sched: Fix softirq time accounting

Peter Zijlstra found a bug in the way softirq time is accounted in
VIRT_CPU_ACCOUNTING on this thread:

   http://lkml.indiana.edu/hypermail//linux/kernel/1009.2/01366.html

The problem is, softirq processing uses local_bh_disable internally. There
is no way, later in the flow, to differentiate between whether softirq is
being processed or is it just that bh has been disabled. So, a hardirq when bh
is disabled results in time being wrongly accounted as softirq.

Looking at the code a bit more, the problem exists in !VIRT_CPU_ACCOUNTING
as well. As account_system_time() in normal tick based accouting also uses
softirq_count, which will be set even when not in softirq with bh disabled.

Peter also suggested solution of using 2*SOFTIRQ_OFFSET as irq count
for local_bh_{disable,enable} and using just SOFTIRQ_OFFSET while softirq
processing. The patch below does that and adds API in_serving_softirq() which
returns whether we are currently processing softirq or not.

Also changes one of the usages of softirq_count in net/sched/cls_cgroup.c
to in_serving_softirq.

Looks like many usages of in_softirq really want in_serving_softirq. Those
changes can be made individually on a case by case basis.

Signed-off-by: Venkatesh Pallipadi <venki@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1286237003-12406-2-git-send-email-venki@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 35 insertions, 18 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 771b518e5f1f..089be8adb074 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3422,7 +3422,7 @@ void account_system_time(struct task_struct *p, int hardirq_offset,
         tmp = cputime_to_cputime64(cputime);
         if (hardirq_count() - hardirq_offset)
                 cpustat->irq = cputime64_add(cpustat->irq, tmp);
-        else if (softirq_count())
+        else if (in_serving_softirq())
                 cpustat->softirq = cputime64_add(cpustat->softirq, tmp);
         else
                 cpustat->system = cputime64_add(cpustat->system, tmp);
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 07b4f1b1a73a..988dfbe6bbe8 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -77,11 +77,21 @@ void wakeup_softirqd(void)
 }
 
 /*
+ * preempt_count and SOFTIRQ_OFFSET usage:
+ * - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving
+ *   softirq processing.
+ * - preempt_count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
+ *   on local_bh_disable or local_bh_enable.
+ * This lets us distinguish between whether we are currently processing
+ * softirq and whether we just have bh disabled.
+ */
+
+/*
  * This one is for softirq.c-internal use,
  * where hardirqs are disabled legitimately:
  */
 #ifdef CONFIG_TRACE_IRQFLAGS
-static void __local_bh_disable(unsigned long ip)
+static void __local_bh_disable(unsigned long ip, unsigned int cnt)
 {
         unsigned long flags;
 
@@ -95,32 +105,43 @@ static void __local_bh_disable(unsigned long ip)
          * We must manually increment preempt_count here and manually
          * call the trace_preempt_off later.
          */
-        preempt_count() += SOFTIRQ_OFFSET;
+        preempt_count() += cnt;
         /*
          * Were softirqs turned off above:
          */
-        if (softirq_count() == SOFTIRQ_OFFSET)
+        if (softirq_count() == cnt)
                 trace_softirqs_off(ip);
         raw_local_irq_restore(flags);
 
-        if (preempt_count() == SOFTIRQ_OFFSET)
+        if (preempt_count() == cnt)
                 trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
 }
 #else /* !CONFIG_TRACE_IRQFLAGS */
-static inline void __local_bh_disable(unsigned long ip)
+static inline void __local_bh_disable(unsigned long ip, unsigned int cnt)
 {
-        add_preempt_count(SOFTIRQ_OFFSET);
+        add_preempt_count(cnt);
         barrier();
 }
 #endif /* CONFIG_TRACE_IRQFLAGS */
 
 void local_bh_disable(void)
 {
-        __local_bh_disable((unsigned long)__builtin_return_address(0));
+        __local_bh_disable((unsigned long)__builtin_return_address(0),
+                                SOFTIRQ_DISABLE_OFFSET);
 }
 
 EXPORT_SYMBOL(local_bh_disable);
 
+static void __local_bh_enable(unsigned int cnt)
+{
+        WARN_ON_ONCE(in_irq());
+        WARN_ON_ONCE(!irqs_disabled());
+
+        if (softirq_count() == cnt)
+                trace_softirqs_on((unsigned long)__builtin_return_address(0));
+        sub_preempt_count(cnt);
+}
+
 /*
  * Special-case - softirqs can safely be enabled in
  * cond_resched_softirq(), or by __do_softirq(),
@@ -128,12 +149,7 @@ EXPORT_SYMBOL(local_bh_disable);
  */
 void _local_bh_enable(void)
 {
-        WARN_ON_ONCE(in_irq());
-        WARN_ON_ONCE(!irqs_disabled());
-
-        if (softirq_count() == SOFTIRQ_OFFSET)
-                trace_softirqs_on((unsigned long)__builtin_return_address(0));
-        sub_preempt_count(SOFTIRQ_OFFSET);
+        __local_bh_enable(SOFTIRQ_DISABLE_OFFSET);
 }
 
 EXPORT_SYMBOL(_local_bh_enable);
@@ -147,13 +163,13 @@ static inline void _local_bh_enable_ip(unsigned long ip)
         /*
          * Are softirqs going to be turned on now:
          */
-        if (softirq_count() == SOFTIRQ_OFFSET)
+        if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)
                 trace_softirqs_on(ip);
         /*
          * Keep preemption disabled until we are done with
          * softirq processing:
          */
-        sub_preempt_count(SOFTIRQ_OFFSET - 1);
+        sub_preempt_count(SOFTIRQ_DISABLE_OFFSET - 1);
 
         if (unlikely(!in_interrupt() && local_softirq_pending()))
                 do_softirq();
@@ -198,7 +214,8 @@ asmlinkage void __do_softirq(void)
         pending = local_softirq_pending();
         account_system_vtime(current);
 
-        __local_bh_disable((unsigned long)__builtin_return_address(0));
+        __local_bh_disable((unsigned long)__builtin_return_address(0),
+                                SOFTIRQ_OFFSET);
         lockdep_softirq_enter();
 
         cpu = smp_processor_id();
@@ -245,7 +262,7 @@ restart:
         lockdep_softirq_exit();
 
         account_system_vtime(current);
-        _local_bh_enable();
+        __local_bh_enable(SOFTIRQ_OFFSET);
 }
 
 #ifndef __ARCH_HAS_DO_SOFTIRQ