linux-kernel - Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on in

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Date:   Thu, 21 Jun 2018 01:11:36 +0900
From:   Byungchul Park <max.byungchul.park@...il.com>
To:     Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Byungchul Park <byungchul.park@....com>,
        jiangshanlai@...il.com, josh@...htriplett.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on in_nmi()

On Thu, Jun 21, 2018 at 12:56 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Wed, Jun 20, 2018 at 11:43:35AM -0400, Steven Rostedt wrote:
>> On Wed, 20 Jun 2018 07:50:58 -0700
>> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
>>
>> > On Wed, Jun 20, 2018 at 05:47:19PM +0900, Byungchul Park wrote:
>> > > Get rid of dependency on ->dynticks_nmi_nesting.
>> > >
>> > > Signed-off-by: Byungchul Park <byungchul.park@....com>
>> > > ---
>> > >  kernel/rcu/tree.c | 22 ++++++++++------------
>> > >  1 file changed, 10 insertions(+), 12 deletions(-)
>> > >
>> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> > > index deb2508..59ae94e 100644
>> > > --- a/kernel/rcu/tree.c
>> > > +++ b/kernel/rcu/tree.c
>> > > @@ -797,6 +797,11 @@ void rcu_nmi_exit(void)
>> > >           return;
>> > >   }
>> > >
>> > > + if (!in_nmi()) {
>> >
>> > Is in_nmi() sufficiently reliable for use here?  In the past, there have
>> > been tracepoints that invoked these functions between the time that the
>> > handlers were entered and the time that software updated the state so that
>> > the various handler-check functions (such as in_nmi()) would return true.
>> >
>> > Steve, has there been any change in this situation?
>>
>> There shouldn't be any "trace events", but what we had to deal with was
>> function tracing. And in the near future, we will be getting "function
>> based events" that will allow you to create an event in any function.
>>
>> That said, even the function tracer shouldn't be called from the time
>> the NMI triggers to "in_nmi()" is set. Because there's some function
>> tracer callbacks that should not be executed from an NMI, and I use
>> in_nmi() to determine if they get called or not.
>
> OK, so in theory this change is safe from a tracing perspective.  But
> it does add conditionals to a fastpath.
>
> Byungchul, is there any reason to make this change other than preparation
> for your second patch?

Sorry again I didn't explain it fully in advance. The only reason is to
prepare for the 2nd. It was harder to read the patch when I made them
into one. But I can make them into one if you don't think so.

>
>                                                         Thanx, Paul
>


-- 
Thanks,
Byungchul