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

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Message-ID: <076566f7-43ff-5cc8-03ba-ebde9c19418c@infradead.org>
Date:   Thu, 24 May 2018 16:26:38 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>,
        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 05/24/2018 04:22 PM, Steven Rostedt wrote:
> 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. :-)

I think that you are confusing "drops it down to" with "drops it down by".
You said the former.  You should say the latter.
IOW, I agree with Joel.

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


-- 
~Randy