linux-kernel - Re: [PATCH RFC tip/core/rcu 4/4] rcu-tasks: Shorten per-grace-period sleep for RCU Tasks Trace

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Message-ID: <CAADnVQK4Rgrzq+cUKCMkr5anZF+UbHmAc7-FH4BjA23aMM03rQ@mail.gmail.com>
Date:   Thu, 10 Sep 2020 20:18:01 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>, dipankar@...ibm.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jiri Olsa <jolsa@...hat.com>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH RFC tip/core/rcu 4/4] rcu-tasks: Shorten per-grace-period
 sleep for RCU Tasks Trace

On Thu, Sep 10, 2020 at 1:20 PM <paulmck@...nel.org> wrote:
>
> From: "Paul E. McKenney" <paulmck@...nel.org>
>
> The various RCU tasks flavors currently wait 100 milliseconds between each
> grace period in order to prevent CPU-bound loops and to favor efficiency
> over latency.  However, RCU Tasks Trace needs to have a grace-period
> latency of roughly 25 milliseconds, which is completely infeasible given
> the 100-millisecond per-grace-period sleep.  This commit therefore reduces
> this sleep duration to 5 milliseconds (or one jiffy, whichever is longer)
> in kernels built with CONFIG_TASKS_TRACE_RCU_READ_MB=y.

The commit log is either misleading or wrong?
If I read the code correctly in CONFIG_TASKS_TRACE_RCU_READ_MB=y
case the existing HZ/10 "paranoid sleep" is preserved.
It's for the MB=n case it is reduced to HZ/200.
Also I don't understand why you're talking about milliseconds but
all numbers are HZ based. HZ/10 gives different number of
milliseconds depending on HZ.