linux-kernel - Re: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only if contended

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Date: Tue, 27 Feb 2024 19:18:55 -0500
From: Waiman Long <longman@...hat.com>
To: Namhyung Kim <namhyung@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 Will Deacon <will@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only
 if contended


On 2/27/24 18:02, Namhyung Kim wrote:
> Hello,
>
> On Mon, Nov 20, 2023 at 12:28 PM Namhyung Kim <namhyung@...nel.org> wrote:
>> Ping!
>>
>> On Wed, Nov 8, 2023 at 1:53 PM Namhyung Kim <namhyung@...nel.org> wrote:
>>> It mistakenly fires lock contention tracepoints always in the writer path.
>>> It should be conditional on the try lock result.
> Can anybody take a look at this?  This makes a large noise
> in the lock contention result.
>
> Thanks,
> Namhyung
>
>>> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
>>> ---
>>>   kernel/locking/percpu-rwsem.c | 11 ++++++++---
>>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
>>> index 185bd1c906b0..6083883c4fe0 100644
>>> --- a/kernel/locking/percpu-rwsem.c
>>> +++ b/kernel/locking/percpu-rwsem.c
>>> @@ -223,9 +223,10 @@ static bool readers_active_check(struct percpu_rw_semaphore *sem)
>>>
>>>   void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
>>>   {
>>> +       bool contended = false;
>>> +
>>>          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);
>>> @@ -234,8 +235,11 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
>>>           * Try set sem->block; this provides writer-writer exclusion.
>>>           * Having sem->block set makes new readers block.
>>>           */
>>> -       if (!__percpu_down_write_trylock(sem))
>>> +       if (!__percpu_down_write_trylock(sem)) {
>>> +               trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
>>>                  percpu_rwsem_wait(sem, /* .reader = */ false);
>>> +               contended = true;
>>> +       }
>>>
>>>          /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
>>>
>>> @@ -247,7 +251,8 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
>>>
>>>          /* Wait for all active readers to complete. */
>>>          rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
>>> -       trace_contention_end(sem, 0);
>>> +       if (contended)
>>> +               trace_contention_end(sem, 0);
>>>   }
>>>   EXPORT_SYMBOL_GPL(percpu_down_write);
>>>
>>> --
>>> 2.42.0.869.gea05f2083d-goog

Yes, that makes sense. Sorry for missing this patch.

Reviewed-by: Waiman Long <longman@...hat.com>