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Commit 75e1056f authored by Venkatesh Pallipadi's avatar Venkatesh Pallipadi Committed by Ingo Molnar
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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: default avatarVenkatesh Pallipadi <venki@google.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1286237003-12406-2-git-send-email-venki@google.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 75dd321d

include/linux/hardirq.h

+5 −0
Original line number Diff line number Diff line
@@ -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?

include/linux/sched.h

+3 −3
Original line number Diff line number Diff line
@@ -2367,7 +2367,7 @@ extern int __cond_resched_lock(spinlock_t *lock); 
extern int __cond_resched_softirq(void);



#define cond_resched_softirq() ({					\

	__might_sleep(__FILE__, __LINE__, SOFTIRQ_OFFSET);	\

	__might_sleep(__FILE__, __LINE__, SOFTIRQ_DISABLE_OFFSET);	\

	__cond_resched_softirq();					\

})

kernel/sched.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

kernel/softirq.c

+34 −17
Original line number Diff line number Diff line
@@ -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 @@ asmlinkage void __do_softirq(void) 
	lockdep_softirq_exit();



	account_system_vtime(current);

	_local_bh_enable();

	__local_bh_enable(SOFTIRQ_OFFSET);

}



#ifndef __ARCH_HAS_DO_SOFTIRQ

net/sched/cls_cgroup.c

+1 −1
Original line number Diff line number Diff line
@@ -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;