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sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity

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When profiling syscall overhead on nohz-full kernels,
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@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: clark@redhat.com
Cc: eric.dumazet@gmail.com
Cc: fweisbec@gmail.com
Cc: luto@amacapital.net
Link: http://lkml.kernel.org/r/1455152907-18495-5-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Rik van Riel 2016-02-10 20:08:27 -05:00 committed by Ingo Molnar
parent 9344c92c2e
commit ff9a9b4c43
1 changed files with 23 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
kernel/sched/cputime.c
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@ -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);
}