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Date: Mon, 28 Mar 2022 10:41:31 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
Waiman Long <longman@...hat.com>,
Boqun Feng <boqun.feng@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Steven Rostedt <rostedt@...dmis.org>,
Byungchul Park <byungchul.park@....com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Arnd Bergmann <arnd@...db.de>,
Radoslaw Burny <rburny@...gle.com>,
linux-arch <linux-arch@...r.kernel.org>,
bpf <bpf@...r.kernel.org>, Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow path
Hi Peter,
On Mon, Mar 28, 2022 at 4:29 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Mar 22, 2022 at 11:57:09AM -0700, Namhyung Kim wrote:
> > Adding the lock contention tracepoints in various lock function slow
> > paths. Note that each arch can define spinlock differently, I only
> > added it only to the generic qspinlock for now.
> >
> > Tested-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > kernel/locking/mutex.c | 3 +++
> > kernel/locking/percpu-rwsem.c | 3 +++
> > kernel/locking/qrwlock.c | 9 +++++++++
> > kernel/locking/qspinlock.c | 5 +++++
> > kernel/locking/rtmutex.c | 11 +++++++++++
> > kernel/locking/rwbase_rt.c | 3 +++
> > kernel/locking/rwsem.c | 9 +++++++++
> > kernel/locking/semaphore.c | 15 ++++++++++++++-
> > 8 files changed, 57 insertions(+), 1 deletion(-)
>
> I had conflicts in rwsem.c due to Waiman's patches, but that was simple
> enough to resolve. However, I had a good look at the other sites and
> ended up with the below...
>
> Yes, I know I'm the one that suggested the percpu thing, but upon
> looking again it missed the largest part of percpu_down_write(), which
> very much includes that RCU grace period and waiting for the readers to
> bugger off
>
> Also, rwbase_rt was missing the entire READ side -- yes, I see that's
> also covered by the rtmuex.c part, but that's on a different address and
> with different flags, and it's very confusing to not have it annotated.
>
> Anyway, I'll queue this patch with the below folded in for post -rc1.
Thanks for doing this, the changes look good.
Namhyung
>
> ---
>
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -155,7 +155,6 @@ static void percpu_rwsem_wait(struct per
> }
> spin_unlock_irq(&sem->waiters.lock);
>
> - trace_contention_begin(sem, LCB_F_PERCPU | (reader ? LCB_F_READ : LCB_F_WRITE));
> while (wait) {
> set_current_state(TASK_UNINTERRUPTIBLE);
> if (!smp_load_acquire(&wq_entry.private))
> @@ -163,7 +162,6 @@ static void percpu_rwsem_wait(struct per
> schedule();
> }
> __set_current_state(TASK_RUNNING);
> - trace_contention_end(sem, 0);
> }
>
> bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
> @@ -174,9 +172,11 @@ bool __sched __percpu_down_read(struct p
> if (try)
> return false;
>
> + trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);
> preempt_enable();
> percpu_rwsem_wait(sem, /* .reader = */ true);
> preempt_disable();
> + trace_contention_end(sem, 0);
>
> return true;
> }
> @@ -219,6 +219,7 @@ void __sched percpu_down_write(struct pe
> {
> might_sleep();
> rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
> + trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
>
> /* Notify readers to take the slow path. */
> rcu_sync_enter(&sem->rss);
> @@ -240,6 +241,7 @@ void __sched percpu_down_write(struct pe
>
> /* Wait for all active readers to complete. */
> rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
> + trace_contention_end(sem, 0);
> }
> EXPORT_SYMBOL_GPL(percpu_down_write);
>
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -35,7 +35,7 @@ void queued_read_lock_slowpath(struct qr
> }
> atomic_sub(_QR_BIAS, &lock->cnts);
>
> - trace_contention_begin(lock, LCB_F_READ | LCB_F_SPIN);
> + trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
>
> /*
> * Put the reader into the wait queue
> @@ -67,7 +67,7 @@ void queued_write_lock_slowpath(struct q
> {
> int cnts;
>
> - trace_contention_begin(lock, LCB_F_WRITE | LCB_F_SPIN);
> + trace_contention_begin(lock, LCB_F_SPIN | LCB_F_WRITE);
>
> /* Put the writer into the wait queue */
> arch_spin_lock(&lock->wait_lock);
> --- a/kernel/locking/rwbase_rt.c
> +++ b/kernel/locking/rwbase_rt.c
> @@ -112,6 +112,8 @@ static int __sched __rwbase_read_lock(st
> * Reader2 to call up_read(), which might be unbound.
> */
>
> + trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
> +
> /*
> * For rwlocks this returns 0 unconditionally, so the below
> * !ret conditionals are optimized out.
> @@ -130,6 +132,8 @@ static int __sched __rwbase_read_lock(st
> raw_spin_unlock_irq(&rtm->wait_lock);
> if (!ret)
> rwbase_rtmutex_unlock(rtm);
> +
> + trace_contention_end(rwb, ret);
> return ret;
> }
>
> @@ -247,7 +251,7 @@ static int __sched rwbase_write_lock(str
> goto out_unlock;
>
> rwbase_set_and_save_current_state(state);
> - trace_contention_begin(rwb, LCB_F_WRITE | LCB_F_RT);
> + trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
> for (;;) {
> /* Optimized out for rwlocks */
> if (rwbase_signal_pending_state(state, current)) {
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