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

[Paul E. McKenney]

On Thu, Sep 10, 2020 at 08:18:01PM -0700, Alexei Starovoitov wrote:
> On Thu, Sep 10, 2020 at 1:20 PM <paulmck@xxxxxxxxxx> wrote:
> >
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> >
> > 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.

Yes, for CONFIG_TASKS_TRACE_RCU_READ_MB=y, the previous 100-millisecond
"paranoid sleep" is preserved.  Preserving previous behavior is of course
especially important for rcupdate.rcu_task_ipi_delay, given that real-time
applications are degraded by IPIs.  And given that we are avoiding IPIs
in this case, speeding up the polling is not all that helpful.

> It's for the MB=n case it is reduced to HZ/200.

Yes, that is, to roughly 5 milliseconds for large HZ or to one jiffy
for HZ<200.  Here, we send IPIs much more aggressively, so polling
more frequently does help a lot.

> 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.

As long as HZ is 10 or greater, HZ/10 jiffies is roughly 100 milliseconds.
In the unlikely event that HZ is less than 10, the code clamps to one
jiffy.  Since schedule_timeout_idle() sleep time is specified in jiffies,
it all works out.

							Thanx, Paul