[PATCH 4/4] sched,time: switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity

From: riel
Date: Wed Feb 10 2016 - 20:09:53 EST


From: Rik van Riel <riel@xxxxxxxxxx>

After removing __acct_update_integrals from the profile,
native_sched_clock remains as the top CPU user. This can be
reduced by moving VIRT_CPU_ACCOUNTING_GEN to jiffy
granularity.

This will reduce timing accuracy on nohz_full CPUs to jiffy
based sampling, just like on normal CPUs. It results in
totally removing native_sched_clock from the profile, and
significantly speeding up the syscall entry and exit path,
as well as irq entry and exit, and kvm guest entry & exit.

Additionally, only call the more expensive functions (and
advance the seqlock) when jiffies actually changed.

This code relies on another CPU advancing jiffies when the
system is busy. On a nohz_full system, this is done by a
housekeeping CPU.

A microbenchmark calling an invalid syscall number 10 million
times in a row speeds up an additional 30% over the numbers
with just the previous patches, for a total speedup of about
40% over 4.4 and 4.5-rc1.

Run times for the microbenchmark:

4.4 3.8 seconds
4.5-rc1 3.7 seconds
4.5-rc1 + first patch 3.3 seconds
4.5-rc1 + first 3 patches 3.1 seconds
4.5-rc1 + all patches 2.3 seconds

A non-NOHZ_FULL cpu (not the housekeeping CPU)
all kernels 1.86 seconds

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
---
kernel/sched/cputime.c | 39 +++++++++++++++++++++++----------------
1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index b2ab2ffb1adc..01d9898bc9a2 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -668,26 +668,25 @@ 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 unsigned long long vtime_delta(struct task_struct *tsk)
+static cputime_t vtime_delta(struct task_struct *tsk)
{
- unsigned long long clock;
+ unsigned long now = READ_ONCE(jiffies);

- clock = local_clock();
- if (clock < tsk->vtime_snap)
+ if (time_before(now, (unsigned long)tsk->vtime_snap))
return 0;

- return clock - tsk->vtime_snap;
+ return jiffies_to_cputime(now - tsk->vtime_snap);
}

static cputime_t get_vtime_delta(struct task_struct *tsk)
{
- unsigned long long delta = vtime_delta(tsk);
+ unsigned long now = READ_ONCE(jiffies);
+ unsigned long delta = now - tsk->vtime_snap;

WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
- tsk->vtime_snap += delta;
+ tsk->vtime_snap = now;

- /* CHECKME: always safe to convert nsecs to cputime? */
- return nsecs_to_cputime(delta);
+ return jiffies_to_cputime(delta);
}

static void __vtime_account_system(struct task_struct *tsk)
@@ -699,6 +698,9 @@ static void __vtime_account_system(struct task_struct *tsk)

void vtime_account_system(struct task_struct *tsk)
{
+ if (!vtime_delta(tsk))
+ return;
+
write_seqcount_begin(&tsk->vtime_seqcount);
__vtime_account_system(tsk);
write_seqcount_end(&tsk->vtime_seqcount);
@@ -707,7 +709,8 @@ 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_delta(tsk))
+ __vtime_account_system(tsk);
if (context_tracking_in_user())
tsk->vtime_snap_whence = VTIME_USER;
write_seqcount_end(&tsk->vtime_seqcount);
@@ -718,16 +721,19 @@ 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_delta(tsk)) {
+ 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_delta(tsk))
+ __vtime_account_system(tsk);
tsk->vtime_snap_whence = VTIME_USER;
write_seqcount_end(&tsk->vtime_seqcount);
}
@@ -742,7 +748,8 @@ 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_delta(tsk))
+ __vtime_account_system(tsk);
current->flags |= PF_VCPU;
write_seqcount_end(&tsk->vtime_seqcount);
}
@@ -772,7 +779,7 @@ 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_snap = jiffies;
write_seqcount_end(&current->vtime_seqcount);
}

@@ -783,7 +790,7 @@ void vtime_init_idle(struct task_struct *t, int cpu)
local_irq_save(flags);
write_seqcount_begin(&t->vtime_seqcount);
t->vtime_snap_whence = VTIME_SYS;
- t->vtime_snap = sched_clock_cpu(cpu);
+ t->vtime_snap = jiffies;
write_seqcount_end(&t->vtime_seqcount);
local_irq_restore(flags);
}
--
2.5.0