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Message-ID: <b3ce0a28-0f94-6d2a-3f88-998da8f870b4@opensource.wdc.com>
Date:   Wed, 10 Aug 2022 06:52:03 -0700
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Subject: Re: [ata] 0568e61225: stress-ng.copy-file.ops_per_sec -15.0%
 regression
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To:     John Garry <john.garry@huawei.com>,
        kernel test robot <oliver.sang@intel.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-ide@vger.kernel.org, lkp@lists.01.org, lkp@intel.com,
        ying.huang@intel.com, feng.tang@intel.com,
        zhengjun.xing@linux.intel.com, fengwei.yin@intel.com
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From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <82dbf4d6-2d43-20ff-22a7-857f9f11a5ce@huawei.com>
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On 2022/08/10 1:33, John Garry wrote:
> On 09/08/2022 15:57, Damien Le Moal wrote:
>>>> As far as I can see, this patch should not make a difference unless the
>>>> ATA shost driver is setting the max_sectors value unnecessarily low.
>>> For __ATA_BASE_SHT, we don't set max_sectors. As such, we default
>>> shost->max_sectors = SCSI_DEFAULT_MAX_SECTORS (=1024) in
>>> scsi_host_alloc(). I assume no shost dma mapping limit applied.
>>>
>>> Then - for example - we could select dev->max_sectors =
>>> ATA_MAX_SECTORS_LBA48 (=65535) in ata_dev_configure().
>>>
>>> So with commit 0568e6122574 we would have final max sectors = 1024, as
>>> opposed to 65535 previously. I guess that the problem is something like
>>> this.
>>>
>>> If so, it seems that we would need to apply the shost dma mapping limit
>>> separately in ata_scsi_dev_config() and not use shost->max_sectors.
>> OK. Will have a look at that.
>>
> 
> We may need to introduce something like shost->max_hw_sectors, which is 
> set according to sht max sectors and dma mapping limits. That could be 
> also used in USB scsiglue slave_configure()
> 
> Or else set max_sectors value for __ATA_BASE_SHT, but I don't know a 
> sane value there considering ATA_MAX_SECTORS_LBA48 gives max_sectors of 
> 65535.
> 
> Damien, please let me know if you need help now. I am just waiting for 
> you to test to prove this theory about dev->max_sectors being capped. I 
> don't have an AHCI setup readily-available for testing - just SAS cards 
> or QEMU.

I am on it.

> 
> Thanks,
> John


-- 
Damien Le Moal
Western Digital Research