Re: [ata] 0568e61225: stress-ng.copy-file.ops_per_sec -15.0% regression

[John Garry]

On 09/08/2022 10:58, John Garry wrote:
> > > commit: 0568e6122574dcc1aded2979cd0245038efe22b6 ("ata: libata-scsi: 
> > > cap ata_device->max_sectors according to shost->max_sectors")
> > > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> > >
> > > in testcase: stress-ng
> > > on test machine: 96 threads 2 sockets Ice Lake with 256G memory
> > > with following parameters:
> > >
> > >     nr_threads: 10%
> > >     disk: 1HDD
> > >     testtime: 60s
> > >     fs: f2fs
> > >     class: filesystem
> > >     test: copy-file
> > >     cpufreq_governor: performance
> > >     ucode: 0xb000280
> >
> > Without knowing what the device adapter is, hard to say where the 
> > problem is. I
> > suspect that with the patch applied, we may be ending up with a small 
> > default
> > max_sectors value, causing overhead due to more commands than necessary.
> >
> > Will check what I see with my test rig.
>
> 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.

thanks,
John