From: Sagi Grimberg <sagi@grimberg.me>
To: Ming Lei <ming.lei@redhat.com>, Chao Leng <lengchao@huawei.com>
Cc: Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
linux-nvme@lists.infradead.org
Subject: Re: [PATCH v3 1/2] blk-mq: add async quiesce interface
Date: Mon, 27 Jul 2020 11:40:24 -0700 [thread overview]
Message-ID: <f23c66b8-8c93-4b4d-56ab-dd17e9ddf4a2@grimberg.me> (raw)
In-Reply-To: <20200727063220.GA1144698@T590>
>>>>>> It is at the end and contains exactly what is needed to synchronize. Not
>>>>> The sync is simply single global synchronize_rcu(), and why bother to add
>>>>> extra >=40bytes for each hctx.
>>>>>
>>>>>> sure what you mean by reuse hctx->srcu?
>>>>> You already reuses hctx->srcu, but not see reason to add extra rcu_synchronize
>>>>> to each hctx for just simulating one single synchronize_rcu().
>>>>
>>>> To sync srcu together, the extra bytes must be needed, seperate blocking
>>>> and non blocking queue to two hctx may be a not good choice.
>>>>
>>>> There is two choice: the struct rcu_synchronize is added in hctx or in srcu.
>>>> Though add rcu_synchronize in srcu has a weakness: the extra bytes is
>>>> not need if which do not need batch sync srcu, I still think it's better
>>>> for the SRCU to provide the batch synchronization interface.
>>>
>>> The 'struct rcu_synchronize' can be allocated from heap or even stack(
>>> if no too many NSs), which is just one shot sync and the API is supposed
>>> to be called in slow path. No need to allocate it as long lifetime variable.
>>> Especially 'struct srcu_struct' has already been too fat.
>>
>> Stack is not suitable, stack can not provide so many space for
>
> Stack is fine if number of NS is small, for example, most of times,
> there is just one NS.
I prefer a single code-path, so stack is not good.
>> many name space. Heap maybe a choice, but need to add abnormal treat
>> when alloc memory failed, Thus io pause time can not be ensured.
>> So the extra space may must be needed for batch srcu sync.
>
> In case of allocation failure, you can switch to synchronize_srcu() simply.
that is possible, but I still prefer to support both srcu and rcu hctx
in a single interface so I don't need to have different code paths in
nvme (or other drivers).
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next prev parent reply other threads:[~2020-07-27 18:40 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-26 0:22 [PATCH v3 0/2] improve quiesce time for large amount of namespaces Sagi Grimberg
2020-07-26 0:23 ` [PATCH v3 1/2] blk-mq: add async quiesce interface Sagi Grimberg
2020-07-26 9:31 ` Ming Lei
2020-07-26 16:27 ` Sagi Grimberg
2020-07-27 2:08 ` Ming Lei
2020-07-27 3:33 ` Chao Leng
2020-07-27 3:50 ` Ming Lei
2020-07-27 5:55 ` Chao Leng
2020-07-27 6:32 ` Ming Lei
2020-07-27 18:40 ` Sagi Grimberg [this message]
2020-07-27 18:38 ` Sagi Grimberg
2020-07-27 18:36 ` Sagi Grimberg
2020-07-27 20:37 ` Jens Axboe
2020-07-27 21:00 ` Sagi Grimberg
2020-07-27 21:05 ` Jens Axboe
2020-07-27 21:21 ` Keith Busch
2020-07-27 21:30 ` Jens Axboe
2020-07-28 1:09 ` Ming Lei
2020-07-26 0:23 ` [PATCH v3 2/2] nvme: improve quiesce time for large amount of namespaces Sagi Grimberg
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