linux-kernel - Re: Query regarding synchronize_sched_expedited and resched

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Message-Id: <20170921153134.GG3521@linux.vnet.ibm.com>
Date:   Thu, 21 Sep 2017 08:31:34 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        sramana@...eaurora.org, prsood@...eaurora.org,
        pkondeti@...eaurora.org, markivx@...eaurora.org
Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu

On Thu, Sep 21, 2017 at 03:55:46PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 18, 2017 at 11:11:05AM -0400, Steven Rostedt wrote:
> > On Sun, 17 Sep 2017 11:37:06 +0530
> > Neeraj Upadhyay <neeraju@...eaurora.org> wrote:
> > 
> > > Hi Paul, how about replacing raw_spin_trylock_irqsave with
> > > raw_spin_lock_irqsave in resched_cpu()? Are there any paths
> > > in RCU code, which depend on trylock check/spinlock recursion?
> > 
> > It looks to me that resched_cpu() was added for nohz full sched
> > balancing, but is not longer used by that. The only user is currently
> > RCU. Perhaps we should change that from a trylock to a lock.
> 
> No, regular NOHZ balancing. NOHZ FULL wasn't conceived back then.
> 
>   46cb4b7c88fa ("sched: dynticks idle load balancing")
> 
> And yeah, its no longer used for that.
> 
> And given RCU is the only user of that thing, I suppose we can indeed
> change it to a full lock.

Thank you!  May I have your ack?

							Thanx, Paul