linux-kernel - Re: [PATCH RFC 02/12] entry/idle: Add a common function for activites during idle entry/exit

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Date:   Sun, 16 Aug 2020 22:17:14 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Kees Cook <keescook@...omium.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        fweisbec@...il.com, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH RFC 02/12] entry/idle: Add a common function for
 activites during idle entry/exit

Hello Peter,

On Sat, Aug 15, 2020 at 10:14:41AM +0200, peterz@...radead.org wrote:
> On Fri, Aug 14, 2020 at 11:18:58PM -0400, Joel Fernandes (Google) wrote:
> > Currently only RCU hooks for idle entry/exit are called. In later
> > patches, kernel-entry protection functionality will be added.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> NAK, rcu_idle_enter() is broken where it is now, it needs to be pushed
> in deeper:
> 
> http://lkml.kernel.org/r/20200807193017.962482579@infradead.org

Thank you for pointing it out. Not a huge problem, a couple ways I can do it:
1. Move the calls to sched_core_unsafe_{enter,exit}() deeper into the idle loop.
2. Keep the calls to sched_core_unsafe_{enter,exit}() where they are now as
   in this patch, but leave out the rcu_idle_{enter,exit}() calls alone so
   they can be moved deeper as you mentioned.

#1 is not necessary for these patches to work and might be overkill.

I'll go the #2 route then. Let me know any other ideas you might have.

What I am trying to do here is to handle a case where task is switching to
idle (say it went to sleep) and wakes up later.

usermode -> syscall (kernel mode - so mark unsafe) -> idle (mark safe).

idle -> syscall wakes (kernel mode - so mark unsafe) -> usermode (mark safe).

Thank you,

 - Joel