linux-kernel - Re: [PATCH 1/1] serial: 8250, disable "too much work" messages

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Message-ID: <20140401144057.63b3ceb5@alan.etchedpixels.co.uk>
Date:	Tue, 1 Apr 2014 14:40:57 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	gregkh@...uxfoundation.org, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] serial: 8250, disable "too much work" messages

On Tue,  1 Apr 2014 13:37:00 +0200
Jiri Slaby <jslaby@...e.cz> wrote:

> The 8250 driver now reports many of these:
>   serial8250: too much work for irq4
> These messages turned out to be common these days with a use of
> virtualization. I tried to increase the limit of processed characters
> in commit e7328ae1848966181a7ac47e8ae6cddbd2cf55f3 (serial: 8250,
> increase PASS_LIMIT) in 2011. It was raised from 256 to 512, but it is
> still not enough, apparently.

A lot of emulations model the queue completely incorrectly. However
simply hiding it with a pr_debug is the wrong answer - it wants fixing.

If we set a large PASS_LIMIT then it's not going to be a big loss on real
hardware - we'll burp for a second or two and continue, but it ought to
cure the virtualisation case.

If it doesn't we've got a bigger problem because it means we are jammed
in the kernel spinning in an IRQ handler feeding data to a fake serial
port that never stops being an IRQ and we end up hanging the virtualised
OS for a long period

If that is happening then we need to actually workaround whatever
crapware emulator is triggering it so we don't hang the guest for long
periods if there is a big I/O.

If its a real port that is jammed our normal time around the loop on the
LPC bus is going to be a shade over 24uS (32uS if TX is jammed on)

So we certainly ought to be able to go a bit higher without major
crisis. Beyond that if it is still tripping then instead of whining we
need to set IIR_NO_INT and set a polling timer to turn the IRQ back on
next timer tick. That way a crappy emulated port can't hang the guest
with a continual stream of data and a busted real one might actually sort
out.

Alan
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