linux-kernel - Re: [PATCH V1 Resend 4/5] tick-sched: don't call update_wall_time() when delta is lesser than tick

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Message-ID: <CAKohponkb+FovSTLuQ0cxEdVMU7JTHYPauhU07=20otAoDiAZA@mail.gmail.com>
Date:	Wed, 16 Apr 2014 09:50:36 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arvind Chauhan <Arvind.Chauhan@....com>,
	Linaro Networking <linaro-networking@...aro.org>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH V1 Resend 4/5] tick-sched: don't call update_wall_time()
 when delta is lesser than tick_period

On 16 April 2014 00:14, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 15 Apr 2014, Viresh Kumar wrote:
>
>> In tick_do_update_jiffies64() we are processing ticks only if delta is greater
>> than tick_period. This is what we are supposed to do here and it broke a bit
>> with this patch:
>>
>> commit 47a1b796306356f358e515149d86baf0cc6bf007
>> Author: John Stultz <john.stultz@...aro.org>
>> Date:   Thu Dec 12 13:10:55 2013 -0800
>>
>>     tick/timekeeping: Call update_wall_time outside the jiffies lock
>
> Please look how I massaged the change log. There is no point in
> copying the whole gunk.

I see.. Nice.

>> With above patch, we might end up calling update_wall_time() even if delta is
>> found to be smaller that tick_period. Fix this by reversing the check and
>> returning early.
>
> Well.
>
>> Cc: <stable@...r.kernel.org> # v3.14+
>> Cc: John Stultz <john.stultz@...aro.org>
>> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
>> ---
>>  kernel/time/tick-sched.c | 32 +++++++++++++++++---------------
>>  1 file changed, 17 insertions(+), 15 deletions(-)
>
> That's not how we do bug fixes if they can be done with 3 lines of
> change. See the commit.

I tried that initially but with these changes as well (which must
be done now ??), which probably makes it more clear ?:

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3cafe7d..0e70b1c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -84,12 +84,12 @@ static void tick_do_update_jiffies64(ktime_t now)

                /* Keep the tick_next_period variable up to date */
                tick_next_period = ktime_add(last_jiffies_update, tick_period);
+
+               write_sequnlock(&jiffies_lock);
+               update_wall_time();
        } else {
                write_sequnlock(&jiffies_lock);
-               return;
        }
-       write_sequnlock(&jiffies_lock);
-       update_wall_time();
 }

 /*
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