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From: Frederic Weisbecker <frederic@kernel.org>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Wanpeng Li <wanpengli@tencent.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Yauheni Kaliuta <yauheni.kaliuta@redhat.com>,
	Ingo Molnar <mingo@kernel.org>, Rik van Riel <riel@redhat.com>
Subject: [PATCH 09/25] kcpustat: Track running task following vtime sequences
Date: Wed, 14 Nov 2018 03:45:53 +0100	[thread overview]
Message-ID: <1542163569-20047-10-git-send-email-frederic@kernel.org> (raw)
In-Reply-To: <1542163569-20047-1-git-send-email-frederic@kernel.org>

In order to make kcpustat vtime aware (ie: work on nohz_full without
freezing), we need to track the task running on the CPU in order to
fetch its vtime delta and add it to the relevant kcpustat field.

The most efficient way to track this task is to use RCU. The task is
assigned on context switch right after we flush the vtime of the previous
task and the next task has been set on vtime.

Things are then prepared to be ordered that way:

             WRITER (ctx switch)                READER
             ------------------            -----------------------
        vtime_seqcount_write_lock(prev)     rcu_read_lock()
        //flush prev vtime                  curr = rcu_dereference(kcpustat->curr)
        vtime_seqcount_write_unlock(prev)   vtime_seqcount_read_start(curr)
                                            //fetch curr vtime
        vtime_seqcount_lock(next)           vtime_seqcount_read_end(curr)
        //Init vtime                        rcu_read_unlock()
        vtime_seqcount_unlock(next)

        rcu_assign_pointer(kcpustat->curr, next)

With this ordering layout, we are sure that we get a sequence with a
coherent couple (task cputime, kcpustat).

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 include/linux/kernel_stat.h |  1 +
 kernel/sched/cputime.c      | 11 ++++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index 7ee2bb4..86fdbce 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -32,6 +32,7 @@ enum cpu_usage_stat {
 };
 
 struct kernel_cpustat {
+	struct task_struct __rcu *curr;
 	u64 cpustat[NR_STATS];
 };
 
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index a0c3a82..2eb313a 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -812,6 +812,7 @@ void vtime_account_idle(struct task_struct *tsk)
 void vtime_task_switch_generic(struct task_struct *prev)
 {
 	struct vtime *vtime = &prev->vtime;
+	struct kernel_cpustat *kcpustat = kcpustat_this_cpu;
 
 	/*
 	 * Flush the prev task vtime, unless it has passed
@@ -835,8 +836,10 @@ void vtime_task_switch_generic(struct task_struct *prev)
 	 * Ignore the next task if it has been preempted after
 	 * vtime_exit_task().
 	 */
-	if (vtime->state == VTIME_DEAD)
+	if (vtime->state == VTIME_DEAD) {
+		rcu_assign_pointer(kcpustat->curr, NULL);
 		return;
+	}
 
 	write_seqcount_begin(&vtime->seqcount);
 	if (is_idle_task(current))
@@ -848,10 +851,13 @@ void vtime_task_switch_generic(struct task_struct *prev)
 	vtime->starttime = sched_clock();
 	vtime->cpu = smp_processor_id();
 	write_seqcount_end(&vtime->seqcount);
+
+	rcu_assign_pointer(kcpustat->curr, current);
 }
 
 void vtime_init_idle(struct task_struct *t, int cpu)
 {
+	struct kernel_cpustat *kcpustat = &kcpustat_cpu(cpu);
 	struct vtime *vtime = &t->vtime;
 	unsigned long flags;
 
@@ -862,6 +868,8 @@ void vtime_init_idle(struct task_struct *t, int cpu)
 	vtime->cpu = cpu;
 	write_seqcount_end(&vtime->seqcount);
 	local_irq_restore(flags);
+
+	rcu_assign_pointer(kcpustat->curr, t);
 }
 
 /*
@@ -885,6 +893,7 @@ void vtime_exit_task(struct task_struct *t)
 	vtime->state = VTIME_DEAD;
 	vtime->cpu = -1;
 	write_seqcount_end(&vtime->seqcount);
+	rcu_assign_pointer(kcpustat_this_cpu->curr, NULL);
 	local_irq_restore(flags);
 }
 
-- 
2.7.4


  parent reply	other threads:[~2018-11-14  2:46 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-14  2:45 [PATCH 00/25] sched/nohz: Make kcpustat vtime aware (Fix kcpustat on nohz_full) Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 01/25] sched/vtime: Fix guest/system mis-accounting on task switch Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 02/25] sched/vtime: Protect idle accounting under vtime seqcount Frederic Weisbecker
2018-11-20 13:19   ` Peter Zijlstra
2018-11-14  2:45 ` [PATCH 03/25] vtime: Rename vtime_account_system() to vtime_account_kernel() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 04/25] vtime: Spare a seqcount lock/unlock cycle on context switch Frederic Weisbecker
2018-11-20 13:25   ` Peter Zijlstra
2019-09-25 14:42     ` Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 05/25] sched/vtime: Record CPU under seqcount for kcpustat needs Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 06/25] sched/cputime: Add vtime idle task state Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 07/25] sched/cputime: Add vtime guest " Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 08/25] vtime: Exit vtime before exit_notify() Frederic Weisbecker
2018-11-20 13:54   ` Peter Zijlstra
2018-11-14  2:45 ` Frederic Weisbecker [this message]
2018-11-20 13:58   ` [PATCH 09/25] kcpustat: Track running task following vtime sequences Peter Zijlstra
2018-11-14  2:45 ` [PATCH 10/25] context_tracking: Remove context_tracking_active() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 11/25] context_tracking: s/context_tracking_is_enabled/context_tracking_enabled() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 12/25] context_tracking: Rename context_tracking_is_cpu_enabled() to context_tracking_enabled_this_cpu() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 13/25] context_tracking: Introduce context_tracking_enabled_cpu() Frederic Weisbecker
2018-11-20 14:02   ` Peter Zijlstra
2018-11-14  2:45 ` [PATCH 14/25] sched/vtime: Rename vtime_accounting_cpu_enabled() to vtime_accounting_enabled_this_cpu() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 15/25] sched/vtime: Introduce vtime_accounting_enabled_cpu() Frederic Weisbecker
2018-11-20 14:04   ` Peter Zijlstra
2018-11-14  2:46 ` [PATCH 16/25] sched/cputime: Allow to pass cputime index on user/guest accounting Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 17/25] sched/cputime: Standardize the kcpustat index based accounting functions Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 18/25] vtime: Track nice-ness on top of context switch Frederic Weisbecker
2018-11-20 14:09   ` Peter Zijlstra
2018-11-14  2:46 ` [PATCH 19/25] sched/vite: Handle nice updates under vtime Frederic Weisbecker
2018-11-20 14:17   ` Peter Zijlstra
2018-11-26 15:53     ` Frederic Weisbecker
2018-11-26 16:11       ` Peter Zijlstra
2018-11-26 18:41         ` Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 20/25] sched/kcpustat: Introduce vtime-aware kcpustat accessor Frederic Weisbecker
2018-11-20 14:23   ` Peter Zijlstra
2018-11-20 22:40     ` Frederic Weisbecker
2018-11-21  8:18       ` Peter Zijlstra
2018-11-21  8:35         ` Peter Zijlstra
2018-11-21 16:33         ` Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 21/25] procfs: Use vtime aware " Frederic Weisbecker
2018-11-20 14:24   ` Peter Zijlstra
2018-11-20 22:31     ` Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 22/25] cpufreq: " Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 23/25] leds: Use vtime aware kcpustat accessors Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 24/25] rackmeter: " Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 25/25] sched/vtime: Clarify vtime_task_switch() argument layout Frederic Weisbecker

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