A15/Octa scheduling problem..

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), May 13, 2013 2:19 pm
Room: Moderated Discussions
Anon (no.delete@this.thanks.com) on May 12, 2013 4:31 pm wrote:
> There is a complex patch on its away apparently, but not the for kernel
> currently used, so it may be a while before we see this fixed.

Quite frankly, the fact that the Exynos 5 currently only works in "either or" configuration almost certainly means that there is something fundamentally wrong with the hardware design, to the point where no amount of "complex patches" can fix it.

And no, I have absolutely no information about the Exynos to actually back that up. But quite frankly, the easiest way by far to do big.little would be to just treat it as CPU hotplug, which Linux has supported for a long time. At which time mixing things should be trivial from a software standpoint.

The fact that that isn't how people do it makes me go: "the hardware is buggered".

The most likely explanation for the buggery is that cache coherency doesn't work between little.big cores. So each cluster of four CPU's is cache-coherent within the cluster, but not across clusters. That is, afaik, what nVidia does for their 4+1 "vSMP" too.

And non-cache-coherent clusters are a bitch to handle. The easiest way to do it is to basically turn off one cluster, flush the caches on it, and turn on another one. Which would explain the reports of "S Lag" on the Exynos 5 octa. Switching between clusters is going to result in user-noticeable latencies.

I understand why you'd do this as a hardware designer - it makes your job oh so much easier. But I don't understand people who think it's actually a good idea in the bigger picture.

I now return you to your regularly scheduled excuse-fest from Wilco about how ARM did it right, and why cache coherency is a bad idea.

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TopicPosted ByDate
Haswell Graphicsyah05/02/13 09:14 AM
  Haswell GraphicsLinus Torvalds05/02/13 10:38 AM
    Haswell GraphicsEmil Briggs05/03/13 05:51 AM
      4K displaysDoug S05/03/13 02:51 PM
        4K displaysEmil Briggs05/03/13 05:01 PM
    Haswell GraphicsGreg05/03/13 07:45 AM
      Haswell Graphicsanon05/05/13 12:32 AM
    Haswell GraphicsRobert Williams05/03/13 04:16 PM
      Haswell GraphicsWilco05/04/13 08:39 PM
        Haswell GraphicsDavid Kanter05/04/13 10:29 PM
          Haswell Graphicsanon05/05/13 12:35 AM
            Haswell GraphicsMichael S05/05/13 03:20 AM
            Haswell GraphicsWilco05/05/13 06:23 PM
          Haswell Graphicsx05/05/13 04:18 AM
            Haswell GraphicsDavid Kanter05/05/13 08:53 AM
              Haswell GraphicsMichael S05/05/13 09:32 AM
                Haswell GraphicsDoug S05/05/13 01:29 PM
              Haswell Graphicsnone05/05/13 02:30 PM
          Haswell GraphicsWilco05/05/13 06:06 PM
            A15, not in phonesDavid Kanter05/06/13 12:23 AM
              A15, not in phonesnone05/06/13 02:10 AM
              A15, not in phonesWilco05/06/13 05:53 AM
                A15, not in phonesDoug S05/06/13 11:32 AM
                  A15, not in phonesWilco05/09/13 02:08 AM
                    A15, not in phonesMichael S05/09/13 02:33 AM
                      links above invisible, sorryMichael S05/09/13 02:37 AM
                        links above invisible, sorryWilco05/12/13 10:20 AM
                          A15/Octa scheduling problem..Anon05/12/13 04:31 PM
                            A15/Octa scheduling problem..none05/12/13 11:26 PM
                              A15/Octa scheduling problem..David Kanter05/13/13 01:00 AM
                                A15/Octa scheduling problem..none05/13/13 02:36 AM
                            A15/Octa scheduling problem..Ricardo B05/13/13 08:42 AM
                            A15/Octa scheduling problem..Linus Torvalds05/13/13 02:19 PM
                              A15/Octa scheduling problem..Anon05/13/13 03:06 PM
                                A15/Octa scheduling problem..Linus Torvalds05/13/13 04:46 PM
                                  A15/Octa scheduling problem..Ricardo B05/13/13 05:12 PM
                                  A15/Octa scheduling problem..Gabriele Svelto05/14/13 01:28 AM
                                    Cache Coherent DMA on ARM A9Konrad Schwarz05/14/13 07:18 AM
                                A15/Octa scheduling problem..Gabriele Svelto05/14/13 01:25 AM
                                  A15/Octa scheduling problem..Anon05/14/13 04:44 PM
                                    A15/Octa scheduling problem..Gabriele Svelto05/15/13 12:24 AM
                            A15/Octa scheduling problem..Ricardo B05/14/13 08:56 AM
                              A15/Octa scheduling problem..Exophase05/14/13 11:21 AM
                    A15, not in phonesDoug S05/09/13 08:02 PM
              A15, not in phonesRichardC05/06/13 09:12 AM
                A15, not in phonesDavid Kanter05/18/13 10:47 AM
                  Example claims that 20nm cost/transistor not happyMark Roulo05/18/13 12:27 PM
                  A15, not in phonesExophase05/18/13 02:41 PM
                    A15, not in phonesMichael S05/18/13 03:20 PM
                      A15, not in phonesExophase05/18/13 07:40 PM
                        A15, not in phonesMichael S05/19/13 03:56 AM
                          A15, not in phonesmpx05/21/13 01:31 AM
                            A15, not in phonesMichael S05/21/13 09:21 AM
                    A15, not in phonesaaron spink05/18/13 05:50 PM
                      A15, not in phonesWilco05/19/13 06:20 AM
                        A15, not in phonesaaron spink05/19/13 11:21 AM
                  A15, not in phonesWilco05/19/13 06:39 AM
                    A15, not in phonesExophase05/19/13 10:13 AM
                      A15, not in phonesWilco05/19/13 01:02 PM
                        A15, not in phonesDavid Kanter05/21/13 08:53 AM
                          A15, not in phonesWilco05/21/13 04:55 PM
                    A15, not in phonesDavid Kanter05/20/13 07:08 PM
                      A15, not in phonesDoug S05/21/13 09:49 AM
                        A15, not in phonesnone05/21/13 01:53 PM
                          A15, not in phonesDoug S05/21/13 08:39 PM
                            A15, not in phonesDavid Kanter05/21/13 11:44 PM
                      A15, not in phonesnone05/21/13 01:36 PM
                      A15, not in phonesWilco05/21/13 06:16 PM
                        A15, not in phonesMichael S05/22/13 02:42 AM
                          A15, not in phonesGabriele Svelto05/22/13 02:59 AM
                          A15, not in phonesWilco05/22/13 04:40 AM
                            A15, not in phonesMichael S05/22/13 07:21 AM
                              Why M0?N Monson05/22/13 12:01 PM
                                Why M0?Dan Fay05/23/13 01:07 PM
                                  Why 8-bit MCUs might be chosenPaul A. Clayton05/23/13 02:10 PM
                                    Why 8-bit MCUs might be chosenDan Fay05/24/13 06:06 AM
                                      Why 8-bit MCUs might be chosenRicardo B05/24/13 07:15 AM
                                    Why 8-bit MCUs might be chosenDavid Hess05/24/13 07:02 AM
                                    Why 8-bit MCUs might be chosenj05/24/13 07:53 AM
                                      Thanks for the feedbackPaul A. Clayton05/24/13 08:36 AM
                                    Why 8-bit MCUs might be chosenFelix05/24/13 08:24 PM
                                      Why 8-bit MCUs might be chosenDavid Hess05/27/13 09:32 PM
                                  Why 8-bit?N Monson05/23/13 11:11 PM
                                    Why 8-bit?Dan Fay05/24/13 06:00 AM
                                      Why 8-bit?Michael S05/24/13 06:41 AM
                                      Why 8-bit?51 is fun05/24/13 08:33 AM
                                        Why 8-bit?Michael S05/24/13 08:45 AM
                                        Why 8-bit?Louis Gerbarg05/25/13 06:50 AM
                                    8051 not the sanest 8-bitterPaul A. Clayton05/24/13 06:17 AM
                                      8051 not the sanest 8-bitterMichael S05/24/13 06:53 AM
                                        8051 not the sanest 8-bitterDavid Hess05/27/13 10:32 PM
                          A15, not in phonesExophase05/22/13 07:03 AM
                        A15, not in phonesDavid Kanter05/22/13 07:32 AM
                          A15, not in phonesExophase05/22/13 08:20 AM
                            A15, not in phonesDavid Kanter05/22/13 11:36 AM
                              A15, not in phonesnone05/23/13 01:52 PM
                  A15, not in phonesVincent Diepeveen05/21/13 09:30 AM
            Haswell GraphicsMichael S05/06/13 12:31 AM
            Haswell GraphicsDoug S05/06/13 12:52 AM
  Haswell GraphicsDoug S05/02/13 10:51 AM
  Haswell GraphicsAlberto05/02/13 10:51 AM
    Haswell Graphicssomeone05/02/13 11:14 AM
      But...Alberto05/03/13 12:49 PM
        Tied off to stop the bleedingsomeone05/04/13 12:21 PM
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