linux-kernel - Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

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Date:   Tue, 21 Nov 2023 11:00:13 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org,
        Michael Jeanson <mjeanson@...icios.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, bpf@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

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