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SubjectRe: [PATCH v4 1/5] tracing: Introduce faultable tracepoints
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On 2023-11-21 10:52, Peter Zijlstra wrote:
> On Tue, Nov 21, 2023 at 03:46:43PM +0100, Peter Zijlstra wrote:
>
>> Why is this such a hard question?
>
> Anyway, recapping from IRC:
>
> preemptible, SRCU:
> counter-array based, GP advances by increasing array index
> and waiting for previous index to drop to 0.
>
> notably, a GP can pass while a task is preempted but not within a
> critical section.
>
> SRCU has smp_mb() in the critical sections to improve GP.

Also:

preemptible only allows blocking when priority inheritance is
guarantees, which excludes doing I/O, and thus page faults.
Otherwise a long I/O could cause the system to OOM.

SRCU allows all kind of blocking, as long as the entire SRCU
domain does not mind waiting for a while before readers complete.

>
> tasks:
> waits for every task to pass schedule()
>
> ensures that any pieces of text rendered unreachable before, is
> actually unused after.
>
> tasks-rude:
> like tasks, but different? build to handle tracing while rcu-idle,
> even though that was already deemed bad?
>
> tasks-tracing-rcu:
> extention of tasks to have critical-sections ? Should this simply be
> tasks?

tasks-trace-rcu is meant to allow tasks to block/take a page fault
within the read-side. It is specialized for tracing and has a single
domain. It does not need the smp_mb on the read-side, which makes it
lower-overhead than SRCU.

Thanks,

Mathieu

>
>
> Can someone complete, please?
>
>
>

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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