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Message-ID: <20170623093341.547ce155@gandalf.local.home>
Date: Fri, 23 Jun 2017 09:33:41 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Mike Galbraith <efault@....de>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>,
linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [patch-rt v2] rtmutex: Fix lock stealing logic
On Fri, 23 Jun 2017 09:37:14 +0200
Mike Galbraith <efault@....de> wrote:
> V2 changes:
> - beautification (ymmv)
> - enable lock stealing when waiter is queued
>
> rtmutex: Fix lock stealing logic
>
> 1. When trying to acquire an rtmutex, we first try to grab it without
> queueing the waiter, and explicitly check for that initial attempt
> in the !waiter path of __try_to_take_rt_mutex(). Checking whether
> the lock taker is top waiter before allowing a steal attempt in that
> path is a thinko: the lock taker has not yet blocked.
>
> 2. It seems wrong to change the definition of rt_mutex_waiter_less()
> to mean less or perhaps equal when we have an rt_mutex_waiter_equal().
>
> Remove the thinko, restore rt_mutex_waiter_less(), implement and use
> rt_mutex_steal() based upon rt_mutex_waiter_less/equal(), moving all
> qualification criteria into the function itself.
>
> Signed-off-by: Mike Galbraith <efault@....de>
> ---
> kernel/locking/rtmutex.c | 76 ++++++++++++++++++++++-------------------------
> 1 file changed, 37 insertions(+), 39 deletions(-)
>
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -236,26 +236,19 @@ static inline bool unlock_rt_mutex_safe(
> }
> #endif
>
> -#define STEAL_NORMAL 0
> -#define STEAL_LATERAL 1
> -
> /*
> * Only use with rt_mutex_waiter_{less,equal}()
> */
> -#define task_to_waiter(p) \
> - &(struct rt_mutex_waiter){ .prio = (p)->prio, .deadline = (p)->dl.deadline }
> +#define task_to_waiter(p) &(struct rt_mutex_waiter) \
> + { .prio = (p)->prio, .deadline = (p)->dl.deadline, .task = (p) }
>
> static inline int
> rt_mutex_waiter_less(struct rt_mutex_waiter *left,
> - struct rt_mutex_waiter *right, int mode)
> + struct rt_mutex_waiter *right)
> {
> - if (mode == STEAL_NORMAL) {
> - if (left->prio < right->prio)
> - return 1;
> - } else {
> - if (left->prio <= right->prio)
> - return 1;
> - }
> + if (left->prio < right->prio)
> + return 1;
> +
> /*
> * If both waiters have dl_prio(), we check the deadlines of the
> * associated tasks.
> @@ -287,6 +280,27 @@ rt_mutex_waiter_equal(struct rt_mutex_wa
> return 1;
> }
>
> +#define STEAL_NORMAL 0
> +#define STEAL_LATERAL 1
> +
> +static inline int
> +rt_mutex_steal(struct rt_mutex *lock, struct rt_mutex_waiter *waiter, int mode)
> +{
> + struct rt_mutex_waiter *top_waiter = rt_mutex_top_waiter(lock);
> +
> + if (waiter == top_waiter || rt_mutex_waiter_less(waiter, top_waiter))
> + return 1;
> +
> + /*
> + * Note that RT tasks are excluded from lateral-steals
> + * to prevent the introduction of an unbounded latency.
> + */
> + if (mode == STEAL_NORMAL || rt_task(waiter->task))
> + return 0;
> +
> + return rt_mutex_waiter_equal(waiter, top_waiter);
> +}
> +
> static void
> rt_mutex_enqueue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
> {
> @@ -298,7 +312,7 @@ rt_mutex_enqueue(struct rt_mutex *lock,
> while (*link) {
> parent = *link;
> entry = rb_entry(parent, struct rt_mutex_waiter, tree_entry);
> - if (rt_mutex_waiter_less(waiter, entry, STEAL_NORMAL)) {
> + if (rt_mutex_waiter_less(waiter, entry)) {
> link = &parent->rb_left;
> } else {
> link = &parent->rb_right;
> @@ -337,7 +351,7 @@ rt_mutex_enqueue_pi(struct task_struct *
> while (*link) {
> parent = *link;
> entry = rb_entry(parent, struct rt_mutex_waiter, pi_tree_entry);
> - if (rt_mutex_waiter_less(waiter, entry, STEAL_NORMAL)) {
> + if (rt_mutex_waiter_less(waiter, entry)) {
> link = &parent->rb_left;
> } else {
> link = &parent->rb_right;
> @@ -847,6 +861,7 @@ static int rt_mutex_adjust_prio_chain(st
> * @task: The task which wants to acquire the lock
> * @waiter: The waiter that is queued to the lock's wait tree if the
> * callsite called task_blocked_on_lock(), otherwise NULL
> + * @mode: Lock steal mode (STEAL_NORMAL, STEAL_LATERAL)
> */
> static int __try_to_take_rt_mutex(struct rt_mutex *lock,
> struct task_struct *task,
> @@ -886,20 +901,16 @@ static int __try_to_take_rt_mutex(struct
> */
> if (waiter) {
> /*
> - * If waiter is not the highest priority waiter of
> - * @lock, give up.
> + * If waiter is not the highest priority waiter of @lock,
> + * or its peer when lateral steal is allowed, give up.
> */
> - if (waiter != rt_mutex_top_waiter(lock)) {
> - /* XXX rt_mutex_waiter_less() ? */
> + if (!rt_mutex_steal(lock, waiter, mode))
> return 0;
> - }
> -
> /*
> * We can acquire the lock. Remove the waiter from the
> * lock waiters tree.
> */
> rt_mutex_dequeue(lock, waiter);
> -
I liked that space.
> } else {
> /*
> * If the lock has waiters already we check whether @task is
> @@ -910,25 +921,12 @@ static int __try_to_take_rt_mutex(struct
> * not need to be dequeued.
> */
> if (rt_mutex_has_waiters(lock)) {
> - struct task_struct *pown = rt_mutex_top_waiter(lock)->task;
> -
> - if (task != pown)
> - return 0;
OK, I see what happened here. Yeah, this will always be true, because
when waiter == NULL, it means that the task isn't on the lock's list
yet, and pown != task is always true.
> -
> - /*
> - * Note that RT tasks are excluded from lateral-steals
> - * to prevent the introduction of an unbounded latency.
> - */
> - if (rt_task(task))
> - mode = STEAL_NORMAL;
> /*
> - * If @task->prio is greater than or equal to
> - * the top waiter priority (kernel view),
> - * @task lost.
> + * If @task->prio is greater than the top waiter
> + * priority (kernel view), or equal to it when a
> + * lateral steal is forbidden, @task lost.
> */
> - if (!rt_mutex_waiter_less(task_to_waiter(task),
> - rt_mutex_top_waiter(lock),
> - mode))
> + if (!rt_mutex_steal(lock, task_to_waiter(task), mode))
> return 0;
> /*
> * The current top waiter stays enqueued. We
Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
-- Steve
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