By: Travis (travis.downs.delete@this.gmail.com), August 21, 2018 3:21 pm
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on August 21, 2018 3:31 pm wrote:
> That kind of silly low-level thing that sounds trivial - but really isn't - is what rseq is about.
> Things that sound so simple that you'd not even realize that they are hard to implement.
Yup. I've seen some ugly stuff to try to emulate this percpu behavior.
For example, mapping the perf_event_mmap_page into the process just to access the perf_event_mmap_page->lock field, which increments on context switches, solely so that threads can detect a context switch by quickly reading this field and update their thread-local CPU value.
Also I've seen a watchdog thread that sits monitoring the scheduler however it can (e.g., via kprobes or just reading /proc//task periodically) and updating every thread's view of its current CPU so that the threads themselves can get the current CPU via a plain read.
Both of these are inherently racy, but for statistics it usually doesn't matter (and even when the primary race happens and you get the wrong CPU number, you usually don't get any missed events because your update also have to race with a thread that thinks it is running on the same CPU as you, and such threads are usually not running.
rseq will be ideal for those use cases.
> That kind of silly low-level thing that sounds trivial - but really isn't - is what rseq is about.
> Things that sound so simple that you'd not even realize that they are hard to implement.
Yup. I've seen some ugly stuff to try to emulate this percpu behavior.
For example, mapping the perf_event_mmap_page into the process just to access the perf_event_mmap_page->lock field, which increments on context switches, solely so that threads can detect a context switch by quickly reading this field and update their thread-local CPU value.
Also I've seen a watchdog thread that sits monitoring the scheduler however it can (e.g., via kprobes or just reading /proc//task periodically) and updating every thread's view of its current CPU so that the threads themselves can get the current CPU via a plain read.
Both of these are inherently racy, but for statistics it usually doesn't matter (and even when the primary race happens and you get the wrong CPU number, you usually don't get any missed events because your update also have to race with a thread that thinks it is running on the same CPU as you, and such threads are usually not running.
rseq will be ideal for those use cases.
Topic | Posted By | Date |
---|---|---|
ARM turns to a god and a hero | AM | 2018/08/16 08:32 AM |
ARM turns to a god and a hero | Maynard Handley | 2018/08/16 08:41 AM |
ARM turns to a god and a hero | Doug S | 2018/08/16 10:11 AM |
ARM turns to a god and a hero | Geoff Langdale | 2018/08/16 10:59 PM |
ARM turns to a god and a hero | dmcq | 2018/08/17 04:12 AM |
ARM is somewhat misleading | Adrian | 2018/08/16 10:56 PM |
It's marketing material | Gabriele Svelto | 2018/08/17 12:00 AM |
It's marketing material | Michael S | 2018/08/17 02:13 AM |
It's marketing material | dmcq | 2018/08/17 04:23 AM |
It's marketing material | Andrei Frumusanu | 2018/08/17 06:25 AM |
It's marketing material | Linus Torvalds | 2018/08/17 10:20 AM |
It's marketing material | Groo | 2018/08/17 12:44 PM |
It's marketing material | Doug S | 2018/08/17 01:14 PM |
promises and deliveries | AM | 2018/08/17 01:32 PM |
promises and deliveries | Passing Through | 2018/08/17 02:02 PM |
Just by way of clarification | Passing Through | 2018/08/17 02:15 PM |
Just by way of clarification | AM | 2018/08/18 11:49 AM |
Just by way of clarification | Passing Through | 2018/08/18 12:34 PM |
This ain't the nineties any longer | Passing Through | 2018/08/18 12:54 PM |
This ain't the nineties any longer | Maynard Handley | 2018/08/18 01:50 PM |
This ain't the nineties any longer | Passing Through | 2018/08/18 02:57 PM |
This ain't the nineties any longer | Passing Through | 2018/09/06 01:42 PM |
This ain't the nineties any longer | Maynard Handley | 2018/09/07 03:10 PM |
This ain't the nineties any longer | Passing Through | 2018/09/07 03:48 PM |
This ain't the nineties any longer | Maynard Handley | 2018/09/07 04:22 PM |
Just by way of clarification | Wilco | 2018/08/18 12:26 PM |
Just by way of clarification | Passing Through | 2018/08/18 12:39 PM |
Just by way of clarification | none | 2018/08/18 09:52 PM |
Just by way of clarification | dmcq | 2018/08/19 07:32 AM |
Just by way of clarification | none | 2018/08/19 07:54 AM |
Just by way of clarification | dmcq | 2018/08/19 10:24 AM |
Just by way of clarification | none | 2018/08/19 10:52 AM |
Just by way of clarification | Gabriele Svelto | 2018/08/19 05:41 AM |
Just by way of clarification | Passing Through | 2018/08/19 08:25 AM |
Whiteboards at Gatwick airport anyone? | Passing Through | 2018/08/20 03:24 AM |
It's marketing material | Michael S | 2018/08/18 10:12 AM |
It's marketing material | Brett | 2018/08/18 04:22 PM |
It's marketing material | Brett | 2018/08/18 04:33 PM |
It's marketing material | Adrian | 2018/08/19 12:21 AM |
A76 | AM | 2018/08/17 01:45 PM |
A76 | Michael S | 2018/08/18 10:20 AM |
A76 | AM | 2018/08/18 11:39 AM |
A76 | Michael S | 2018/08/18 11:49 AM |
A76 | AM | 2018/08/18 12:06 PM |
A76 | Doug S | 2018/08/18 12:43 PM |
A76 | Maynard Handley | 2018/08/18 01:42 PM |
A76 | Maynard Handley | 2018/08/18 03:22 PM |
Why write zeros when one can use metadata? | Paul A. Clayton | 2018/08/18 05:19 PM |
Why write zeros when one can use metadata? | Maynard Handley | 2018/08/19 10:12 AM |
Dictionary compress might apply to memcopy | Paul A. Clayton | 2018/08/19 12:45 PM |
Instructions for zeroing | Konrad Schwarz | 2018/08/30 05:37 AM |
Instructions for zeroing | Maynard Handley | 2018/08/30 07:41 AM |
Instructions for zeroing | Adrian | 2018/08/30 10:37 AM |
dcbz -> dcbzl (was: Instructions for zeroing) | hobold | 2018/08/31 12:50 AM |
dcbz -> dcbzl (was: Instructions for zeroing) | dmcq | 2018/09/01 04:28 AM |
A76 | Travis | 2018/08/19 10:36 AM |
A76 | Maynard Handley | 2018/08/19 11:22 AM |
A76 | Travis | 2018/08/19 01:07 PM |
A76 | Maynard Handley | 2018/08/19 05:24 PM |
Remote atomics | matthew | 2018/08/19 11:51 AM |
Remote atomics | Michael S | 2018/08/19 12:58 PM |
Remote atomics | matthew | 2018/08/19 01:32 PM |
Remote atomics | Michael S | 2018/08/19 01:36 PM |
Remote atomics | matthew | 2018/08/19 01:48 PM |
Remote atomics | Michael S | 2018/08/19 02:16 PM |
Remote atomics | Ricardo B | 2018/08/20 09:05 AM |
Remote atomics | dmcq | 2018/08/19 01:33 PM |
Remote atomics | Travis | 2018/08/19 01:32 PM |
Remote atomics | Michael S | 2018/08/19 01:46 PM |
Remote atomics | Travis | 2018/08/19 04:35 PM |
Remote atomics | Michael S | 2018/08/20 02:29 AM |
Remote atomics | matthew | 2018/08/19 06:58 PM |
Remote atomics | anon | 2018/08/19 11:59 PM |
Remote atomics | Travis | 2018/08/20 09:26 AM |
Remote atomics | Travis | 2018/08/20 08:57 AM |
Remote atomics | Linus Torvalds | 2018/08/20 03:29 PM |
Fitting time slices to execution phases | Paul A. Clayton | 2018/08/21 08:09 AM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 01:34 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 02:31 PM |
Fitting time slices to execution phases | Gabriele Svelto | 2018/08/21 02:54 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 03:26 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 03:21 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 03:39 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 03:59 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 04:13 PM |
Fitting time slices to execution phases | anon | 2018/08/21 03:27 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 05:02 PM |
Fitting time slices to execution phases | Etienne | 2018/08/22 01:28 AM |
Fitting time slices to execution phases | Gabriele Svelto | 2018/08/22 02:07 PM |
Fitting time slices to execution phases | Travis | 2018/08/22 03:00 PM |
Fitting time slices to execution phases | anon | 2018/08/22 05:52 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 03:37 PM |
Is preventing misuse that complex? | Paul A. Clayton | 2018/08/23 04:42 AM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/23 11:46 AM |
Is preventing misuse that complex? | Travis | 2018/08/23 12:29 PM |
Is preventing misuse that complex? | Travis | 2018/08/23 12:33 PM |
Is preventing misuse that complex? | Jeff S. | 2018/08/24 06:57 AM |
Is preventing misuse that complex? | Travis | 2018/08/24 07:47 AM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/23 01:30 PM |
Is preventing misuse that complex? | Travis | 2018/08/23 02:11 PM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/24 12:00 PM |
Is preventing misuse that complex? | Gabriele Svelto | 2018/08/24 12:25 PM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/24 12:33 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 02:54 PM |
rseq: holy grail rwlock? | Travis | 2018/08/21 02:18 PM |
rseq: holy grail rwlock? | Linus Torvalds | 2018/08/21 02:59 PM |
rseq: holy grail rwlock? | Travis | 2018/08/21 03:27 PM |
rseq: holy grail rwlock? | Linus Torvalds | 2018/08/21 04:10 PM |
rseq: holy grail rwlock? | Travis | 2018/08/21 05:21 PM |
ARM design houses | Michael S | 2018/08/21 04:07 AM |
ARM design houses | Wilco | 2018/08/22 11:38 AM |
ARM design houses | Michael S | 2018/08/22 01:21 PM |
ARM design houses | Wilco | 2018/08/22 02:23 PM |
ARM design houses | Michael S | 2018/08/29 12:58 AM |
Qualcomm's core naming scheme really, really sucks | Heikki Kultala | 2018/08/29 01:19 AM |
A76 | Maynard Handley | 2018/08/18 01:07 PM |
A76 | Michael S | 2018/08/18 01:32 PM |
A76 | Maynard Handley | 2018/08/18 01:52 PM |
A76 | Michael S | 2018/08/18 02:04 PM |
ARM is somewhat misleading | juanrga | 2018/08/17 12:20 AM |
Surprised?? | Alberto | 2018/08/17 12:52 AM |
Surprised?? | Alberto | 2018/08/17 01:10 AM |
Surprised?? | none | 2018/08/17 01:46 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 06:30 AM |
Garbage talk | Michael S | 2018/08/17 06:43 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 08:51 AM |
Garbage talk | Michael S | 2018/08/18 10:29 AM |
Garbage talk | Adrian | 2018/08/17 07:28 AM |
Garbage talk | Alberto | 2018/08/17 08:20 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 08:48 AM |
Garbage talk | Adrian | 2018/08/17 09:17 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 09:36 AM |
Garbage talk | Adrian | 2018/08/17 01:53 PM |
Garbage talk | Andrei Frumusanu | 2018/08/17 11:17 PM |
More like a religion he?? ARM has an easy life :) | Alberto | 2018/08/17 08:13 AM |
More like a religion he?? ARM has an easy life :) | Andrei Frumusanu | 2018/08/17 08:34 AM |
More like a religion he?? ARM has an easy life :) | Alberto | 2018/08/17 09:03 AM |
More like a religion he?? ARM has an easy life :) | Andrei Frumusanu | 2018/08/17 09:43 AM |
More like a religion he?? ARM has an easy life :) | Doug S | 2018/08/17 01:17 PM |
15W phone SoCs | AM | 2018/08/17 02:04 PM |
More like a religion he?? ARM has an easy life :) | Maynard Handley | 2018/08/17 11:29 AM |
my future stuff will be better than your old stuff, hey I'm a god at last (NT) | Eric Bron | 2018/08/18 02:34 AM |
my future stuff will be better than your old stuff, hey I'm a god at last | none | 2018/08/18 07:34 AM |