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Date: Fri, 28 Jul 2006 12:38:33 +0100 (BST)
From: Esben Nielsen <nielsen.esben@...glemail.com>
To: "Paul E. McKenney" <paulmck@...ibm.com>
cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
rostedt@...dmis.org, dipankar@...ibm.com, billh@...ppy.monkey.org,
nielsen.esben@...glemail.com, mingo@...e.hu, tytso@...ibm.com,
dvhltc@...ibm.com
Subject: Re: [RFC, PATCH, -rt] Early prototype RCU priority-boost patch
Hi,
I have considered an idea to make this work with the PI: Add the ability
to at a waiter not refering to a lock to the PI list. I think a few
subsystems can use that if they temporarely want to boost a task in a
consistend way (HR-timers is one). After a little renaming getting the
boosting part seperated out of rt_mutex_waiter:
struct prio_booster {
struct plist_node booster_list_entry;
};
void add_prio_booster(struct task_struct *, struct prio_booster *booster);
void remove_prio_booster(struct task_struct *, struct prio_booster
*booster);
void change_prio_booster(struct task_struct *, struct prio_booster
*booster, int new_prio);
(these functions takes care of doing/triggering a lock chain traversal if
needed) and change
struct rt_mutext_waiter {
...
struct prio_booster booster;
...
};
There are issues with lock orderings between task->pi_lock (which should
be renamed to task->prio_lock) and rq->lock. The lock ordering probably
have to be reversed, thus integrating the boosting system directly into
the scheduler instead of into rtmutex-subsystem.
Esben
On Thu, 27 Jul 2006, Paul E. McKenney wrote:
> Hello!
>
> This is a very crude not-for-inclusion patch that boosts priority of
> RCU read-side critical sections, but only when they are preempted, and
> only to the highest non-RT priority. The rcu_read_unlock() primitive
> does the unboosting. There are a large number of things that this patch
> does -not- do, including:
>
> o Boost RCU read-side critical sections to the highest possible
> priority. One might wish to do this in OOM situations. Or
> if the grace period is extending too long. I played with this
> a bit some months back, see:
>
> http://www.rdrop.com/users/paulmck/patches/RCUboost-20.patch
>
> to see what I was thinking. Or similarly-numbered patches,
> see http://www.rdrop.com/users/paulmck/patches for the full
> list. Lots of subtly broken approaches for those who are
> interested in subtle breakage.
>
> One must carefully resolve races between boosting and the
> to-be-boosted task slipping out of its RCU read-side critical
> section. My thought has been to grab the to-be-boosted task
> by the throat, and only boost it if it is (1) still in an
> RCU read-side critical section and (2) not running. If you
> try boosting a thread that is already running, the races between
> boosting and rcu_read_unlock() are insanely complex, particularly
> for implementations of rcu_read_unlock() that don't use atomic
> instructions or memory barriers. ;-)
>
> Much better to either have the thread boost itself or to make
> sure the thread is not running if having someone else boost it.
>
> o Boost RCU read-side critical sections that must block waiting
> for a non-raw spinlock. The URL noted above shows one approach
> I was messing with some time back.
>
> o Boost RCU read-side critical sections based on the priority of
> tasks doing synchronize_rcu() and call_rcu(). (This was something
> Steve Rostedt suggested at OLS.) One thing at a time! ;-)
>
> o Implementing preemption thresholding, as suggested by Bill Huey.
> I am taking the coward's way out on this for the moment in order
> to improve the odds of getting something useful done (as opposed
> to getting something potentially even more useful only half done).
>
> Anyway, the following patch compiles and passes lightweight "smoke" tests.
> It almost certainly has fatal flaws -- for, example, I don't see how it
> would handle yet another task doing a lock-based priority boost between
> the time the task is RCU-boosted and the time it de-boosts itself in
> rcu_read_unlock().
>
> Again, not for inclusion in its present form, but any enlightenment would
> be greatly appreciated.
>
> (Thomas, you did ask for this!!!)
>
> Thanx, Paul
>
> Signed-off-by: Paul E. McKenney <paulmck@...ibm.com> (but not for inclusion)
> ---
>
> include/linux/init_task.h | 1 +
> include/linux/rcupdate.h | 2 ++
> include/linux/sched.h | 3 +++
> kernel/rcupreempt.c | 11 +++++++++++
> kernel/sched.c | 8 ++++++++
> 5 files changed, 25 insertions(+)
>
> diff -urpNa -X dontdiff linux-2.6.17-rt7/include/linux/init_task.h linux-2.6.17-rt7-rcubp/include/linux/init_task.h
> --- linux-2.6.17-rt7/include/linux/init_task.h 2006-07-27 14:29:55.000000000 -0700
> +++ linux-2.6.17-rt7-rcubp/include/linux/init_task.h 2006-07-27 14:34:20.000000000 -0700
> @@ -89,6 +89,7 @@ extern struct group_info init_groups;
> .prio = MAX_PRIO-20, \
> .static_prio = MAX_PRIO-20, \
> .normal_prio = MAX_PRIO-20, \
> + .rcu_prio = MAX_PRIO, \
> .policy = SCHED_NORMAL, \
> .cpus_allowed = CPU_MASK_ALL, \
> .mm = NULL, \
> diff -urpNa -X dontdiff linux-2.6.17-rt7/include/linux/rcupdate.h linux-2.6.17-rt7-rcubp/include/linux/rcupdate.h
> --- linux-2.6.17-rt7/include/linux/rcupdate.h 2006-07-27 14:29:55.000000000 -0700
> +++ linux-2.6.17-rt7-rcubp/include/linux/rcupdate.h 2006-07-27 14:34:20.000000000 -0700
> @@ -175,6 +175,8 @@ extern int rcu_needs_cpu(int cpu);
>
> #else /* #ifndef CONFIG_PREEMPT_RCU */
>
> +#define RCU_PREEMPT_BOOST_PRIO MAX_USER_RT_PRIO /* Initial boost level. */
> +
> #define rcu_qsctr_inc(cpu)
> #define rcu_bh_qsctr_inc(cpu)
> #define call_rcu_bh(head, rcu) call_rcu(head, rcu)
> diff -urpNa -X dontdiff linux-2.6.17-rt7/include/linux/sched.h linux-2.6.17-rt7-rcubp/include/linux/sched.h
> --- linux-2.6.17-rt7/include/linux/sched.h 2006-07-27 14:29:55.000000000 -0700
> +++ linux-2.6.17-rt7-rcubp/include/linux/sched.h 2006-07-27 14:34:20.000000000 -0700
> @@ -851,6 +851,9 @@ struct task_struct {
> int oncpu;
> #endif
> int prio, static_prio, normal_prio;
> +#ifdef CONFIG_PREEMPT_RCU
> + int rcu_prio;
> +#endif
> struct list_head run_list;
> prio_array_t *array;
>
> diff -urpNa -X dontdiff linux-2.6.17-rt7/kernel/rcupreempt.c linux-2.6.17-rt7-rcubp/kernel/rcupreempt.c
> --- linux-2.6.17-rt7/kernel/rcupreempt.c 2006-07-27 14:29:55.000000000 -0700
> +++ linux-2.6.17-rt7-rcubp/kernel/rcupreempt.c 2006-07-27 14:34:20.000000000 -0700
> @@ -147,6 +147,17 @@ rcu_read_lock(void)
> atomic_inc(current->rcu_flipctr2);
> smp_mb__after_atomic_inc(); /* might optimize out... */
> }
> + if (unlikely(current->rcu_prio <= RCU_PREEMPT_BOOST_PRIO)) {
> + int new_prio = MAX_PRIO;
> +
> + current->rcu_prio = MAX_PRIO;
> + if (new_prio > current->static_prio)
> + new_prio = current->static_prio;
> + if (new_prio > current->normal_prio)
> + new_prio = current->normal_prio;
> + /* How to account for lock-based prio boost? */
> + rt_mutex_setprio(current, new_prio);
> + }
> }
> trace_special((unsigned long) current->rcu_flipctr1,
> (unsigned long) current->rcu_flipctr2,
> diff -urpNa -X dontdiff linux-2.6.17-rt7/kernel/sched.c linux-2.6.17-rt7-rcubp/kernel/sched.c
> --- linux-2.6.17-rt7/kernel/sched.c 2006-07-27 14:29:55.000000000 -0700
> +++ linux-2.6.17-rt7-rcubp/kernel/sched.c 2006-07-27 14:58:40.000000000 -0700
> @@ -3685,6 +3685,14 @@ asmlinkage void __sched preempt_schedule
> return;
>
> need_resched:
> +#ifdef CONFIG_PREEMPT_RT
> + if (unlikely(current->rcu_read_lock_nesting > 0) &&
> + (current->rcu_prio > RCU_PREEMPT_BOOST_PRIO)) {
> + current->rcu_prio = RCU_PREEMPT_BOOST_PRIO;
> + if (current->rcu_prio < current->prio)
> + rt_mutex_setprio(current, current->rcu_prio);
> + }
> +#endif
> local_irq_disable();
> add_preempt_count(PREEMPT_ACTIVE);
> /*
>
-
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