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Message-ID: <d293d88c-b83b-a955-de5e-db775f20c1e1@amd.com>
Date: Wed, 10 Jul 2024 23:11:51 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>
CC: Peter Zijlstra <peterz@...radead.org>, Joel Fernandes <joelaf@...gle.com>,
Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>, 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>, Will Deacon
<will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng
<boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya
<Metin.Kaya@....com>, Xuewen Yan <xuewen.yan94@...il.com>, Thomas Gleixner
<tglx@...utronix.de>, Daniel Lezcano <daniel.lezcano@...aro.org>,
<kernel-team@...roid.com>, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v11 1/7] locking/mutex: Remove wakeups from under
mutex::wait_lock
Hello John,
On 7/10/2024 2:01 AM, John Stultz wrote:
> From: Peter Zijlstra <peterz@...radead.org>
>
> 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@...alina.io>
> 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: 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: Metin Kaya <Metin.Kaya@....com>
> Cc: Xuewen Yan <xuewen.yan94@...il.com>
> Cc: K Prateek Nayak <kprateek.nayak@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.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>
> Tested-by: K Prateek Nayak <kprateek.nayak@....com>
> Tested-by: Metin Kaya <metin.kaya@....com>
> Acked-by: Davidlohr Bueso <dave@...olabs.net>
> Reviewed-by: Metin Kaya <metin.kaya@....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.
> v6:
> * Made tweaks suggested by Waiman Long
> v7:
> * Fixups to pass wake_qs down for PREEMPT_RT logic
> v10:
> * Switched preempt_enable to be lower close to the unlock as
> suggested by Valentin
> * Added additional preempt_disable coverage around the wake_q
> calls as again noted by Valentin
> ---
> kernel/locking/mutex.c | 17 +++++++++++++----
> kernel/locking/rtmutex.c | 30 +++++++++++++++++++++---------
> kernel/locking/rwbase_rt.c | 8 +++++++-
> kernel/locking/rwsem.c | 4 ++--
> kernel/locking/spinlock_rt.c | 3 ++-
> kernel/locking/ww_mutex.h | 29 ++++++++++++++++++-----------
> 6 files changed, 63 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index cbae8c0b89ab..4269da1f3ef5 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -575,6 +575,7 @@ __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;
> @@ -625,7 +626,7 @@ __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;
> }
> @@ -645,7 +646,7 @@ __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;
> }
> @@ -681,6 +682,11 @@ __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 */
> + if (!wake_q_empty(&wake_q)) {
nit.
This checks seems unnecessary (to my untrained eye). Any harm in
skipping it and simply doing a wake_up_q() followed by wake_q_init()
unconditionally?
> + wake_up_q(&wake_q);
> + wake_q_init(&wake_q);
> + }
> schedule_preempt_disabled();
>
> first = __mutex_waiter_is_first(lock, &waiter);
> @@ -714,7 +720,7 @@ __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);
> @@ -730,6 +736,7 @@ __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;
>
> @@ -741,6 +748,7 @@ __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;
> }
> @@ -951,9 +959,10 @@ 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();
> }
>
> #ifndef CONFIG_DEBUG_LOCK_ALLOC
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 88d08eeb8bc0..7a85d9bfa972 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -34,13 +34,15 @@
>
> static inline int __ww_mutex_add_waiter(struct rt_mutex_waiter *waiter,
> struct rt_mutex *lock,
> - struct ww_acquire_ctx *ww_ctx)
> + struct ww_acquire_ctx *ww_ctx,
> + struct wake_q_head *wake_q)
> {
> return 0;
> }
>
> static inline void __ww_mutex_check_waiters(struct rt_mutex *lock,
> - struct ww_acquire_ctx *ww_ctx)
> + struct ww_acquire_ctx *ww_ctx,
> + struct wake_q_head *wake_q)
> {
> }
>
> @@ -1207,6 +1209,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
> struct rt_mutex_waiter *top_waiter = waiter;
> struct rt_mutex_base *next_lock;
> int chain_walk = 0, res;
> + DEFINE_WAKE_Q(wake_q);
>
> lockdep_assert_held(&lock->wait_lock);
>
> @@ -1245,7 +1248,10 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
>
> /* Check whether the waiter should back out immediately */
> rtm = container_of(lock, struct rt_mutex, rtmutex);
> - res = __ww_mutex_add_waiter(waiter, rtm, ww_ctx);
> + preempt_disable();
> + res = __ww_mutex_add_waiter(waiter, rtm, ww_ctx, &wake_q);
> + wake_up_q(&wake_q);
> + preempt_enable();
I'm trying to understand this - we enter task_blocks_on_rt_mutex() with
"wait_lock" held (I believe the lockdep_assert_held() in the previous
hunk checks for the same). I walked down the call chain (although
briefly) and could only spot "task->pi_lock" being locked and unlocked
before this call to "wake_up_q()" but the "wait_lock" seems to be held
throughout, only being unlocked and locked again for
"rt_mutex_adjust_prio_chain()" later down.
Did I miss something or is disabling preemption for this specific hunk
enough to enable safe nesting?
--
Thanks and Regards,
Prateek
> if (res) {
> raw_spin_lock(&task->pi_lock);
> rt_mutex_dequeue(lock, waiter);
> [..snip..]
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