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Date: Thu, 3 Oct 2019 18:17:45 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Wanpeng Li <wanpengli@...cent.com>,
Thomas Gleixner <tglx@...utronix.de>,
Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
Rik van Riel <riel@...hat.com>
Subject: [PATCH 2/2] vtime: Spare a seqcount lock/unlock cycle on context switch
On context switch we are locking the vtime seqcount of the scheduling-out
task twice:
* On vtime_task_switch_common(), when we flush the pending vtime through
vtime_account_system()
* On arch_vtime_task_switch() to reset the vtime state.
This is pointless as these actions can be performed without the need
to unlock/lock in the middle. The reason these steps are separated is to
consolidate a very small amount of common code between
CONFIG_VIRT_CPU_ACCOUNTING_GEN and CONFIG_VIRT_CPU_ACCOUNTING_NATIVE.
Performance in this fast path is definetly a priority over artificial
code factorization so split the task switch code between GEN and
NATIVE and mutualize the parts than can run under a single seqcount
locked block.
As a side effect, vtime_account_idle() becomes included in the seqcount
protection. This happens to be a welcome preparation in order to
properly support kcpustat under vtime in the future and fetch
CPUTIME_IDLE without race.
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Yauheni Kaliuta <yauheni.kaliuta@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Rik van Riel <riel@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Wanpeng Li <wanpengli@...cent.com>
Cc: Ingo Molnar <mingo@...nel.org>
---
include/linux/vtime.h | 32 ++++++++++++++++----------------
kernel/sched/cputime.c | 34 +++++++++++++++++++++-------------
2 files changed, 37 insertions(+), 29 deletions(-)
diff --git a/include/linux/vtime.h b/include/linux/vtime.h
index 2fd247f90408..d9160ab3667a 100644
--- a/include/linux/vtime.h
+++ b/include/linux/vtime.h
@@ -14,8 +14,12 @@ struct task_struct;
* vtime_accounting_cpu_enabled() definitions/declarations
*/
#if defined(CONFIG_VIRT_CPU_ACCOUNTING_NATIVE)
+
static inline bool vtime_accounting_cpu_enabled(void) { return true; }
+extern void vtime_task_switch(struct task_struct *prev);
+
#elif defined(CONFIG_VIRT_CPU_ACCOUNTING_GEN)
+
/*
* Checks if vtime is enabled on some CPU. Cputime readers want to be careful
* in that case and compute the tickless cputime.
@@ -36,33 +40,29 @@ static inline bool vtime_accounting_cpu_enabled(void)
return false;
}
+
+extern void vtime_task_switch_generic(struct task_struct *prev);
+
+static inline void vtime_task_switch(struct task_struct *prev)
+{
+ if (vtime_accounting_cpu_enabled())
+ vtime_task_switch_generic(prev);
+}
+
#else /* !CONFIG_VIRT_CPU_ACCOUNTING */
+
static inline bool vtime_accounting_cpu_enabled(void) { return false; }
-#endif
+static inline void vtime_task_switch(struct task_struct *prev) { }
+#endif
/*
* Common vtime APIs
*/
#ifdef CONFIG_VIRT_CPU_ACCOUNTING
-
-#ifdef __ARCH_HAS_VTIME_TASK_SWITCH
-extern void vtime_task_switch(struct task_struct *prev);
-#else
-extern void vtime_common_task_switch(struct task_struct *prev);
-static inline void vtime_task_switch(struct task_struct *prev)
-{
- if (vtime_accounting_cpu_enabled())
- vtime_common_task_switch(prev);
-}
-#endif /* __ARCH_HAS_VTIME_TASK_SWITCH */
-
extern void vtime_account_kernel(struct task_struct *tsk);
extern void vtime_account_idle(struct task_struct *tsk);
-
#else /* !CONFIG_VIRT_CPU_ACCOUNTING */
-
-static inline void vtime_task_switch(struct task_struct *prev) { }
static inline void vtime_account_kernel(struct task_struct *tsk) { }
#endif /* !CONFIG_VIRT_CPU_ACCOUNTING */
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index b45932e27857..cef23c211f41 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -405,9 +405,10 @@ static inline void irqtime_account_process_tick(struct task_struct *p, int user_
/*
* Use precise platform statistics if available:
*/
-#ifdef CONFIG_VIRT_CPU_ACCOUNTING
+#ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
+
# ifndef __ARCH_HAS_VTIME_TASK_SWITCH
-void vtime_common_task_switch(struct task_struct *prev)
+void vtime_task_switch(struct task_struct *prev)
{
if (is_idle_task(prev))
vtime_account_idle(prev);
@@ -418,10 +419,7 @@ void vtime_common_task_switch(struct task_struct *prev)
arch_vtime_task_switch(prev);
}
# endif
-#endif /* CONFIG_VIRT_CPU_ACCOUNTING */
-
-#ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
/*
* Archs that account the whole time spent in the idle task
* (outside irq) as idle time can rely on this and just implement
@@ -731,19 +729,25 @@ static void vtime_account_guest(struct task_struct *tsk,
}
}
-void vtime_account_kernel(struct task_struct *tsk)
+static void __vtime_account_kernel(struct task_struct *tsk,
+ struct vtime *vtime)
{
- struct vtime *vtime = &tsk->vtime;
-
- if (!vtime_delta(vtime))
- return;
-
- write_seqcount_begin(&vtime->seqcount);
/* We might have scheduled out from guest path */
if (tsk->flags & PF_VCPU)
vtime_account_guest(tsk, vtime);
else
vtime_account_system(tsk, vtime);
+}
+
+void vtime_account_kernel(struct task_struct *tsk)
+{
+ struct vtime *vtime = &tsk->vtime;
+
+ if (!vtime_delta(vtime))
+ return;
+
+ write_seqcount_begin(&vtime->seqcount);
+ __vtime_account_kernel(tsk, vtime);
write_seqcount_end(&vtime->seqcount);
}
@@ -804,11 +808,15 @@ void vtime_account_idle(struct task_struct *tsk)
account_idle_time(get_vtime_delta(&tsk->vtime));
}
-void arch_vtime_task_switch(struct task_struct *prev)
+void vtime_task_switch_generic(struct task_struct *prev)
{
struct vtime *vtime = &prev->vtime;
write_seqcount_begin(&vtime->seqcount);
+ if (is_idle_task(prev))
+ vtime_account_idle(prev);
+ else
+ __vtime_account_kernel(prev, vtime);
vtime->state = VTIME_INACTIVE;
write_seqcount_end(&vtime->seqcount);
--
2.23.0
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