtime_delta(tsk);
        account_idle_time(delta_cpu);
 }
 
@@ -773,6 +794,7 @@ void arch_vtime_task_switch(struct task_struct *prev)
        write_seqcount_begin(&current->vtime_seqcount);
        current->vtime_snap_whence = VTIME_SYS;
        current->vtime_snap = sched_clock_cpu(smp_processor_id());
+       current->vtime_jiffies = jiffies;
        write_seqcount_end(&current->vtime_seqcount);
 }
 
@@ -784,6 +806,7 @@ void vtime_init_idle(struct task_struct *t, int cpu)
        write_seqcount_begin(&t->vtime_seqcount);
        t->vtime_snap_whence = VTIME_SYS;
        t->vtime_snap = sched_clock_cpu(cpu);
+       t->vtime_jiffies = jiffies;
        write_seqcount_end(&t->vtime_seqcount);
        local_irq_restore(flags);
 }
-- 
2.5.0