Commit ff9a9b4c authored by Rik van Riel's avatar Rik van Riel Committed by Ingo Molnar
Browse files

sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity 



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: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarThomas 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: default avatarIngo Molnar <mingo@kernel.org>
parent 9344c92c

kernel/sched/cputime.c

+23 −16
Original line number Diff line number Diff line
@@ -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);
@@ -707,6 +709,7 @@ void vtime_account_system(struct task_struct *tsk) 
void vtime_gen_account_irq_exit(struct task_struct *tsk)

{

	write_seqcount_begin(&tsk->vtime_seqcount);

	if (vtime_delta(tsk))

		__vtime_account_system(tsk);

	if (context_tracking_in_user())

		tsk->vtime_snap_whence = VTIME_USER;
@@ -718,15 +721,18 @@ 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;

	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);

	if (vtime_delta(tsk))

		__vtime_account_system(tsk);

	tsk->vtime_snap_whence = VTIME_USER;

	write_seqcount_end(&tsk->vtime_seqcount);
@@ -742,6 +748,7 @@ void vtime_guest_enter(struct task_struct *tsk) 
	 * that can thus safely catch up with a tickless delta.

	 */

	write_seqcount_begin(&tsk->vtime_seqcount);

	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);

}

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