sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity

Rik van Riel · Ingo Molnar · commit ff9a9b4c4334 · 2016-02-29T09:53:10.000+01:00
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>
diff --git 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);
 }
