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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.
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