[tip:sched/core] sched: Fix softirq time accounting

tip-bot for Venkatesh Pallipadi
2010-10-18T19:25:07Z

Commit-ID: 75e1056f5c57050415b64cb761a3acc35d91f013
Gitweb: http://git.kernel.org/tip/75e1056f5c57050415b64cb761a3acc35d91f013
Author: Venkatesh Pallipadi <venki@google.com>
AuthorDate: Mon, 4 Oct 2010 17:03:16 -0700
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Mon, 18 Oct 2010 20:52:20 +0200

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>
---
include/linux/hardirq.h | 5 ++++
include/linux/sched.h | 6 ++--
kernel/sched.c | 2 +-
kernel/softirq.c | 51 +++++++++++++++++++++++++++++++---------------
net/sched/cls_cgroup.c | 2 +-
5 files changed, 44 insertions(+), 22 deletions(-)

diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index d5b3876..e37a77c 100644
--- 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
--- 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
--- 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
--- 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
diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index 78ef2c5..37dff78 100644
--- 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;