From: Waiman Long <waiman.long@hpe.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, <linux-kernel@vger.kernel.org>,
<x86@kernel.org>, <linux-alpha@vger.kernel.org>,
<linux-ia64@vger.kernel.org>, <linux-s390@vger.kernel.org>,
<linux-arch@vger.kernel.org>, Davidlohr Bueso <dave@stgolabs.net>,
Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [RFC PATCH-tip v2 3/6] locking/rwsem: Enable count-based spinning on reader
Date: Wed, 15 Jun 2016 15:28:03 -0400 [thread overview]
Message-ID: <5761AC43.5080707@hpe.com> (raw)
In-Reply-To: <20160615173830.GR30921@twins.programming.kicks-ass.net>
On 06/15/2016 01:38 PM, Peter Zijlstra wrote:
> On Tue, Jun 14, 2016 at 06:48:06PM -0400, Waiman Long wrote:
>> static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> {
>> - bool taken = false;
>> + bool taken = false, can_spin;
> I would place the variables without assignment first.
Sure, easy change.
>
>> + int loopcnt;
>>
>> preempt_disable();
>>
>> @@ -409,6 +412,8 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> if (!osq_lock(&sem->osq))
>> goto done;
>>
>> + loopcnt = sem->rspin_enabled ? RWSEM_RSPIN_THRESHOLD : 0;
>> +
>> /*
>> * Optimistically spin on the owner field and attempt to acquire the
>> * lock whenever the owner changes. Spinning will be stopped when:
>> @@ -416,7 +421,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> * 2) readers own the lock as we can't determine if they are
>> * actively running or not.
>> */
>> - while (rwsem_spin_on_owner(sem)) {
>> + while ((can_spin = rwsem_spin_on_owner(sem)) || loopcnt) {
>> /*
>> * Try to acquire the lock
>> */
>> @@ -425,13 +430,16 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> break;
>> }
>>
>> + if (!can_spin&& loopcnt)
>> + loopcnt--;
> This seems to suggest 'can_spin' is a bad name, because if we cannot
> spin, we do in fact spin anyway?
>
> Maybe call it write_spin or something, which makes it clear that if its
> not a write spin we'll do a read spin?
I am fine with the write_spin name.
>
> Also, isn't this the wrong level to do loopcnt at?
> rwsem_spin_on_owner() can have spend any amount of cycles spinning. So
> you're not counting loops of similar unit.
The loopcnt does not include time spinning on writer. It will be
decremented only if the lock is owned by reader (can_spin == false). I
will clarify that with additional comments.
>> + /*
>> + * Was owner a reader?
>> + */
>> + if (rwsem_owner_is_reader(sem->owner)) {
>> + /*
>> + * Update rspin_enabled for reader spinning
> full stop and newline?
Sure.
>
>> + * Increment by 1 if successfully& decrement by 8 if
>> + * unsuccessful.
> This is bloody obvious from the code, explain why, not what the code
> does.
Will clarify the comment.
>> The decrement amount is kind of arbitrary
>> + * and can be adjusted if necessary.
>> + */
>> + if (taken&& (sem->rspin_enabled< RWSEM_RSPIN_ENABLED_MAX))
>> + sem->rspin_enabled++;
>> + else if (!taken)
>> + sem->rspin_enabled = (sem->rspin_enabled>= 8)
>> + ? sem->rspin_enabled - 8 : 0;
> This is unreadable and against coding style.
I will fix the coding style problem.
Cheers,
Longman
next prev parent reply other threads:[~2016-06-15 19:43 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-14 22:48 [RFC PATCH-tip v2 0/6] locking/rwsem: Enable reader optimistic spinning Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Waiman Long
2016-06-15 8:04 ` Boqun Feng
2016-06-15 17:18 ` Peter Zijlstra
2016-06-15 19:01 ` Waiman Long
2016-06-16 2:19 ` Boqun Feng
2016-06-16 10:16 ` Will Deacon
2016-06-16 21:35 ` Waiman Long
2016-06-17 0:48 ` Boqun Feng
2016-06-17 15:26 ` Waiman Long
2016-06-17 15:45 ` Will Deacon
2016-06-17 18:17 ` Waiman Long
2016-06-18 8:46 ` Boqun Feng
2016-06-20 7:59 ` Will Deacon
2016-06-15 16:56 ` Davidlohr Bueso
2016-06-15 17:12 ` Peter Zijlstra
2016-06-15 18:27 ` Davidlohr Bueso
2016-06-15 18:40 ` Peter Zijlstra
2016-06-15 18:56 ` Davidlohr Bueso
2016-06-17 1:11 ` Davidlohr Bueso
2016-06-17 14:28 ` Waiman Long
2016-06-17 16:29 ` Davidlohr Bueso
2016-06-17 16:46 ` Davidlohr Bueso
2016-06-15 19:08 ` Waiman Long
2016-06-15 20:04 ` Waiman Long
2016-06-15 21:59 ` Peter Zijlstra
2016-06-14 22:48 ` [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP Waiman Long
2016-06-15 17:22 ` Peter Zijlstra
2016-06-15 19:17 ` Waiman Long
2016-06-16 2:14 ` Davidlohr Bueso
2016-06-16 21:25 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 3/6] locking/rwsem: Enable count-based spinning on reader Waiman Long
2016-06-15 17:38 ` Peter Zijlstra
2016-06-15 19:28 ` Waiman Long [this message]
2016-06-14 22:48 ` [RFC PATCH-tip v2 4/6] locking/rwsem: move down rwsem_down_read_failed function Waiman Long
2016-06-15 17:40 ` Peter Zijlstra
2016-06-15 19:21 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation Waiman Long
2016-06-15 17:43 ` Peter Zijlstra
2016-06-15 19:31 ` Waiman Long
2016-06-15 21:57 ` Peter Zijlstra
2016-06-15 17:45 ` Peter Zijlstra
2016-06-15 19:35 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 6/6] locking/rwsem: Enable spinning readers Waiman Long
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