| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <4897C281.6090701@novell.com>
Date: Mon, 04 Aug 2008 23:01:21 -0400
From: Gregory Haskins <ghaskins@...ell.com>
To: Gregory Haskins <ghaskins@...ell.com>
CC: mingo@...e.hu, paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
tglx@...utronix.de, rosted@...dmis.org,
linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
gregory.haskins@...il.com,
David Holmes - Sun Microsystems <David.Holmes@....COM>
Subject: Re: [PATCH RT RFC 7/7] rtmutex: pi-boost locks as late as possible
Gregory Haskins wrote:
> Gregory Haskins wrote:
>> Adaptive-locking technology often times acquires the lock by
>> spinning on a running-owner instead of sleeping. It is unecessary
>> to go through pi-boosting if the owner is of equal or (logically)
>> lower priority. Therefore, we can save some significant overhead
>> by deferring the boost until absolutely necessary. This has shown
>> to improve overall performance in PREEMPT_RT
>>
>> Special thanks to Peter Morreale for suggesting the optimization to
>> only consider skipping the boost if the owner is >= to current
>>
>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>> CC: Peter Morreale <pmorreale@...ell.com>
>>
>
> I received feedback that this prologue was too vague to accurately
> describe what this patch does and why it is not broken to use this
> optimization. Therefore, here is the new prologue:
>
> -----------------------
> From: Gregory Haskins <ghaskins@...ell.com>
>
> rtmutex: pi-boost locks as late as possible
>
> PREEMPT_RT replaces most spinlock_t instances with a preemptible
> real-time lock that supports priority inheritance. An uncontended
> (fastpath) acquisition of this lock has no more overhead than
> its non-rt spinlock_t counterpart. However, the contended case
> has considerably more overhead so that the lock can maintain
> proper priority queue order and support pi-boosting of the lock
> owner, yet remaining fully preemptible.
>
> Instrumentation shows that the majority of acquisitions under most
> workloads falls either into the fastpath category, or the adaptive
> spin category within the slowpath. The necessity to pi-boost a
> lock-owner should be sufficiently rare, yet the slow-path path
> blindly incurs this overhead in 100% of contentions.
>
> Therefore, this patch intends to capitalize on this observation
> in order to reduce overhead and improve acquisition throughput.
> It is important to note that real-time latency is still treated
> as a higher order constraint than throughput, so the full
> pi-protocol is observed using new carefully constructed rules
> around the old concepts.
>
> 1) We check the priority of the owner relative to the waiter on
> each spin of the lock (if we are not boosted already). If the
> owner's effective priority is logically less than the waiters
> priority, we must boost them.
>
> 2) We check the priority of ourselves against our current queue
> position on the waiters-list (if we are not boosted already).
> If our priority was changed, we need to re-queue ourselves to
> update our position.
>
> 3) We break out of the adaptive-spin if either of the above
> conditions (1), (2) change so that we can re-evaluate the
> lock conditions.
>
> 4) We must enter pi-boost mode if, at any time, we decide to
> voluntarily preempt since we are losing our ability to
> dynamically process the conditions above.
>
> Note: We still fully support priority inheritance with this
> protocol, even if we defer the low-level calls to adjust priority.
> The difference is really in terms of being a pro-active protocol
> (boost on entry), verses a reactive protocol (boost when
> necessary). The upside to the latter is that we don't take a
> penalty for pi when it is not necessary. The downside is that we
> technically leave the owner exposed to getting preempted, even if
> our waiter is the highest priority task in the system.
David Holmes (CC'd) pointed out that this statement is a little vague
and confusing as well. The question is: how could the owner be exposed
to preemption since it would presumably be running at or above the
waiters priority or we would have boosted it already? The answer is
that this is in reference to the fact that the owner may have its
priority lowered after we have already made the decision to defer boosting.
Therefore, my updated statement should read:
"The downside is that we technically leave the owner exposed to getting
preempted *should it get asynchronously deprioritized*, even if ...."
As I go on to explain below, this deboosting could only happen as the
result of a setscheduler() call, which I assert should not be cause for
concern. However, I wanted to highlight this phenomenon in the interest
of full disclosure since it is technically a difference in behavior from
the original algorithm. I will update the header with this edit for
clarity.
Thanks for the review, David!
-Greg
> When this
> happens, the owner would be immediately boosted (because we would
> hit the "oncpu" condition, and subsequently follow the voluntary
> preempt path which boosts the owner). Therefore, inversion is
> fully prevented, but we have the extra latency of the
> preempt/boost/wakeup that could have been avoided in the proactive
> model.
>
> However, the design of the algorithm described above constrains the
> probability of this phenomenon occurring to setscheduler()
> operations. Since rt-locks do not support being interrupted by
> signals or timeouts, waiters only depart via the acquisition path.
> And while acquisitions do deboost the owner, the owner also
> changes simultaneously, rending the deboost moot relative to the
> other waiters.
>
> What this all means is that the downside to this implementation is
> that a high-priority waiter *may* see an extra latency (equivalent
> to roughly two wake-ups) if the owner has its priority reduced via
> setscheduler() while it holds the lock. The penalty is
> deterministic, arguably small enough, and sufficiently rare that I
> do not believe it should be an issue.
>
> Note: If the concept of other exit paths are ever introduced in the
> future, simply adapting the condition to look at owner->normal_prio
> instead of owner->prio should once again constrain the limitation
> to setscheduler().
>
> Special thanks to Peter Morreale for suggesting the optimization to
> only consider skipping the boost if the owner is >= to current.
>
> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
> CC: Peter Morreale <pmorreale@...ell.com>
>
>
>> ---
>>
>> include/linux/rtmutex.h | 1 kernel/rtmutex.c | 195
>> ++++++++++++++++++++++++++++++++++++-----------
>> kernel/rtmutex_common.h | 1 3 files changed, 153 insertions(+),
>> 44 deletions(-)
>>
>> diff --git a/include/linux/rtmutex.h b/include/linux/rtmutex.h
>> index d984244..1d98107 100644
>> --- a/include/linux/rtmutex.h
>> +++ b/include/linux/rtmutex.h
>> @@ -33,6 +33,7 @@ struct rt_mutex {
>> struct pi_node node;
>> struct pi_sink snk;
>> int prio;
>> + int boosters;
>> } pi;
>> #ifdef CONFIG_DEBUG_RT_MUTEXES
>> int save_state;
>> diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
>> index 0f64298..de213ac 100644
>> --- a/kernel/rtmutex.c
>> +++ b/kernel/rtmutex.c
>> @@ -76,14 +76,15 @@ rt_mutex_set_owner(struct rt_mutex *lock, struct
>> task_struct *owner,
>> {
>> unsigned long val = (unsigned long)owner | mask;
>>
>> - if (rt_mutex_has_waiters(lock)) {
>> + if (lock->pi.boosters) {
>> struct task_struct *prev_owner = rt_mutex_owner(lock);
>>
>> rtmutex_pi_owner(lock, prev_owner, 0);
>> rtmutex_pi_owner(lock, owner, 1);
>> + }
>>
>> + if (rt_mutex_has_waiters(lock))
>> val |= RT_MUTEX_HAS_WAITERS;
>> - }
>>
>> lock->owner = (struct task_struct *)val;
>> }
>> @@ -177,7 +178,7 @@ static inline int rtmutex_pi_update(struct
>> pi_sink *snk,
>>
>> spin_lock_irqsave(&lock->wait_lock, iflags);
>>
>> - if (rt_mutex_has_waiters(lock)) {
>> + if (lock->pi.boosters) {
>> owner = rt_mutex_owner(lock);
>>
>> if (owner && owner != RT_RW_READER) {
>> @@ -206,6 +207,7 @@ static void init_pi(struct rt_mutex *lock)
>> pi_node_init(&lock->pi.node);
>>
>> lock->pi.prio = MAX_PRIO;
>> + lock->pi.boosters = 0;
>> pi_source_init(&lock->pi.src, &lock->pi.prio);
>> lock->pi.snk = rtmutex_pi_snk;
>>
>> @@ -303,6 +305,16 @@ static inline int try_to_take_rt_mutex(struct
>> rt_mutex *lock)
>> return do_try_to_take_rt_mutex(lock, STEAL_NORMAL);
>> }
>>
>> +static inline void requeue_waiter(struct rt_mutex *lock,
>> + struct rt_mutex_waiter *waiter)
>> +{
>> + BUG_ON(!waiter->task);
>> +
>> + plist_del(&waiter->list_entry, &lock->wait_list);
>> + plist_node_init(&waiter->list_entry, waiter->pi.prio);
>> + plist_add(&waiter->list_entry, &lock->wait_list);
>> +}
>> +
>> /*
>> * These callbacks are invoked whenever a waiter has changed priority.
>> * So we should requeue it within the lock->wait_list
>> @@ -343,11 +355,8 @@ static inline int
>> rtmutex_waiter_pi_update(struct pi_sink *snk,
>> * pi list. Therefore, if waiter->pi.prio has changed since we
>> * queued ourselves, requeue it.
>> */
>> - if (waiter->task && waiter->list_entry.prio != waiter->pi.prio) {
>> - plist_del(&waiter->list_entry, &lock->wait_list);
>> - plist_node_init(&waiter->list_entry, waiter->pi.prio);
>> - plist_add(&waiter->list_entry, &lock->wait_list);
>> - }
>> + if (waiter->task && waiter->list_entry.prio != waiter->pi.prio)
>> + requeue_waiter(lock, waiter);
>>
>> spin_unlock_irqrestore(&lock->wait_lock, iflags);
>>
>> @@ -359,20 +368,9 @@ static struct pi_sink rtmutex_waiter_pi_snk = {
>> .update = rtmutex_waiter_pi_update,
>> };
>>
>> -/*
>> - * This must be called with lock->wait_lock held.
>> - */
>> -static int add_waiter(struct rt_mutex *lock,
>> - struct rt_mutex_waiter *waiter,
>> - unsigned long *flags)
>> +static void boost_lock(struct rt_mutex *lock,
>> + struct rt_mutex_waiter *waiter)
>> {
>> - int has_waiters = rt_mutex_has_waiters(lock);
>> -
>> - waiter->task = current;
>> - waiter->lock = lock;
>> - waiter->pi.prio = current->prio;
>> - plist_node_init(&waiter->list_entry, waiter->pi.prio);
>> - plist_add(&waiter->list_entry, &lock->wait_list);
>> waiter->pi.snk = rtmutex_waiter_pi_snk;
>>
>> /*
>> @@ -397,35 +395,28 @@ static int add_waiter(struct rt_mutex *lock,
>> * If we previously had no waiters, we are transitioning to
>> * a mode where we need to boost the owner
>> */
>> - if (!has_waiters) {
>> + if (!lock->pi.boosters) {
>> struct task_struct *owner = rt_mutex_owner(lock);
>> rtmutex_pi_owner(lock, owner, 1);
>> }
>>
>> - spin_unlock_irqrestore(&lock->wait_lock, *flags);
>> - task_pi_update(current, 0);
>> - spin_lock_irqsave(&lock->wait_lock, *flags);
>> -
>> - return 0;
>> + lock->pi.boosters++;
>> + waiter->pi.boosted = 1;
>> }
>>
>> -/*
>> - * Remove a waiter from a lock
>> - *
>> - * Must be called with lock->wait_lock held
>> - */
>> -static void remove_waiter(struct rt_mutex *lock,
>> - struct rt_mutex_waiter *waiter)
>> +static void deboost_lock(struct rt_mutex *lock,
>> + struct rt_mutex_waiter *waiter,
>> + struct task_struct *p)
>> {
>> - struct task_struct *p = waiter->task;
>> + BUG_ON(!waiter->pi.boosted);
>>
>> - plist_del(&waiter->list_entry, &lock->wait_list);
>> - waiter->task = NULL;
>> + waiter->pi.boosted = 0;
>> + lock->pi.boosters--;
>>
>> /*
>> * We can stop boosting the owner if there are no more waiters
>> */
>> - if (!rt_mutex_has_waiters(lock)) {
>> + if (!lock->pi.boosters) {
>> struct task_struct *owner = rt_mutex_owner(lock);
>> rtmutex_pi_owner(lock, owner, 0);
>> }
>> @@ -446,6 +437,51 @@ static void remove_waiter(struct rt_mutex *lock,
>> }
>>
>> /*
>> + * This must be called with lock->wait_lock held.
>> + */
>> +static void _add_waiter(struct rt_mutex *lock,
>> + struct rt_mutex_waiter *waiter)
>> +{
>> + waiter->task = current;
>> + waiter->lock = lock;
>> + waiter->pi.prio = current->prio;
>> + plist_node_init(&waiter->list_entry, waiter->pi.prio);
>> + plist_add(&waiter->list_entry, &lock->wait_list);
>> +}
>> +
>> +static int add_waiter(struct rt_mutex *lock,
>> + struct rt_mutex_waiter *waiter,
>> + unsigned long *flags)
>> +{
>> + _add_waiter(lock, waiter);
>> +
>> + boost_lock(lock, waiter);
>> +
>> + spin_unlock_irqrestore(&lock->wait_lock, *flags);
>> + task_pi_update(current, 0);
>> + spin_lock_irqsave(&lock->wait_lock, *flags);
>> +
>> + return 0;
>> +}
>> +
>> +/*
>> + * Remove a waiter from a lock
>> + *
>> + * Must be called with lock->wait_lock held
>> + */
>> +static void remove_waiter(struct rt_mutex *lock,
>> + struct rt_mutex_waiter *waiter)
>> +{
>> + struct task_struct *p = waiter->task;
>> +
>> + plist_del(&waiter->list_entry, &lock->wait_list);
>> + waiter->task = NULL;
>> +
>> + if (waiter->pi.boosted)
>> + deboost_lock(lock, waiter, p);
>> +}
>> +
>> +/*
>> * Wake up the next waiter on the lock.
>> *
>> * Remove the top waiter from the current tasks waiter list and from
>> @@ -558,6 +594,24 @@ static int adaptive_wait(struct rt_mutex_waiter
>> *waiter,
>> if (orig_owner != rt_mutex_owner(waiter->lock))
>> return 0;
>>
>> + /* Special handling for when we are not in pi-boost mode */
>> + if (!waiter->pi.boosted) {
>> + /*
>> + * Are we higher priority than the owner? If so
>> + * we should bail out immediately so that we can
>> + * pi boost them.
>> + */
>> + if (current->prio < orig_owner->prio)
>> + return 0;
>> +
>> + /*
>> + * Did our priority change? If so, we need to
>> + * requeue our position in the list
>> + */
>> + if (waiter->pi.prio != current->prio)
>> + return 0;
>> + }
>> +
>> /* Owner went to bed, so should we */
>> if (!task_is_current(orig_owner))
>> return 1;
>> @@ -599,6 +653,7 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
>> unsigned long saved_state, state, flags;
>> struct task_struct *orig_owner;
>> int missed = 0;
>> + int boosted = 0;
>>
>> init_waiter(&waiter);
>>
>> @@ -631,26 +686,54 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
>> }
>> missed = 1;
>>
>> + orig_owner = rt_mutex_owner(lock);
>> +
>> /*
>> * waiter.task is NULL the first time we come here and
>> * when we have been woken up by the previous owner
>> * but the lock got stolen by an higher prio task.
>> */
>> - if (!waiter.task) {
>> - add_waiter(lock, &waiter, &flags);
>> + if (!waiter.task)
>> + _add_waiter(lock, &waiter);
>> +
>> + /*
>> + * We only need to pi-boost the owner if they are lower
>> + * priority than us. We dont care if this is racy
>> + * against priority changes as we will break out of
>> + * the adaptive spin anytime any priority changes occur
>> + * without boosting enabled.
>> + */
>> + if (!waiter.pi.boosted && current->prio < orig_owner->prio) {
>> + boost_lock(lock, &waiter);
>> + boosted = 1;
>> +
>> + spin_unlock_irqrestore(&lock->wait_lock, flags);
>> + task_pi_update(current, 0);
>> + spin_lock_irqsave(&lock->wait_lock, flags);
>> +
>> /* Wakeup during boost ? */
>> if (unlikely(!waiter.task))
>> continue;
>> }
>>
>> /*
>> + * If we are not currently pi-boosting the lock, we have to
>> + * monitor whether our priority changed since the last
>> + * time it was recorded and requeue ourselves if it moves.
>> + */
>> + if (!waiter.pi.boosted && waiter.pi.prio != current->prio) {
>> + waiter.pi.prio = current->prio;
>> +
>> + requeue_waiter(lock, &waiter);
>> + }
>> +
>> + /*
>> * Prevent schedule() to drop BKL, while waiting for
>> * the lock ! We restore lock_depth when we come back.
>> */
>> saved_flags = current->flags & PF_NOSCHED;
>> current->lock_depth = -1;
>> current->flags &= ~PF_NOSCHED;
>> - orig_owner = rt_mutex_owner(lock);
>> get_task_struct(orig_owner);
>> spin_unlock_irqrestore(&lock->wait_lock, flags);
>>
>> @@ -664,6 +747,24 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
>> * barrier which we rely upon to ensure current->state
>> * is visible before we test waiter.task.
>> */
>> + if (waiter.task && !waiter.pi.boosted) {
>> + spin_lock_irqsave(&lock->wait_lock, flags);
>> +
>> + /*
>> + * We get here if we have not yet boosted
>> + * the lock, yet we are going to sleep. If
>> + * we are still pending (waiter.task != 0),
>> + * then go ahead and boost them now
>> + */
>> + if (waiter.task) {
>> + boost_lock(lock, &waiter);
>> + boosted = 1;
>> + }
>> +
>> + spin_unlock_irqrestore(&lock->wait_lock, flags);
>> + task_pi_update(current, 0);
>> + }
>> +
>> if (waiter.task)
>> schedule_rt_mutex(lock);
>> } else
>> @@ -696,7 +797,8 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
>> spin_unlock_irqrestore(&lock->wait_lock, flags);
>>
>> /* Undo any pi boosting, if necessary */
>> - task_pi_update(current, 0);
>> + if (boosted)
>> + task_pi_update(current, 0);
>>
>> debug_rt_mutex_free_waiter(&waiter);
>> }
>> @@ -708,6 +810,7 @@ static void noinline __sched
>> rt_spin_lock_slowunlock(struct rt_mutex *lock)
>> {
>> unsigned long flags;
>> + int deboost = 0;
>>
>> spin_lock_irqsave(&lock->wait_lock, flags);
>>
>> @@ -721,12 +824,16 @@ rt_spin_lock_slowunlock(struct rt_mutex *lock)
>> return;
>> }
>>
>> + if (lock->pi.boosters)
>> + deboost = 1;
>> +
>> wakeup_next_waiter(lock, 1);
>>
>> spin_unlock_irqrestore(&lock->wait_lock, flags);
>>
>> - /* Undo pi boosting when necessary */
>> - task_pi_update(current, 0);
>> + if (deboost)
>> + /* Undo pi boosting when necessary */
>> + task_pi_update(current, 0);
>> }
>>
>> void __lockfunc rt_spin_lock(spinlock_t *lock)
>> diff --git a/kernel/rtmutex_common.h b/kernel/rtmutex_common.h
>> index 7bf32d0..34e2381 100644
>> --- a/kernel/rtmutex_common.h
>> +++ b/kernel/rtmutex_common.h
>> @@ -55,6 +55,7 @@ struct rt_mutex_waiter {
>> struct {
>> struct pi_sink snk;
>> int prio;
>> + int boosted;
>> } pi;
>> #ifdef CONFIG_DEBUG_RT_MUTEXES
>> unsigned long ip;
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe
>> linux-kernel" in
>> the body of a message to majordomo@...r.kernel.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at http://www.tux.org/lkml/
>>
>
>
Download attachment "signature.asc" of type "application/pgp-signature" (258 bytes)
Powered by blists - more mailing lists