linux-kernel - Re: [PATCH 22/71] ncr5380: Eliminate selecting state

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Message-Id: <201511252259.26792.linux@rainbow-software.org>
Date:	Wed, 25 Nov 2015 22:59:24 +0100
From:	Ondrej Zary <linux@...nbow-software.org>
To:	Finn Thain <fthain@...egraphics.com.au>
Cc:	"James E.J. Bottomley" <JBottomley@...n.com>,
	Michael Schmitz <schmitzmic@...il.com>,
	linux-m68k@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 22/71] ncr5380: Eliminate selecting state

On Wednesday 25 November 2015 04:17:09 Finn Thain wrote:
> 
> On Tue, 24 Nov 2015, Ondrej Zary wrote:
> 
> > On Wednesday 18 November 2015 09:35:17 Finn Thain wrote:
> > > Linux v2.1.105 changed the algorithm for polling for the BSY signal
> > > in NCR5380_select() and NCR5380_main().
> > > 
> > > Presently, this code has a bug. Back then, NCR5380_set_timer(hostdata, 1)
> > > meant reschedule main() after sleeping for 10 ms. Repeated 25 times this
> > > provided the recommended 250 ms selection time-out delay. This got broken
> > > when HZ became configurable.
> > > 
> > > We could fix this but there's no need to reschedule the main loop. This
> > > BSY polling presently happens when the NCR5380_main() work queue item
> > > calls NCR5380_select(), which in turn schedules NCR5380_main(), which
> > > calls NCR5380_select() again, and so on.
> > > 
> > > This algorithm is a deviation from the simpler one in atari_NCR5380.c.
> > > The extra complexity and state is pointless. There's no reason to
> > > stop selection half-way and return to to the main loop when the main
> > > loop can do nothing useful until selection completes.
> > > 
> > > So just poll for BSY. We can sleep while polling now that we have a
> > > suitable workqueue.
> > 
> > Bisecting slow module initialization pointed to this commit.
> 
> That's disappointing. This patch removed some nasty code. Anyway, thanks 
> for taking the trouble to bisect.
> 
> > 
> > Before this commit (2 seconds):
> > [   60.317374] scsi host2: Generic NCR5380/NCR53C400 SCSI, io_port 0x0, n_io_port 0, base 0xd8000, irq 0, can_queue 16, cmd_per_lun 2, sg_tablesize 128, this_id 7, flags { NCR53C400 }, USLEEP_POLL 3, USLEEP_SLEEP 50, options { AUTOPROBE_IRQ PSEUDO_DMA }
> > [   60.780715] scsi 2:0:1:0: Direct-Access     QUANTUM  LP240S GM240S01X 4.6  PQ: 0 ANSI: 2 CCS
> > [   62.606260] sd 2:0:1:0: Attached scsi generic sg1 type 0
> > 
> > 
> > After this commit (22 seconds):
> > [  137.511711] scsi host2: Generic NCR5380/NCR53C400 SCSI, io_port 0x0, n_io_port 0, base 0xd8000, irq 0, can_queue 16, cmd_per_lun 2, sg_tablesize 128, this_id 7, flags { NCR53C400 }, USLEEP_POLL 3, USLEEP_SLEEP 50, options { AUTOPROBE_IRQ PSEUDO_DMA }
> > [  145.028532] clocksource: timekeeping watchdog: Marking clocksource 'tsc' as unstable because the skew is too large:
> > [  145.029767] clocksource:                       'acpi_pm' wd_now: a49738 wd_last: f4da04 mask: ffffff
> > [  145.029828] clocksource:                       'tsc' cs_now: 2ea624698e cs_last: 2c710aa17f mask: ffffffffffffffff
> > [  145.032733] clocksource: Switched to clocksource acpi_pm
> 
> I figured that it was okay to sleep from an unbound CPU-intensive 
> workqueue but doing so seems to cause problems. (See also patch 66/71 
> "Fix soft lockups".)
> 
> Perhaps a kthread is needed instead of a workqueue? (This workqueue 
> already has it's own kthread, but top shows that it doesn't accrue CPU 
> time.)
> 
> > [  145.236951] scsi 2:0:1:0: Direct-Access     QUANTUM  LP240S GM240S01X 4.6  PQ: 0 ANSI: 2 CCS
> > [  159.959308] sd 2:0:1:0: Attached scsi generic sg1 type 0
> > 
> > 
> 
> This problem doesn't show up on my hardware, and I'd like to know where 
> those 22 seconds are being spent. Would you please apply the entire series 
> and add,
> #define NDEBUG (NDEBUG_ARBITRATION | NDEBUG_SELECTION | NDEBUG_MAIN)
> to the top of g_NCR5380.c and send me the messages logged during modprobe?

 [  397.014581] scsi host2: Generic NCR5380/NCR53C400 SCSI, io_port 0x0, n_io_port 0, base 0xd8000, irq 0, can_queue 16, cmd_per_lun 2, sg_tablesize 128, this_id 7, flags { NO_DMA_FIXUP }, options { AUTOPROBE_IRQ PSEUDO_DMA }
[  412.099695] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  412.103625] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  412.110503] scsi 2:0:1:0: Direct-Access     QUANTUM  LP240S GM240S01X 4.6  PQ: 0 ANSI: 2 CCS
[  412.110892] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  412.114154] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  412.119733] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  427.198108] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  442.276586] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  457.354592] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  472.432999] STATUS_REG: 00
BASR: 08
ICR: 00
MODE: 00
[  487.513027] sd 2:0:1:0: Attached scsi generic sg1 type 0



-- 
Ondrej Zary
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