On 23 September 2014 19:17:20 CEST, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >On 09/23/2014 04:24 AM, Frans Klaver wrote: >> On Wed, Sep 17, 2014 at 02:13:03PM +0200, Frans Klaver wrote: >>> On Wed, Sep 17, 2014 at 08:01:08AM -0400, Peter Hurley wrote: >>>> On 09/16/2014 04:50 AM, Frans Klaver wrote: >>>>> On Mon, Sep 15, 2014 at 01:31:56PM -0400, Peter Hurley wrote: >>>>>> On 09/15/2014 11:39 AM, Peter Hurley wrote: >>>>>>> On 09/15/2014 10:00 AM, Frans Klaver wrote: >>>>>>>> At 3.6Mbaud, with slightly over 2Mbit/s data coming in, we see >1600 uart >>>>>>>> rx buffer overflows within 30 seconds. Threading the interrupt >handling reduces >>>>>>>> this to about 170 overflows in 10 minutes. >>>>>>> >>>>>>> Why is the threadirqs kernel boot option not sufficient? >>>>>>> Or conversely, shouldn't this be selectable? >>>>>> >>>>> >>>>> I wasn't aware of the threadirqs boot option. I also wouldn't know >if >>>>> this should be selectable. What would be a reason to favor the >>>>> non-threaded irq over the threaded irq? >>>> >>>> Not everyone cares enough about serial to dedicate kthreads to it >:) >>> >>> Fair enough. In that light, we might not care enough about other >>> subsystems to dedicate kthreads to it :). Selectable seems >reasonable in >>> that case. >>> >>> >>>>>> Also, do you see the same performance differential when you >implement this >>>>>> in the 8250 driver (that is, on top of Sebastian's omap->8250 >conversion)? >>>>>> >>>>> >>>>> I haven't gotten Sebastian's driver to work properly yet on the >console. >>>>> There was no reason for me yet to throw my omap changes on top of >>>>> Sebastian's queue. >> >> Doing the threaded interrupt change on the 8250 driver doesn't seem >as >> trivial. Unless I'm mistaken, that version of this patch would mess >with >> all other 8250 based serial drivers, if it's done properly. >Incidentally >> I did try using threadirqs, but that didn't give my any significant >> results. I mostly noticed a difference in the console. >> >> >>>>> >>>>>>> PS - To overflow the 64 byte RX FIFO at those data rates means >interrupt >>>>>>> latency in excess of 250us? >>>>> >>>>> At 3686400 baud it should take about 174 us to fill a 64 byte >buffer. I >>>>> haven't done any measurements on the interrupt latency though. If >you >>>>> consider that we're sending about 1kB of data, 240 times a second, >we're >>>>> spending a lot of time reading data from the uart. I can imagine >the >>>>> system has other work to do as well. >>>> >>>> System work should not keep irqs from being serviced. Even 174us is >a long >>>> time not to service an interrupt. Maybe console writes are keeping >the isr >>>> from running? >>> >>> That's quite possible. I'll have to redo the test setup I had for >this to >>> give you a decent answer. I'll have to do that anyway as Sebastian's >>> 8250 conversion improves. >> >> I haven't had time yet to look into this any further. I'll accept >that >> this patch may fix a case most people aren't the least interested in. >> I'll also happily accept that I probably need a better argumentation >> than "this works better for us".Would it make sense to drop this >patch >> and resubmit the other three? As I mentioned in the previous run, I >> think these are useful in any case. > >I would've thought the first 2 patches had already been picked up >because >they fix div-by-zero faults. I've had no confirmation of that happening so far. I also don't know if I should expect one. Who'd take these patches? Tony? >I don't really have a problem with the patch (except for it should be >selectable, even if that's just a CONFIG_ setting). At the same time, >the performance results don't really make sense; so if there's actually >an underlying problem, I'd rather that get addressed (and the long >interrupt latency may be the underlying problem). Your questions got me thinking a bit more. I concluded that it's hard to define why this difference in performance is so big. I only got to it by just trying and seeing what would happen. >As far as the 8250 driver and threaded irqs go, I just was hoping for >another data point with a simple hard-coded test jig, not a full-blown >patch series for all of them. Sorry for the misunderstanding. Ah yes. I'll see what I can do about it. It does make sense to get that data point somehow. Thanks, Frans > >Regards, >Peter Hurley > >_______________________________________________ >linux-arm-kernel mailing list >linux-arm-kernel@xxxxxxxxxxxxxxxxxxx >http://lists.infradead.org/mailman/listinfo/linux-arm-kernel -- 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
