Re: [RFC PATCH 2/7] RDMA/rxe: Convert the triple tasklets to workqueues

From: Bob Pearson <rpearsonhpe@gmail.com>
To: "matsuda-daisuke@fujitsu.com" <matsuda-daisuke@fujitsu.com>,
	'Bart Van Assche' <bvanassche@acm.org>,
	Yanjun Zhu <yanjun.zhu@linux.dev>,
	"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
	"leonro@nvidia.com" <leonro@nvidia.com>,
	"jgg@nvidia.com" <jgg@nvidia.com>,
	"zyjzyj2000@gmail.com" <zyjzyj2000@gmail.com>
Cc: "nvdimm@lists.linux.dev" <nvdimm@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"yangx.jy@fujitsu.com" <yangx.jy@fujitsu.com>,
	"lizhijian@fujitsu.com" <lizhijian@fujitsu.com>,
	"y-goto@fujitsu.com" <y-goto@fujitsu.com>
Subject: Re: [RFC PATCH 2/7] RDMA/rxe: Convert the triple tasklets to workqueues
Date: Mon, 12 Sep 2022 14:52:27 -0500	[thread overview]
Message-ID: <4a9f2b42-f230-b951-f03b-588b94cd39a6@gmail.com> (raw)
In-Reply-To: <TYCPR01MB8455D739FC9FB034E3485C87E5449@TYCPR01MB8455.jpnprd01.prod.outlook.com>

On 9/12/22 02:58, matsuda-daisuke@fujitsu.com wrote:
> On Mon, Sep 12, 2022 12:09 AM Bart Van Assche wrote:
>> On 9/11/22 00:10, Yanjun Zhu wrote:
>>> I also implemented a workqueue for rxe. IMO, can we add a variable to
>>> decide to use tasklet or workqueue?
>>>
>>> If user prefer using tasklet, he can set the variable to use
>>> tasklet. And the default is tasklet. Set the variable to another
>>> value to use workqueue.
> 
> That's an interesting idea, but I am not sure how users specify it.
> IIRC, tasklets are generated when rdma link is added, typically by 
> executing ' rdma link add' command. I don't think we can add
> an device specific option to the utility(iproute2/rdma).
> 
>>
>> I'm in favor of removing all uses of the tasklet mechanism because of
>> the disadvantages of that mechanism. See also:
>> * "Eliminating tasklets" (https://lwn.net/Articles/239633/).
>> * "Modernizing the tasklet API" (https://lwn.net/Articles/830964/).
>> * Sebastian Andrzej Siewior's opinion about tasklets
>> (https://lore.kernel.org/all/YvovfXMJQAUBsvBZ@linutronix.de/).
> 
> I am also in favor of using workqueues alone not only because of the
> disadvantages above but also to avoid complexity. I would like to know
> if there is anybody who will bothered by the change especially in terms
> of performance.
> 
> Thanks,
> Daisuke
> 
>>
>> Thanks,
>>
>> Bart.
> 

The HPE patch set for work queues (I should send it in) kept the ability to run tasklets or work queues.
The reason was that the change was motivated by a benchmarking exercise and we found that the performance
of tasklets was noticeably better for one IO stream but for 2 or more IO streams work queues were better because we
could place the work on separate cpus. Tasklets have a tendency to bunch up on the same cpu. I am interested in
how Matsuda got better/same performance for work queues.

Bob