linux-kernel - Re: Query regarding synchronize_sched_expedited and resched

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Message-ID: <20170921135546.x3q3zeicbo2vtjvk@hirez.programming.kicks-ass.net>
Date:   Thu, 21 Sep 2017 15:55:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Neeraj Upadhyay <neeraju@...eaurora.org>,
        paulmck@...ux.vnet.ibm.com, 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 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.