From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751921AbdIUNzz (ORCPT <rfc822;w@1wt.eu>);
        Thu, 21 Sep 2017 09:55:55 -0400
Received: from merlin.infradead.org ([205.233.59.134]:36774 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751606AbdIUNzx (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 21 Sep 2017 09:55:53 -0400
Date: Thu, 21 Sep 2017 15:55:46 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>, paulmck@linux.vnet.ibm.com,
        josh@joshtriplett.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, linux-kernel@vger.kernel.org,
        sramana@codeaurora.org, prsood@codeaurora.org, pkondeti@codeaurora.org,
        markivx@codeaurora.org
Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu
Message-ID: <20170921135546.x3q3zeicbo2vtjvk@hirez.programming.kicks-ass.net>
References: <8f33e48e-ac6d-2c88-e16f-20b698c06292@codeaurora.org>
 <20170917010015.GW3521@linux.vnet.ibm.com>
 <ede28a3e-7708-f5e7-8480-f339c7c37fc5@codeaurora.org>
 <20170918111105.15f687da@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170918111105.15f687da@gandalf.local.home>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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@codeaurora.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.