linux-kernel - Re: [PATCH v3 66/77] ncr5380: Fix soft lockups

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Date:	Wed, 23 Dec 2015 13:42:54 +1300
From:	Michael Schmitz <schmitzmic@...il.com>
To:	Finn Thain <fthain@...egraphics.com.au>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	"James E.J. Bottomley" <JBottomley@...n.com>,
	"Linux/m68k" <linux-m68k@...r.kernel.org>,
	scsi <linux-scsi@...r.kernel.org>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Russell King <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 66/77] ncr5380: Fix soft lockups

I'd like to think that, too - probably true for the Atari TT SCSI case
(can do scatter-gather, can do more than one command per LUN). Worse
for the Falcon SCSI which is the only one I can test (no
scatter-gather, one command per LUN, interrupt shared with IDE and IDE
driver locked out while SCSI command handled).

But that only affects balancing of I/O between IDE and SCSI drivers.
Is that what you are worried about, Alan?

Happy to test whether limiting max_sectors makes a difference in the DMA case.

Cheers,

  Michael



On Wed, Dec 23, 2015 at 2:47 AM, Finn Thain <fthain@...egraphics.com.au> wrote:
>
> On Tue, 22 Dec 2015, One Thousand Gnomes wrote:
>
>> On Tue, 22 Dec 2015 12:18:44 +1100 Finn Thain
>> <fthain@...egraphics.com.au> wrote:
>>
>> > Because of the rudimentary design of the chip, it is necessary to poll
>> > the SCSI bus signals during PIO and this tends to hog the CPU. The
>> > driver will accept new commands while others execute, and this causes
>> > a soft lockup because the workqueue item will not terminate until the
>> > issue queue is emptied.
>> >
>> > When exercising dmx3191d using sequential IO from dd, the driver is
>> > sent 512 KiB WRITE commands and 128 KiB READs. For a PIO transfer, the
>> > rate is is only about 300 KiB/s, so these are long-running commands.
>> > And although PDMA may run at several MiB/s, interrupts are disabled
>> > for the duration of the transfer.
>> >
>> > Fix the unresponsiveness and soft lockup issues by calling
>> > cond_resched() after each command is completed and by limiting
>> > max_sectors for drivers that don't implement real DMA.
>>
>> Is there a reason for not doing some limiting in the DMA case too. A
>> 512K write command even with DMA on a low end 68K box introduces a
>> second of latency before another I/O can be scheduled ?
>
> The DMA case is the atari_scsi case. I'd like to think that atari_scsi
> would have only the latency issues that might be expected from any SCSI-2
> host adapter driver.
>
> Unlike PDMA, interrupts are not disabled for these DMA transfers. Note
> that this patch isn't really relevant to DMA, because the main loop
> iterates only when done == 0, that is, !hostdata->dmalen.
>
> --
>
>>
>> Alan
>
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