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

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Date:   Thu, 24 May 2018 17:14:47 -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 04:26 PM, Randy Dunlap wrote:
> 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.


Please forget this.  After reading the numbers, your comments look correct.


-- 
~Randy