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

Finn Thain
2015-11-25T03:17:09Z

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?

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