From: Waiman Long <longman@redhat.com>
To: Hillf Danton <hdanton@sina.com>
Cc: "Mukesh Ojha" <quic_mojha@quicinc.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Ingo Molnar" <mingo@redhat.com>, "Will Deacon" <will@kernel.org>,
"Boqun Feng" <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com,
"Ting11 Wang 王婷" <wangting11@xiaomi.com>
Subject: Re: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff
Date: Tue, 18 Oct 2022 22:49:02 -0400 [thread overview]
Message-ID: <086fd2bd-0d24-9f7b-8264-448f1e27c3b5@redhat.com> (raw)
In-Reply-To: <20221019022934.1166-1-hdanton@sina.com>
On 10/18/22 22:29, Hillf Danton wrote:
> On 18 Oct 2022 20:39:59 -0400 Waiman Long <longman@redhat.com>
>> On 10/18/22 19:51, Hillf Danton wrote:
>>> On 18 Oct 2022 13:37:20 -0400 Waiman Long <longman@redhat.com>
>>>> On 10/18/22 10:13, Mukesh Ojha wrote:
>>>>> On 10/18/2022 4:44 PM, Hillf Danton wrote:
>>>>>> On 17 Oct 2022 17:13:55 -0400 Waiman Long <longman@redhat.com>
>>>>>>> @@ -1067,13 +1119,33 @@ rwsem_down_read_slowpath(struct rw_semaphore
>>>>>>> return sem;
>>>>>>> }
>>>>>>> adjustment += RWSEM_FLAG_WAITERS;
>>>>>>> + } else if ((count & RWSEM_FLAG_HANDOFF) &&
>>>>>>> + ((count & RWSEM_LOCK_MASK) == RWSEM_READER_BIAS)) {
>>>>>> Could a couple of CPUs go read slow path in parallel?
>>>>>>
>>>> This is under wait_lock protection. So no parallel execution is possible.
>>> They individually add RWSEM_READER_BIAS to count before taking wait_lock,
>>> and the check for BIAS here does not cover the case of readers in parallel.
>>> Is this intended?
>>>
>>> Hillf
>> As I said in the patch description, the lock handoff can only be done if
>> we can be sure that there is no other active locks outstanding with the
>> handoff bit set. If at the time of the check, another reader come in and
>> adds its RWSEM_READER_BIAS, the check fail and the cpu will proceed to
>> put its waiter in the queue and begin sleeping. Hopefully, the last one
>> left will find that count has only its RWSEM_READER_BIAS and it can
>> start the handoff process.
> If handoff grants rwsem to a read waiter then the read fast path may revive.
I don't quite understand what you mean by "read fast path may revive".
> And at the time of the check, multiple readers do not break handoff IMO.
I am not saying that multiple readers will break handoff. They will just
delay it until all their temporary RWSEM_READ_BIAS are taken off.
Cheers,
Longman
next prev parent reply other threads:[~2022-10-19 2:49 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-17 21:13 [PATCH v3 0/5] lockinig/rwsem: Fix rwsem bugs & enable true lock handoff Waiman Long
2022-10-17 21:13 ` [PATCH v3 1/5] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Waiman Long
2022-10-24 13:17 ` Peter Zijlstra
2022-10-24 13:50 ` Waiman Long
2022-10-17 21:13 ` [PATCH v3 2/5] locking/rwsem: Limit # of null owner retries for handoff writer Waiman Long
2022-10-24 13:31 ` Peter Zijlstra
2022-10-24 15:55 ` Waiman Long
2022-10-25 11:22 ` Peter Zijlstra
2022-10-25 11:48 ` Peter Zijlstra
2022-10-25 19:55 ` Waiman Long
2022-10-25 20:14 ` Peter Zijlstra
2022-10-26 1:44 ` Waiman Long
[not found] ` <20221025145843.2953-1-hdanton@sina.com>
2022-10-25 19:00 ` Waiman Long
2022-10-17 21:13 ` [PATCH v3 3/5] locking/rwsem: Change waiter->hanodff_set to a handoff_state enum Waiman Long
2022-10-17 21:13 ` [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Waiman Long
2022-10-17 21:13 ` [PATCH v3 5/5] locking/rwsem: Update handoff lock events tracking Waiman Long
[not found] ` <20221018111424.1007-1-hdanton@sina.com>
2022-10-18 14:13 ` [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Mukesh Ojha
2022-10-18 17:37 ` Waiman Long
[not found] ` <20221018235138.1088-1-hdanton@sina.com>
2022-10-19 0:39 ` Waiman Long
[not found] ` <20221019022934.1166-1-hdanton@sina.com>
2022-10-19 2:49 ` Waiman Long [this message]
[not found] ` <20221019070559.1220-1-hdanton@sina.com>
2022-10-19 15:02 ` Waiman Long
2022-10-24 16:18 ` Waiman Long
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