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>

diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index d5b3876..e37a77c 100644 (file)
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -64,6 +64,8 @@
 #define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT)
 #define NMI_OFFSET     (1UL << NMI_SHIFT)
 
+#define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET)
+
 #ifndef PREEMPT_ACTIVE
 #define PREEMPT_ACTIVE_BITS    1
 #define PREEMPT_ACTIVE_SHIFT   (NMI_SHIFT + NMI_BITS)
@@ -82,10 +84,13 @@
 /*
  * Are we doing bottom half or hardware interrupt processing?
  * Are we in a softirq context? Interrupt context?
+ * in_softirq - Are we currently processing softirq or have bh disabled?
+ * in_serving_softirq - Are we currently processing softirq?
  */
 #define in_irq()               (hardirq_count())
 #define in_softirq()           (softirq_count())
 #define in_interrupt()         (irq_count())
+#define in_serving_softirq()   (softirq_count() & SOFTIRQ_OFFSET)
 
 /*
  * Are we in NMI context?

diff --git a/include/linux/sched.h b/include/linux/sched.h
index cdf5669..8744e50 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2366,9 +2366,9 @@ extern int __cond_resched_lock(spinlock_t *lock);
 
 extern int __cond_resched_softirq(void);
 
-#define cond_resched_softirq() ({                              \
-       __might_sleep(__FILE__, __LINE__, SOFTIRQ_OFFSET);      \
-       __cond_resched_softirq();                               \
+#define cond_resched_softirq() ({                                      \
+       __might_sleep(__FILE__, __LINE__, SOFTIRQ_DISABLE_OFFSET);      \
+       __cond_resched_softirq();                                       \
 })
 
 /*

diff --git a/kernel/sched.c b/kernel/sched.c
index 771b518..089be8a 100644 (file)
--- 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 07b4f1b..988dfbe 100644 (file)
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -76,12 +76,22 @@ void wakeup_softirqd(void)
                wake_up_process(tsk);
 }
 
+/*
+ * 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

diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index 78ef2c5..37dff78 100644 (file)
--- a/net/sched/cls_cgroup.c
+++ b/net/sched/cls_cgroup.c
@@ -123,7 +123,7 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
         * calls by looking at the number of nested bh disable calls because
         * softirqs always disables bh.
         */
-       if (softirq_count() != SOFTIRQ_OFFSET) {
+       if (in_serving_softirq()) {
                /* If there is an sk_classid we'll use that. */
                if (!skb->sk)
                        return -1;