linux-kernel - Re: [ata] 0568e61225: stress-ng.copy-file.ops_per

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Date:   Wed, 17 Aug 2022 08:55:00 -0700
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     John Garry <john.garry@...wei.com>,
        Oliver Sang <oliver.sang@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-ide@...r.kernel.org, lkp@...ts.01.org, lkp@...el.com,
        ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...ux.intel.com, fengwei.yin@...el.com
Subject: Re: [ata] 0568e61225: stress-ng.copy-file.ops_per_sec -15.0%
 regression

On 2022/08/16 13:44, John Garry wrote:
> On 16/08/2022 21:02, Damien Le Moal wrote:
>>> ou confirm this? Thanks!
>>>
>>> On this basis, it appears that max_hw_sectors_kb is getting capped from
>>> scsi default @ 1024 sectors by commit 0568e61225. If it were getting
>>> capped by swiotlb mapping limit then that would give us 512 sectors -
>>> this value is fixed.
>>>
>>> So for my SHT change proposal I am just trying to revert to previous
>>> behaviour in 5.19 - make max_hw_sectors_kb crazy big again.
>> I reread the entire thing and I think I got things reverted here. The perf
>> regression happens with the 512/512 settings, while the original 1280/32767
>> before your patches was OK.
> 
> Right, that's as I read it. It would be useful for Oliver to confirm the 
> results.
> 
>> So is your patch defining the optimal mapping size
>> cause the reduction to 512/512.
> 
> The optimal mapping size only affects specifically sas controllers, so I 
> think that we can ignore that one for now. The reduction to 512/512 
> comes from the change in ata_scsi_dev_config().
> 
>> It would mean that for ATA, we need a sane
>> default mapping instead of SCSI default 1024 sectors.
> 
> Right
> 
>> Now I understand your
>> proposed change using ATA_MAX_SECTORS_LBA48.
>>
>> However, that would be correct only for LBA48 capable drives.
>> ata_dev_configure() already sets dev->max_sectors correctly according to the
>> drive type, capabilities and eventual quirks. So the problem comes from the
>> libata-scsi change:
>>
>> dev->max_sectors = min(dev->max_sectors, sdev->host->max_sectors);
>>
>> when sdev->host->max_sectors is 0 (not initialized).
> 
> That cannot happen. If sht->max_sectors is 0, then we set 
> shost->max_sectors at SCSI default 1024 sectors in scsi_host_alloc()
> 
> For my proposed change, dev->max_sectors would still be initialized in 
> ata_dev_configure() according to drive type, etc. And it should be <= 
> LBA48 max sectors (=65535).
> 
> So then in ata_scsi_dev_config():
> 
> dev->max_sectors = min(dev->max_sectors, sdev->host->max_sectors)
> 
> this only has an impact for ahci controllers if sdev->host->max_sectors 
> was capped according to host dma dev max mapping size.

Got it. I think your fix is fine then. It brings everything the defaults to what
they were before the dma max mapping patches.

> 
> I will admit that this is not ideal. An alt approach is to change 
> ata_scsi_dev_config() to cap the dev max_sectors only according to shost 
> dma dev mapping limit (similar to scsi_add_host_with_dma()), but that 
> would not work for a controller like ipr, which does have a geniune 
> max_sectors limit (which we should respect).
> 
> Thanks,
> John
> 
> 
>> So maybe simply changing
>> this line to:
>>
>> dev->max_sectors = min_not_zero(dev->max_sectors, sdev->host->max_sectors);
>>
>> would do the trick ? Any particular adapter driver that needs a mapping cap on
>> the adpter max mapping size can still set sdev->host->max_sectors as needed, and
>> we keep the same defaults as before when it is not set. Thoughts ? Or am I
>> missing something else ?
>>
>>
>>>> The regression may come not from commands becoming tiny, but from the fact that
>>>> after the patch, max_sectors_kb is too large,
>>> I don't think it is, but need confirmation.
>>>
>>>> causing a lot of overhead with
>>>> qemu swiotlb mapping and slowing down IO processing.
>>>> Above, it can be seen that we ed up with max_sectors_kb being 1280, which is the
>>>> default for most scsi disks (including ATA drives). That is normal. But before
>>>> that, it was 512, which likely better fits qemu swiotlb and does not generate
>>> Again, I don't think this this is the case. Need confirmation.
>>>
>>>> overhead. So the above fix will not change anything I think...
> 


-- 
Damien Le Moal
Western Digital Research