Re: [PATCH RFC v7 10/12] megaraid_sas: switch fusion adapters to MQ

From: John Garry <john.garry@huawei.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>, <axboe@kernel.dk>,
	<jejb@linux.ibm.com>, <martin.petersen@oracle.com>,
	<don.brace@microsemi.com>,
	Sumit Saxena <sumit.saxena@broadcom.com>, <bvanassche@acm.org>,
	<hare@suse.com>, <hch@lst.de>,
	Shivasharan Srikanteshwara 
	<shivasharan.srikanteshwara@broadcom.com>,
	<linux-block@vger.kernel.org>, <linux-scsi@vger.kernel.org>,
	<esc.storagedev@microsemi.com>, <chenxiang66@hisilicon.com>,
	"PDL,MEGARAIDLINUX" <megaraidlinux.pdl@broadcom.com>
Subject: Re: [PATCH RFC v7 10/12] megaraid_sas: switch fusion adapters to MQ
Date: Thu, 23 Jul 2020 18:29:01 +0100	[thread overview]
Message-ID: <f4a896a3-756e-68bb-7700-cab1e5523c81@huawei.com> (raw)
In-Reply-To: <20200723140758.GA957464@T590>

>> As I see, since megaraid will have 1:1 mapping of CPU to hw queue, will
>> there only ever possibly a single bit set in ctx_map? If so, it seems a
>> waste to always check every sbitmap map. But adding logic for this may
>> negate any possible gains.
> 
> It really depends on min and max cpu id in the map, then sbitmap
> depth can be reduced to (max - min + 1). I'd suggest to double check that
> cost of sbitmap_any_bit_set() really matters.

Hi Ming,

I'm not sure that reducing the search range would help much, as we still 
need to load some indexes of map[], and at best this may be reduced from 
2/3 -> 1 elements, depending on nr_cpus.

> 
>>
>>>>>> for
>>>>>>> none because request may not be dispatched successfully by direct
>>>> issue.
>>>>>> When block layer attempt posting request to h/w queue directly (for
>>>>>> ioscheduler=none) and if it fails, it is calling
>>>>>> blk_mq_request_bypass_insert().
>>>>>> blk_mq_request_bypass_insert function will start the h/w queue from
>>>>>> submission context. Do we still have an issue if we skip running hw
>>>>>> queue from completion ?
>>>>> The thing is that we can't guarantee that direct issue or adding request
>>>> into
>>>>> hctx->dispatch is always done for MQ/none, for example, request still
>>>>> can be added to sw queue from blk_mq_flush_plug_list() when mq plug is
>>>>> applied.
>>>> I see even blk_mq_sched_insert_requests() from blk_mq_flush_plug_list make
>>>> sure it run the h/w queue. If all the submission path which deals with s/w
>>>> queue make sure they run h/w queue, can't we remove blk_mq_run_hw_queues()
>>>> from scsi_end_request ?
>>> No, one purpose of blk_mq_run_hw_queues() is for rerun queue in case that
>>> dispatch budget is running out of in submission path, and sdev->device_busy is
>>> shared by all hw queues on this scsi device.
>>>
>>> I posted one patch for avoiding it in scsi_end_request() before, looks it
>>> never lands upstream:
>>>
>>
>> I saw that you actually posted the v3:
>> https://lore.kernel.org/linux-scsi/BL0PR2101MB11230C5F70151037B23C0C35CE2D0@BL0PR2101MB1123.namprd21.prod.outlook.com/
>> And it no longer applies, due to the changes in scsi_mq_get_budget(), I
>> think, which look non-trivial. Any chance to repost?
> 
> OK, will post V4.

Thanks!