linux-kernel - Re: [RFC] rcu: use killable versions of swait

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Date:   Thu, 15 Jun 2017 11:55:57 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC] rcu: use killable versions of swait

"Luis R. Rodriguez" <mcgrof@...nel.org> writes:

> On Thu, Jun 15, 2017 at 9:22 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
>> On Thu, Jun 15, 2017 at 05:50:39PM +0200, Luis R. Rodriguez wrote:
>>> On Wed, Jun 14, 2017 at 04:43:45PM -0700, Paul E. McKenney wrote:
>>> > On Wed, Jun 14, 2017 at 04:06:39PM -0700, Luis R. Rodriguez wrote:

>>> > > Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>
>>> > > ---
>>> > >
>>> > > The killable swaits were just posted [1] as part of a series where SIGCHLD
>>> > > was detected as interrupting and killing kernel calls waiting using
>>> > > non-killable swaits [1]. The fragility here made curious about other callers
>>> > > and seeing if they really meant to use such broad wait which captures a lot
>>> > > of signals.
>>> > >
>>> > > I can't see why we'd want to have these killed by other signals, specialy
>>> > > since it seems we don't even check for the return value... Granted to abort
>>> > > properly we'd have to check for the return value for -ERESTARTSYS, but yeah,
>>> > > none of this is done, so it would seem we don't want fragile signals
>>> > > interrupting these ?
>>> >
>>> > The later WARN_ON(signal_pending(current)) complains if a signal somehow
>>> > makes it to this task.  Assuming that the signal is nonfatal, anyway.
>>>
>>> I see, how about just using swait_event_timeout() and removing the WARN_ON()?
>>> Is there a reason for having the interruptible ?
>>
>> If sleeping-uninterruptible kthreads are now excluded from the load average,
>> no reason.  But if sleeping-uninterruptible kthreads are still included in
>> the load average, it must stay interruptible.
>
> Got it!

There is now TASK_IDLE that is uninterruptible and does not contribute
to load avearage.  see: task_contributes_to_load.

So a swait_event_idle() could be written for this case.

Eric