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Commit 25e2d8c1 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Ingo Molnar
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sched/cputime: Fix ksoftirqd cputime accounting regression 



irq_time_read() returns the irqtime minus the ksoftirqd time. This
is necessary because irq_time_read() is used to substract the IRQ time
from the sum_exec_runtime of a task. If we were to include the softirq
time of ksoftirqd, this task would substract its own CPU time everytime
it updates ksoftirqd->sum_exec_runtime which would therefore never
progress.

But this behaviour got broken by:

  a499a5a1 ("sched/cputime: Increment kcpustat directly on irqtime account")

... which now includes ksoftirqd softirq time in the time returned by
irq_time_read().

This has resulted in wrong ksoftirqd cputime reported to userspace
through /proc/stat and thus "top" not showing ksoftirqd when it should
after intense networking load.

ksoftirqd->stime happens to be correct but it gets scaled down by
sum_exec_runtime through task_cputime_adjusted().

To fix this, just account the strict IRQ time in a separate counter and
use it to report the IRQ time.

Reported-and-tested-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Acked-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Link: http://lkml.kernel.org/r/1493129448-5356-1-git-send-email-fweisbec@gmail.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent ea839b41

kernel/sched/cputime.c

+16 −11
Original line number Diff line number Diff line
@@ -34,6 +34,18 @@ void disable_sched_clock_irqtime(void) 
	sched_clock_irqtime = 0;

}



static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,

				  enum cpu_usage_stat idx)

{

	u64 *cpustat = kcpustat_this_cpu->cpustat;



	u64_stats_update_begin(&irqtime->sync);

	cpustat[idx] += delta;

	irqtime->total += delta;

	irqtime->tick_delta += delta;

	u64_stats_update_end(&irqtime->sync);

}



/*

 * Called before incrementing preempt_count on {soft,}irq_enter

 * and before decrementing preempt_count on {soft,}irq_exit.
@@ -41,7 +53,6 @@ void disable_sched_clock_irqtime(void) 
void irqtime_account_irq(struct task_struct *curr)

{

	struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);

	u64 *cpustat = kcpustat_this_cpu->cpustat;

	s64 delta;

	int cpu;
@@ -52,22 +63,16 @@ void irqtime_account_irq(struct task_struct *curr) 
	delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;

	irqtime->irq_start_time += delta;



	u64_stats_update_begin(&irqtime->sync);

	/*

	 * We do not account for softirq time from ksoftirqd here.

	 * We want to continue accounting softirq time to ksoftirqd thread

	 * in that case, so as not to confuse scheduler with a special task

	 * that do not consume any time, but still wants to run.

	 */

	if (hardirq_count()) {

		cpustat[CPUTIME_IRQ] += delta;

		irqtime->tick_delta += delta;

	} else if (in_serving_softirq() && curr != this_cpu_ksoftirqd()) {

		cpustat[CPUTIME_SOFTIRQ] += delta;

		irqtime->tick_delta += delta;

	}



	u64_stats_update_end(&irqtime->sync);

	if (hardirq_count())

		irqtime_account_delta(irqtime, delta, CPUTIME_IRQ);

	else if (in_serving_softirq() && curr != this_cpu_ksoftirqd())

		irqtime_account_delta(irqtime, delta, CPUTIME_SOFTIRQ);

}

EXPORT_SYMBOL_GPL(irqtime_account_irq);

kernel/sched/sched.h

+7 −2
Original line number Diff line number Diff line
@@ -1869,6 +1869,7 @@ static inline void nohz_balance_exit_idle(unsigned int cpu) { } 


#ifdef CONFIG_IRQ_TIME_ACCOUNTING

struct irqtime {

	u64			total;

	u64			tick_delta;

	u64			irq_start_time;

	struct u64_stats_sync	sync;
@@ -1876,16 +1877,20 @@ struct irqtime { 


DECLARE_PER_CPU(struct irqtime, cpu_irqtime);



/*

 * Returns the irqtime minus the softirq time computed by ksoftirqd.

 * Otherwise ksoftirqd's sum_exec_runtime is substracted its own runtime

 * and never move forward.

 */

static inline u64 irq_time_read(int cpu)

{

	struct irqtime *irqtime = &per_cpu(cpu_irqtime, cpu);

	u64 *cpustat = kcpustat_cpu(cpu).cpustat;

	unsigned int seq;

	u64 total;



	do {

		seq = __u64_stats_fetch_begin(&irqtime->sync);

		total = cpustat[CPUTIME_SOFTIRQ] + cpustat[CPUTIME_IRQ];

		total = irqtime->total;

	} while (__u64_stats_fetch_retry(&irqtime->sync, seq));



	return total;