Date: Tue, 22 Dec 2015 11:39:03 +0000
From: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
To: Finn Thain <fthain@telegraphics.com.au>
Cc: "James E.J. Bottomley" <JBottomley@odin.com>,
        Michael Schmitz <schmitzmic@gmail.com>, <linux-m68k@vger.kernel.org>,
        <linux-scsi@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Russell King <linux@arm.linux.org.uk>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 66/77] ncr5380: Fix soft lockups
Message-ID: <20151222113903.06422dbe@lxorguk.ukuu.org.uk>
In-Reply-To: <20151222011755.965510468@telegraphics.com.au>
References: <20151222011737.980475848@telegraphics.com.au>
	<20151222011755.965510468@telegraphics.com.au>
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On Tue, 22 Dec 2015 12:18:44 +1100
Finn Thain <fthain@telegraphics.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 ?

Alan
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