linux-kernel - Re: [PATCH v4] rcu: Speed up calling of RCU tasks callbacks

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Message-ID: <20180524192247.759f7572@gandalf.local.home>
Date:   Thu, 24 May 2018 19:22:47 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v4] rcu: Speed up calling of RCU tasks callbacks

On Thu, 24 May 2018 16:19:18 -0700
Joel Fernandes <joel@...lfernandes.org> wrote:

> On Thu, May 24, 2018 at 06:49:46PM -0400, Steven Rostedt wrote:
> > 
> > From: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > 
> > Joel Fernandes found that the synchronize_rcu_tasks() was taking a
> > significant amount of time. He demonstrated it with the following test:
> > 
> >  # cd /sys/kernel/tracing
> >  # while [ 1 ]; do x=1; done &
> >  # echo '__schedule_bug:traceon' > set_ftrace_filter
> >  # time echo '!__schedule_bug:traceon' > set_ftrace_filter;
> > 
> > real	0m1.064s
> > user	0m0.000s
> > sys	0m0.004s
> > 
> > Where it takes a little over a second to perform the synchronize,
> > because there's a loop that waits 1 second at a time for tasks to get
> > through their quiescent points when there's a task that must be waited
> > for.
> > 
> > After discussion we came up with a simple way to wait for holdouts but
> > increase the time for each iteration of the loop but no more than a
> > full second.
> > 
> > With the new patch we have:
> > 
> >  # time echo '!__schedule_bug:traceon' > set_ftrace_filter;
> > 
> > real	0m0.131s
> > user	0m0.000s
> > sys	0m0.004s
> > 
> > Which drops it down to 13% of what the original wait time was.  
> 
> Should be 90% of original?

That would be if I said "drops it down X" but I said "drops it down to
X of what the original wait time was". And 0.131 is 13% of 1.064. :-)

> Other than minor change log change, looks good to me:
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Awesome, thanks!

-- Steve