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Date: Tue, 22 Aug 2023 15:11:40 -0400
From: Waiman Long <longman@...hat.com>
To: John Stultz <jstultz@...gle.com>,
LKML <linux-kernel@...r.kernel.org>
Cc: Joel Fernandes <joelaf@...gle.com>,
Qais Yousef <qyousef@...gle.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Valentin Schneider <vschneid@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Zimuzo Ezeozue <zezeozue@...gle.com>,
Youssef Esmat <youssefesmat@...gle.com>,
Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>,
"Paul E . McKenney" <paulmck@...nel.org>, kernel-team@...roid.com
Subject: Re: [PATCH v5 02/19] locking/mutex: Removes wakeups from under
mutex::wait_lock
On 8/19/23 02:08, John Stultz wrote:
> In preparation to nest mutex::wait_lock under rq::lock we need to remove
> wakeups from under it.
>
> Cc: Joel Fernandes <joelaf@...gle.com>
> Cc: Qais Yousef <qyousef@...gle.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
> Cc: Youssef Esmat <youssefesmat@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: "Paul E . McKenney" <paulmck@...nel.org>
> Cc: kernel-team@...roid.com
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> [Heavily changed after 55f036ca7e74 ("locking: WW mutex cleanup") and
> 08295b3b5bee ("locking: Implement an algorithm choice for Wound-Wait
> mutexes")]
> Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> [jstultz: rebased to mainline, added extra wake_up_q & init
> to avoid hangs, similar to Connor's rework of this patch]
> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---
> v5:
> * Reverted back to an earlier version of this patch to undo
> the change that kept the wake_q in the ctx structure, as
> that broke the rule that the wake_q must always be on the
> stack, as its not safe for concurrency.
> ---
> kernel/locking/mutex.c | 15 ++++++++++++---
> kernel/locking/ww_mutex.h | 29 ++++++++++++++++++-----------
> 2 files changed, 30 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index d973fe6041bf..118b6412845c 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -570,6 +570,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> struct lockdep_map *nest_lock, unsigned long ip,
> struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
> {
> + DEFINE_WAKE_Q(wake_q);
> struct mutex_waiter waiter;
> struct ww_mutex *ww;
> int ret;
> @@ -620,7 +621,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> */
> if (__mutex_trylock(lock)) {
> if (ww_ctx)
> - __ww_mutex_check_waiters(lock, ww_ctx);
> + __ww_mutex_check_waiters(lock, ww_ctx, &wake_q);
>
> goto skip_wait;
> }
> @@ -640,7 +641,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> * Add in stamp order, waking up waiters that must kill
> * themselves.
> */
> - ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
> + ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx, &wake_q);
> if (ret)
> goto err_early_kill;
> }
> @@ -676,6 +677,10 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> }
>
> raw_spin_unlock(&lock->wait_lock);
> + /* Make sure we do wakeups before calling schedule */
> + wake_up_q(&wake_q);
> + wake_q_init(&wake_q);
> +
The wake_q may have task to wake up only in the case of ww_mutex which
is a minority in the kernel. IOW, wake_up_q() which is a function call
will do nothing in most cases. From an optimization point of view, it is
better to do a "!wake_q_empty(&wake_q)" check before calling wake_up_q().
> schedule_preempt_disabled();
>
> first = __mutex_waiter_is_first(lock, &waiter);
> @@ -709,7 +714,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> */
> if (!ww_ctx->is_wait_die &&
> !__mutex_waiter_is_first(lock, &waiter))
> - __ww_mutex_check_waiters(lock, ww_ctx);
> + __ww_mutex_check_waiters(lock, ww_ctx, &wake_q);
> }
>
> __mutex_remove_waiter(lock, &waiter);
> @@ -725,6 +730,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> ww_mutex_lock_acquired(ww, ww_ctx);
>
> raw_spin_unlock(&lock->wait_lock);
> + wake_up_q(&wake_q);
> preempt_enable();
> return 0;
>
> @@ -736,6 +742,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> raw_spin_unlock(&lock->wait_lock);
> debug_mutex_free_waiter(&waiter);
> mutex_release(&lock->dep_map, ip);
> + wake_up_q(&wake_q);
> preempt_enable();
> return ret;
> }
> @@ -946,9 +953,11 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
> if (owner & MUTEX_FLAG_HANDOFF)
> __mutex_handoff(lock, next);
>
> + preempt_disable();
> raw_spin_unlock(&lock->wait_lock);
>
> wake_up_q(&wake_q);
> + preempt_enable();
> }
I think it looks better to put the preempt_disable() right before
raw_spin_lock() for proper nesting.
Cheers,
Longman
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