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Date: Thu, 2 May 2013 10:32:57 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Julian Anastasov <ja@....bg>
Cc: Peter Zijlstra <peterz@...radead.org>,
Simon Horman <horms@...ge.net.au>,
Eric Dumazet <eric.dumazet@...il.com>,
Ingo Molnar <mingo@...hat.com>, lvs-devel@...r.kernel.org,
netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
linux-kernel@...r.kernel.org,
Pablo Neira Ayuso <pablo@...filter.org>,
Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper
On Thu, May 02, 2013 at 06:54:05PM +0300, Julian Anastasov wrote:
>
> Hello,
>
> On Thu, 2 May 2013, Peter Zijlstra wrote:
>
> > On Wed, May 01, 2013 at 09:22:08PM +0300, Julian Anastasov wrote:
> > > > +extern int __cond_resched_rcu(void);
> > > > +
> > > > +#define cond_resched_rcu() ({ \
> > > > + __might_sleep(__FILE__, __LINE__, 0); \
> > >
> > > I see your goal. But digging into __might_sleep()
> > > I see that rcu_sleep_check() will scream for the non-preempt
> > > case because we are under rcu_read_lock.
> >
> >
> > #ifdef CONFIG_PREEMPT_RCU
> > #define PREEMPT_RCU_OFFSET 0
> > #else
> > #define PREEMPT_RCU_OFFSET 1
> > #endif
> >
> > #define cond_resched_rcu() ({ \
> > __might_sleep(__FILE__, __LINE__, PREEMPT_RCU_OFFSET); \
> > __cond_resched_rcu(); \
> > })
> >
> > Should work I think..
>
> I implemented your idea.
>
> I tested the following patch in 2 variants,
> TINY_RCU and CONFIG_TREE_PREEMPT_RCU. I see the
Could you please also try CONFIG_TREE_RCU?
> error if extra rcu_read_lock is added for testing.
>
> I'm using the PREEMPT_ACTIVE flag to indicate
> that we are already under lock. It should work because
> __might_sleep is not called with such bit. I also tried to
> add new flag in include/linux/hardirq.h but PREEMPT_ACTIVE
> depends on the arch, so this alternative looked difficult to
> implement.
>
> include/linux/rcupdate.h | 7 ++++---
> include/linux/sched.h | 14 ++++++++++++++
> kernel/sched/core.c | 20 ++++++++++++++++++--
> 3 files changed, 36 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index b758ce1..b594759 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -480,9 +480,10 @@ static inline void rcu_preempt_sleep_check(void)
> }
> #endif /* #else #ifdef CONFIG_PROVE_RCU */
>
> -#define rcu_sleep_check() \
> +#define rcu_sleep_check(locked) \
> do { \
> - rcu_preempt_sleep_check(); \
> + if (!(locked)) \
> + rcu_preempt_sleep_check(); \
> rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map), \
> "Illegal context switch in RCU-bh" \
> " read-side critical section"); \
> @@ -494,7 +495,7 @@ static inline void rcu_preempt_sleep_check(void)
> #else /* #ifdef CONFIG_PROVE_RCU */
>
> #define rcu_lockdep_assert(c, s) do { } while (0)
> -#define rcu_sleep_check() do { } while (0)
> +#define rcu_sleep_check(locked) do { } while (0)
>
> #endif /* #else #ifdef CONFIG_PROVE_RCU */
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e692a02..027deea 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2608,6 +2608,20 @@ extern int __cond_resched_softirq(void);
> __cond_resched_softirq(); \
> })
>
> +#ifdef CONFIG_PREEMPT_RCU
> +#define PREEMPT_RCU_OFFSET 1
> +#else
> +#define PREEMPT_RCU_OFFSET PREEMPT_CHECK_OFFSET
> +#endif
> +
> +extern int __cond_resched_rcu(void);
> +
> +#define cond_resched_rcu() ({ \
> + __might_sleep(__FILE__, __LINE__, PREEMPT_ACTIVE | \
> + PREEMPT_RCU_OFFSET); \
> + __cond_resched_rcu(); \
> +})
> +
> /*
> * Does a critical section need to be broken due to another
> * task waiting?: (technically does not depend on CONFIG_PREEMPT,
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 67d0465..2724be7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2793,7 +2793,7 @@ static inline void schedule_debug(struct task_struct *prev)
> */
> if (unlikely(in_atomic_preempt_off() && !prev->exit_state))
> __schedule_bug(prev);
> - rcu_sleep_check();
> + rcu_sleep_check(0);
>
> profile_hit(SCHED_PROFILING, __builtin_return_address(0));
>
> @@ -4364,6 +4364,20 @@ int __sched __cond_resched_softirq(void)
> }
> EXPORT_SYMBOL(__cond_resched_softirq);
>
> +int __sched __cond_resched_rcu(void)
> +{
> +#ifndef CONFIG_PREEMPT_RCU
> + if (should_resched()) {
> + rcu_read_unlock();
> + __cond_resched();
> + rcu_read_lock();
> + return 1;
> + }
> +#endif
> + return 0;
> +}
> +EXPORT_SYMBOL(__cond_resched_rcu);
> +
> /**
> * yield - yield the current processor to other threads.
> *
> @@ -7062,7 +7076,9 @@ void __might_sleep(const char *file, int line, int preempt_offset)
> {
> static unsigned long prev_jiffy; /* ratelimiting */
>
> - rcu_sleep_check(); /* WARN_ON_ONCE() by default, no rate limit reqd. */
> + /* WARN_ON_ONCE() by default, no rate limit reqd. */
> + rcu_sleep_check(preempt_offset & PREEMPT_ACTIVE);
Color me confused.
>From what I can see, the two values passed in through preempt_offset
are PREEMPT_LOCK_OFFSET and SOFTIRQ_DISABLE_OFFSET. PREEMPT_ACTIVE
is normally a high-order bit, above PREEMPT_MASK, SOFTIRQ_MASK, and
HARDIRQ_MASK.
PREEMPT_LOCK_OFFSET and SOFTIRQ_DISABLE_OFFSET have only low-order bits,
so I don't see how rcu_sleep_check() is passed anything other than zero.
Am I going blind, or what?
Thanx, Paul
> + preempt_offset &= ~PREEMPT_ACTIVE;
> if ((preempt_count_equals(preempt_offset) && !irqs_disabled()) ||
> system_state != SYSTEM_RUNNING || oops_in_progress)
> return;
> --
> 1.7.3.4
>
>
> Regards
>
> --
> Julian Anastasov <ja@....bg>
> --
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