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:

  a499a5a14db ("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: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Jesper 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: Ingo Molnar <mingo@kernel.org>

2 files changed, 23 insertions, 13 deletions

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index f3778e2b46c8..aea3135c5d90 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -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);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 5cbf92214ad8..767aab3505a8 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -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;