Remote atomics

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), August 20, 2018 4:29 pm
Room: Moderated Discussions
matthew (nobody.delete@this.example.com) on August 19, 2018 7:58 pm wrote:
>
> Solaris had the ability for a thread to tell the kernel it was holding a mutex and that it shouldn't
> be preempted until it had dropped the lock. There have been several attempts to add something
> like that to Linux, but none have succeeded yet. Nothing with hardware assists either.

Linux now (merged into the latest released kernel version, 4.18) actually has what could be seen as the reverse of that: "rseq" aka restartable sequences.

It doesn't disable preemption (which is crazy and all kinds of stupid), but it does allow user space to see if it has been preempted, and mark certain sequences to be done atomically. And if preemption happens, the sequence gets aborted.

So you can think of rseq as kind of like the OS equivalent of transactional memory, but instead of the transactional sequence being aborted on a cache conflict, it gets aborted on preemption.

That allows you to do certain per-cpu things in user space (as opposed to per-thread).

And that, in turn, can be a big deal when you have 4 cores, but 4 million threads. You don't want to have the memory overhead of per-thread allocations, when all you really wanted was the cache advantages of per-cpu counters.

It's a pretty limited use-case, and I don't expect normal users to really ever see it. But it is designed to allow for things like per-cpu malloc libraries etc, and a few other very specific situations where you can take advantage of it.

We'll see if people end up taking advantage of it. The downside with a lot of clever interfaces is that because they are non-standard, you really don't see people using them unless there is a big win or unless you can transparently hide them in a library with absolutely zero downside from the portable standard approach.

Which is basically what seems to have killed transactional memory. The library approach (HLE) ends up performing horribly in many real-life situations, so it's not possible to use as a direct transparent replacement, and the full transactional model is so non-portable that it's not worth spending effort on.

We'll see if rseq can do better.

Linus
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TopicPosted ByDate
ARM turns to a god and a heroAM2018/08/16 09:32 AM
  ARM turns to a god and a heroMaynard Handley2018/08/16 09:41 AM
    ARM turns to a god and a heroDoug S2018/08/16 11:11 AM
    ARM turns to a god and a heroGeoff Langdale2018/08/16 11:59 PM
      ARM turns to a god and a herodmcq2018/08/17 05:12 AM
  ARM is somewhat misleadingAdrian2018/08/16 11:56 PM
    It's marketing materialGabriele Svelto2018/08/17 01:00 AM
      It's marketing materialMichael S2018/08/17 03:13 AM
        It's marketing materialdmcq2018/08/17 05:23 AM
          It's marketing materialAndrei Frumusanu2018/08/17 07:25 AM
        It's marketing materialLinus Torvalds2018/08/17 11:20 AM
          It's marketing materialGroo2018/08/17 01:44 PM
            It's marketing materialDoug S2018/08/17 02:14 PM
          promises and deliveriesAM2018/08/17 02:32 PM
            promises and deliveriesPassing Through2018/08/17 03:02 PM
              Just by way of clarification Passing Through2018/08/17 03:15 PM
                Just by way of clarification AM2018/08/18 12:49 PM
                  Just by way of clarification Passing Through2018/08/18 01:34 PM
                    This ain't the nineties any longerPassing Through2018/08/18 01:54 PM
                      This ain't the nineties any longerMaynard Handley2018/08/18 02:50 PM
                        This ain't the nineties any longerPassing Through2018/08/18 03:57 PM
                          This ain't the nineties any longerPassing Through2018/09/06 02:42 PM
                            This ain't the nineties any longerMaynard Handley2018/09/07 04:10 PM
                              This ain't the nineties any longerPassing Through2018/09/07 04:48 PM
                                This ain't the nineties any longerMaynard Handley2018/09/07 05:22 PM
                Just by way of clarification Wilco2018/08/18 01:26 PM
                  Just by way of clarification Passing Through2018/08/18 01:39 PM
                  Just by way of clarification none2018/08/18 10:52 PM
                    Just by way of clarification dmcq2018/08/19 08:32 AM
                      Just by way of clarification none2018/08/19 08:54 AM
                        Just by way of clarification dmcq2018/08/19 11:24 AM
                          Just by way of clarification none2018/08/19 11:52 AM
                  Just by way of clarification Gabriele Svelto2018/08/19 06:41 AM
                    Just by way of clarification Passing Through2018/08/19 09:25 AM
                      Whiteboards at Gatwick airport anyone? Passing Through2018/08/20 04:24 AM
          It's marketing materialMichael S2018/08/18 11:12 AM
          It's marketing materialBrett2018/08/18 05:22 PM
            It's marketing materialBrett2018/08/18 05:33 PM
              It's marketing materialAdrian2018/08/19 01:21 AM
        A76AM2018/08/17 02:45 PM
          A76Michael S2018/08/18 11:20 AM
            A76AM2018/08/18 12:39 PM
              A76Michael S2018/08/18 12:49 PM
                A76AM2018/08/18 01:06 PM
                  A76Doug S2018/08/18 01:43 PM
                    A76Maynard Handley2018/08/18 02:42 PM
                      A76Maynard Handley2018/08/18 04:22 PM
                        Why write zeros when one can use metadata?Paul A. Clayton2018/08/18 06:19 PM
                          Why write zeros when one can use metadata?Maynard Handley2018/08/19 11:12 AM
                            Dictionary compress might apply to memcopyPaul A. Clayton2018/08/19 01:45 PM
                        Instructions for zeroingKonrad Schwarz2018/08/30 06:37 AM
                          Instructions for zeroingMaynard Handley2018/08/30 08:41 AM
                          Instructions for zeroingAdrian2018/08/30 11:37 AM
                            dcbz -> dcbzl (was: Instructions for zeroing)hobold2018/08/31 01:50 AM
                              dcbz -> dcbzl (was: Instructions for zeroing)dmcq2018/09/01 05:28 AM
                      A76Travis2018/08/19 11:36 AM
                        A76Maynard Handley2018/08/19 12:22 PM
                          A76Travis2018/08/19 02:07 PM
                            A76Maynard Handley2018/08/19 06:24 PM
                        Remote atomicsmatthew2018/08/19 12:51 PM
                          Remote atomicsMichael S2018/08/19 01:58 PM
                            Remote atomicsmatthew2018/08/19 02:32 PM
                              Remote atomicsMichael S2018/08/19 02:36 PM
                                Remote atomicsmatthew2018/08/19 02:48 PM
                                  Remote atomicsMichael S2018/08/19 03:16 PM
                                    Remote atomicsRicardo B2018/08/20 10:05 AM
                            Remote atomicsdmcq2018/08/19 02:33 PM
                          Remote atomicsTravis2018/08/19 02:32 PM
                            Remote atomicsMichael S2018/08/19 02:46 PM
                              Remote atomicsTravis2018/08/19 05:35 PM
                                Remote atomicsMichael S2018/08/20 03:29 AM
                            Remote atomicsmatthew2018/08/19 07:58 PM
                              Remote atomicsanon2018/08/20 12:59 AM
                                Remote atomicsTravis2018/08/20 10:26 AM
                              Remote atomicsTravis2018/08/20 09:57 AM
                              Remote atomicsLinus Torvalds2018/08/20 04:29 PM
                                Fitting time slices to execution phasesPaul A. Clayton2018/08/21 09:09 AM
                                  Fitting time slices to execution phasesLinus Torvalds2018/08/21 02:34 PM
                                    Fitting time slices to execution phasesLinus Torvalds2018/08/21 03:31 PM
                                      Fitting time slices to execution phasesGabriele Svelto2018/08/21 03:54 PM
                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 04:26 PM
                                      Fitting time slices to execution phasesTravis2018/08/21 04:21 PM
                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 04:39 PM
                                          Fitting time slices to execution phasesTravis2018/08/21 04:59 PM
                                            Fitting time slices to execution phasesLinus Torvalds2018/08/21 05:13 PM
                                      Fitting time slices to execution phasesanon2018/08/21 04:27 PM
                                        Fitting time slices to execution phasesLinus Torvalds2018/08/21 06:02 PM
                                          Fitting time slices to execution phasesEtienne2018/08/22 02:28 AM
                                        Fitting time slices to execution phasesGabriele Svelto2018/08/22 03:07 PM
                                          Fitting time slices to execution phasesTravis2018/08/22 04:00 PM
                                          Fitting time slices to execution phasesanon2018/08/22 06:52 PM
                                    Fitting time slices to execution phasesTravis2018/08/21 04:37 PM
                                    Is preventing misuse that complex?Paul A. Clayton2018/08/23 05:42 AM
                                      Is preventing misuse that complex?Linus Torvalds2018/08/23 12:46 PM
                                        Is preventing misuse that complex?Travis2018/08/23 01:29 PM
                                          Is preventing misuse that complex?Travis2018/08/23 01:33 PM
                                            Is preventing misuse that complex?Jeff S.2018/08/24 07:57 AM
                                              Is preventing misuse that complex?Travis2018/08/24 08:47 AM
                                          Is preventing misuse that complex?Linus Torvalds2018/08/23 02:30 PM
                                            Is preventing misuse that complex?Travis2018/08/23 03:11 PM
                                              Is preventing misuse that complex?Linus Torvalds2018/08/24 01:00 PM
                                                Is preventing misuse that complex?Gabriele Svelto2018/08/24 01:25 PM
                                                  Is preventing misuse that complex?Linus Torvalds2018/08/24 01:33 PM
                                  Fitting time slices to execution phasesTravis2018/08/21 03:54 PM
                                rseq: holy grail rwlock?Travis2018/08/21 03:18 PM
                                  rseq: holy grail rwlock?Linus Torvalds2018/08/21 03:59 PM
                                    rseq: holy grail rwlock?Travis2018/08/21 04:27 PM
                                      rseq: holy grail rwlock?Linus Torvalds2018/08/21 05:10 PM
                                        rseq: holy grail rwlock?Travis2018/08/21 06:21 PM
                  ARM design housesMichael S2018/08/21 05:07 AM
                    ARM design housesWilco2018/08/22 12:38 PM
                      ARM design housesMichael S2018/08/22 02:21 PM
                        ARM design housesWilco2018/08/22 03:23 PM
                          ARM design housesMichael S2018/08/29 01:58 AM
                            Qualcomm's core naming scheme really, really sucksHeikki Kultala2018/08/29 02:19 AM
                A76Maynard Handley2018/08/18 02:07 PM
                  A76Michael S2018/08/18 02:32 PM
                    A76Maynard Handley2018/08/18 02:52 PM
                      A76Michael S2018/08/18 03:04 PM
    ARM is somewhat misleadingjuanrga2018/08/17 01:20 AM
    Surprised??Alberto2018/08/17 01:52 AM
      Surprised??Alberto2018/08/17 02:10 AM
      Surprised??none2018/08/17 02:46 AM
      Garbage talkAndrei Frumusanu2018/08/17 07:30 AM
        Garbage talkMichael S2018/08/17 07:43 AM
          Garbage talkAndrei Frumusanu2018/08/17 09:51 AM
            Garbage talkMichael S2018/08/18 11:29 AM
        Garbage talkAdrian2018/08/17 08:28 AM
          Garbage talkAlberto2018/08/17 09:20 AM
          Garbage talkAndrei Frumusanu2018/08/17 09:48 AM
            Garbage talkAdrian2018/08/17 10:17 AM
              Garbage talkAndrei Frumusanu2018/08/17 10:36 AM
                Garbage talkAdrian2018/08/17 02:53 PM
                  Garbage talkAndrei Frumusanu2018/08/18 12:17 AM
        More like a religion he?? ARM has an easy life :)Alberto2018/08/17 09:13 AM
          More like a religion he?? ARM has an easy life :)Andrei Frumusanu2018/08/17 09:34 AM
            More like a religion he?? ARM has an easy life :)Alberto2018/08/17 10:03 AM
              More like a religion he?? ARM has an easy life :)Andrei Frumusanu2018/08/17 10:43 AM
              More like a religion he?? ARM has an easy life :)Doug S2018/08/17 02:17 PM
              15W phone SoCsAM2018/08/17 03:04 PM
          More like a religion he?? ARM has an easy life :)Maynard Handley2018/08/17 12:29 PM
  my future stuff will be better than your old stuff, hey I'm a god at last (NT)Eric Bron2018/08/18 03:34 AM
    my future stuff will be better than your old stuff, hey I'm a god at lastnone2018/08/18 08:34 AM
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