linux-kernel - Re: [PATCH] rcu: Check the range of jiffies_till_{first,next}

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Date:   Sun, 3 Jun 2018 16:51:06 +0900
From:   Byungchul Park <max.byungchul.park@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Byungchul Park <byungchul.park@....com>, jiangshanlai@...il.com,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        josh@...htriplett.org, rostedt@...dmis.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        kernel-team@...roid.com
Subject: Re: [PATCH] rcu: Check the range of jiffies_till_{first,next}_fqs
 when setting them

On Sun, Jun 3, 2018 at 3:23 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
> On Sun, Jun 03, 2018 at 02:38:04PM +0900, Byungchul Park wrote:
>> On Sun, Jun 3, 2018 at 12:58 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
>> > On Fri, Jun 01, 2018 at 11:03:09AM +0900, Byungchul Park wrote:
>> >> Currently, the range of jiffies_till_{first,next}_fqs are checked and
>> >> adjusted on and on in the loop of rcu_gp_kthread on runtime.
>> >>
>> >> However, it's enough to check them only when setting them, not every
>> >> time in the loop. So make them handled on a setting time via sysfs.
>> >>
>> >> Signed-off-by: Byungchul Park <byungchul.park@....com>
>> >> ---
>> >>  kernel/rcu/tree.c | 45 ++++++++++++++++++++++++++++++++-------------
>> >>  1 file changed, 32 insertions(+), 13 deletions(-)
>> >>
>> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> >> index 4e96761..eb54d7d 100644
>> >> --- a/kernel/rcu/tree.c
>> >> +++ b/kernel/rcu/tree.c
>> >> @@ -518,8 +518,38 @@ void rcu_all_qs(void)
>> >>  static ulong jiffies_till_next_fqs = ULONG_MAX;
>> >>  static bool rcu_kick_kthreads;
>> >>
>> >> -module_param(jiffies_till_first_fqs, ulong, 0644);
>> >> -module_param(jiffies_till_next_fqs, ulong, 0644);
>> >> +static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
>> >> +{
>> >> +     ulong j;
>> >> +     int ret = kstrtoul(val, 0, &j);
>> >> +
>> >> +     if (!ret)
>> >> +             WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : j);
>> >> +     return ret;
>> >> +}
>> >> +
>> >> +static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param *kp)
>> >> +{
>> >> +     ulong j;
>> >> +     int ret = kstrtoul(val, 0, &j);
>> >> +
>> >> +     if (!ret)
>> >> +             WRITE_ONCE(*(ulong *)kp->arg, (j > HZ) ? HZ : (j ?: 1));
>> >> +     return ret;
>> >> +}
>> >
>> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> >
>> > Also, can we not combine the 2 param_set_ handlers as well?
>> >
>> > Only thing we would be giving up is that jiffies_till_first_fqs = 0 wouldn't
>> > be allowed (if we go with the param_set_next handler to be the common one)
>> > but don't think that's a useful/valid usecase since jiffies_till_first_fqs is
>> > set to a sane non-0 value anyway at boot up because of rcu_init_geometry
>> > anyway.. Thoughts?
>>
>> Excuse me. Which code in rcu_init_geometry() makes jiffies_till_first_fqs
>> be a non-zero in case called with jiffies_till_first_fqs == 0?
>
> What do you mean? I think you misunderstood. I didn't say value of 0 is being
> handled at boot up. What I said is its initialized to something sane that's
> non-zero:
>
> If you see, jiffies_till_first_fqs is assigned to ULONG_MAX at compile time:
> static ulong jiffies_till_first_fqs = ULONG_MAX;
>
> Then in rcu_init_geometry, we have:
>         d = RCU_JIFFIES_TILL_FORCE_QS + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
>         if (jiffies_till_first_fqs == ULONG_MAX)
>                         jiffies_till_first_fqs = d;
>
> On my system, jiffies_till_first_fqs is assigned to 3 because of this at
> boot.
>
>> Furthermore, what if we want to change the value through sysfs to zero
>> on runtime?
>
> My patch was just a suggestion. I didn't know if anyone would want to force
> it to 0 or not and was wondering what the value in doing so was at the cost
> of adding one more function to handle it. It basically says if you set to 0,
> that you never want to wait for a timeout before forcing a qs for the first
> time? Then why are we calling swait_event_idle_timeout anyway? Wouldn't a
> saner timeout be something not zero?

I'm sorry I tried but don't understand your point :(

Did you happen to read the following?

https://lkml.org/lkml/2018/5/29/99

If yes, it would be appreciated if you let me know what I'm missing.

-- 
Thanks,
Byungchul