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SubjectRe: [PATCH 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

/me wonders what's up with partial Cc's today..

On Wed, Jun 13, 2018 at 09:47:44AM +0200, Thomas Hellstrom wrote:
> The current Wound-Wait mutex algorithm is actually not Wound-Wait but
> Wait-Die. Implement also Wound-Wait as a per-ww-class choice. Wound-Wait
> is, contrary to Wait-Die a preemptive algorithm and is known to generate
> fewer backoffs. Testing reveals that this is true if the
> number of simultaneous contending transactions is small.
> As the number of simultaneous contending threads increases, Wait-Wound
> becomes inferior to Wait-Die in terms of elapsed time.
> Possibly due to the larger number of held locks of sleeping transactions.
>
> Update documentation and callers.
>
> Timings using git://people.freedesktop.org/~thomash/ww_mutex_test
> tag patch-18-06-04
>
> Each thread runs 100000 batches of lock / unlock 800 ww mutexes randomly
> chosen out of 100000. Four core Intel x86_64:
>
> Algorithm #threads Rollbacks time
> Wound-Wait 4 ~100 ~17s.
> Wait-Die 4 ~150000 ~19s.
> Wound-Wait 16 ~360000 ~109s.
> Wait-Die 16 ~450000 ~82s.

> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> index 39fda195bf78..6278077f288b 100644
> --- a/include/linux/ww_mutex.h
> +++ b/include/linux/ww_mutex.h
> @@ -8,6 +8,8 @@
> *
> * Wound/wait implementation:
> * Copyright (C) 2013 Canonical Ltd.
> + * Choice of algorithm:
> + * Copyright (C) 2018 WMWare Inc.
> *
> * This file contains the main data structure and API definitions.
> */
> @@ -23,15 +25,17 @@ struct ww_class {
> struct lock_class_key mutex_key;
> const char *acquire_name;
> const char *mutex_name;
> + bool is_wait_die;
> };

No _Bool in composites please.

> struct ww_acquire_ctx {
> struct task_struct *task;
> unsigned long stamp;
> unsigned acquired;
> + bool wounded;

Again.

> + struct ww_class *ww_class;
> #ifdef CONFIG_DEBUG_MUTEXES
> unsigned done_acquire;
> - struct ww_class *ww_class;
> struct ww_mutex *contending_lock;
> #endif
> #ifdef CONFIG_DEBUG_LOCK_ALLOC

> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 2048359f33d2..b449a012c6f9 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -290,12 +290,47 @@ __ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
> (a->stamp != b->stamp || a > b);
> }
>
> +/*
> + * Wound the lock holder transaction if it's younger than the contending
> + * transaction, and there is a possibility of a deadlock.
> + * Also if the lock holder transaction isn't the current transaction,

Comma followed by a capital?

> + * Make sure it's woken up in case it's sleeping on another ww mutex.

> + */
> +static bool __ww_mutex_wound(struct mutex *lock,
> + struct ww_acquire_ctx *ww_ctx,
> + struct ww_acquire_ctx *hold_ctx)
> +{
> + struct task_struct *owner =
> + __owner_task(atomic_long_read(&lock->owner));

Did you just spell __mutex_owner() wrong?

> +
> + lockdep_assert_held(&lock->wait_lock);
> +
> + if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) &&
> + ww_ctx->acquired > 0) {
> + WRITE_ONCE(hold_ctx->wounded, true);
> + if (owner != current) {
> + /*
> + * wake_up_process() inserts a write memory barrier to

It does no such thing. But yes, it does ensure the wakee sees all prior
stores IFF the wakeup happened.

> + * make sure owner sees it is wounded before
> + * TASK_RUNNING in case it's sleeping on another
> + * ww_mutex. Note that owner points to a valid
> + * task_struct as long as we hold the wait_lock.
> + */

What exactly are you trying to say here ?

I'm thinking this is the pairing barrier to the smp_mb() below, with
your list_empty() thing? Might make sense to write a single coherent
comment and refer to the other location.

> + wake_up_process(owner);
> + }
> + return true;
> + }
> +
> + return false;
> +}
> +
> /*
> * Wake up any waiters that may have to back off when the lock is held by the
> * given context.
> *
> * Due to the invariants on the wait list, this can only affect the first
> - * waiter with a context.
> + * waiter with a context, unless the Wound-Wait algorithm is used where
> + * also subsequent waiters with a context main wound the lock holder.
> *
> * The current task must not be on the wait list.
> */
> @@ -303,6 +338,7 @@ static void __sched
> __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
> {
> struct mutex_waiter *cur;
> + bool is_wait_die = ww_ctx->ww_class->is_wait_die;
>
> lockdep_assert_held(&lock->wait_lock);
>
> @@ -310,13 +346,14 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
> if (!cur->ww_ctx)
> continue;
>
> - if (cur->ww_ctx->acquired > 0 &&
> + if (is_wait_die && cur->ww_ctx->acquired > 0 &&
> __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
> debug_mutex_wake_waiter(lock, cur);
> wake_up_process(cur->task);
> }
>
> - break;
> + if (is_wait_die || __ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
> + break;
> }
> }
>
> @@ -338,12 +375,17 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
> * and keep spinning, or it will acquire wait_lock, add itself
> * to waiter list and sleep.
> */
> - smp_mb(); /* ^^^ */
> + smp_mb(); /* See comments above and below. */
>
> /*
> - * Check if lock is contended, if not there is nobody to wake up
> + * Check if lock is contended, if not there is nobody to wake up.
> + * Checking MUTEX_FLAG_WAITERS is not enough here,

That seems like a superfluous thing to say. It makes sense in the
context of this patch because we change the FLAG check into a list
check, but the resulting comment/code looks odd.

> since we need to
> + * order against the lock->ctx check in __ww_mutex_wound called from
> + * __ww_mutex_add_waiter. We can use list_empty without taking the
> + * wait_lock, given the memory barrier above and the list_empty
> + * documentation.

I don't trust documentation. Please reason about implementation.

> */
> - if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
> + if (likely(list_empty(&lock->base.wait_list)))
> return;
>
> /*
> @@ -653,6 +695,17 @@ __ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
> struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
> struct mutex_waiter *cur;
>
> + /*
> + * If we miss a wounded == true here, we will have a pending

Explain how we can miss that.

> + * TASK_RUNNING and pick it up on the next schedule fall-through.
> + */
> + if (!ctx->ww_class->is_wait_die) {
> + if (READ_ONCE(ctx->wounded))
> + goto deadlock;
> + else
> + return 0;
> + }
> +
> if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
> goto deadlock;
>
> @@ -683,12 +736,15 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
> {
> struct mutex_waiter *cur;
> struct list_head *pos;
> + bool is_wait_die;
>
> if (!ww_ctx) {
> list_add_tail(&waiter->list, &lock->wait_list);
> return 0;
> }
>
> + is_wait_die = ww_ctx->ww_class->is_wait_die;
> +
> /*
> * Add the waiter before the first waiter with a higher stamp.
> * Waiters without a context are skipped to avoid starving
> @@ -701,7 +757,7 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>
> if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
> /* Back off immediately if necessary. */
> - if (ww_ctx->acquired > 0) {
> + if (is_wait_die && ww_ctx->acquired > 0) {
> #ifdef CONFIG_DEBUG_MUTEXES
> struct ww_mutex *ww;
>
> @@ -721,13 +777,26 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
> * Wake up the waiter so that it gets a chance to back
> * off.
> */
> - if (cur->ww_ctx->acquired > 0) {
> + if (is_wait_die && cur->ww_ctx->acquired > 0) {
> debug_mutex_wake_waiter(lock, cur);
> wake_up_process(cur->task);
> }
> }
>
> list_add_tail(&waiter->list, pos);
> + if (!is_wait_die) {
> + struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
> +
> + /*
> + * Make sure a racing lock taker sees a non-empty waiting list
> + * before we read ww->ctx, so that if we miss ww->ctx, the
> + * racing lock taker will call __ww_mutex_wake_up_for_backoff()
> + * and wound itself.
> + */
> + smp_mb();
> + __ww_mutex_wound(lock, ww_ctx, ww->ctx);
> + }
> +
> return 0;
> }
>
> @@ -750,6 +819,14 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> if (use_ww_ctx && ww_ctx) {
> if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
> return -EALREADY;
> +
> + /*
> + * Reset the wounded flag after a backoff.
> + * No other process can race and wound us here since they
> + * can't have a valid owner pointer at this time
> + */
> + if (ww_ctx->acquired == 0)
> + ww_ctx->wounded = false;
> }
>
> preempt_disable();
> @@ -858,6 +935,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> acquired:
> __set_current_state(TASK_RUNNING);
>
> + /* We stole the lock. Need to check wounded status. */
> + if (use_ww_ctx && ww_ctx && !ww_ctx->ww_class->is_wait_die &&
> + !__mutex_waiter_is_first(lock, &waiter))
> + __ww_mutex_wakeup_for_backoff(lock, ww_ctx);
> +
> mutex_remove_waiter(lock, &waiter, current);
> if (likely(list_empty(&lock->wait_list)))
> __mutex_clear_flag(lock, MUTEX_FLAGS);

I can't say I'm a fan. I'm already cursing the ww_mutex stuff every time
I have to look at it, and you just made it worse spagethi.


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