On 04/01/2014 03:40 PM, One Thousand Gnomes wrote: > On Tue, 1 Apr 2014 13:37:00 +0200 > Jiri Slaby <jslaby@xxxxxxx> 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. So, according to Takashi's measurements, we would need over 15000 loops on a single port. Of course, this value is highly dependent on a system. On my system, it is like 7 times lower (2100). And it lasts ~300ms here. I suppose a limit like 32k loops is way too much and I just should go and implement the polling. Or what about adding inter-character sleeps to qemu to correspond to the speed? I can do that too, but I am not sure if limiting the throughput will be accepted by them. thanks, -- js suse labs -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
