linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 19 Jul 2018 15:17:27 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>,
        mhillenb@...zon.de, linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Thu, Jul 19, 2018 at 08:16:47AM +0200, David Woodhouse wrote:
> 
> 
> On Wed, 2018-07-18 at 20:11 -0700, Paul E. McKenney wrote:
> > 
> > > That is interesting. As I replied to Paul, we are already calling
> > > rcu_user_enter/exit() on guest_enter/exit_irqsoff(). So I'm wondering why
> > > you're seeing such an optimization by repeating those calls.
> > > 
> > > Perhaps the rcu_user_* somehow aren't actually called from
> > > __context_tracking_enter()...? Some bug in context tracking?
> > > Otherwise it's a curious side effect.
> > 
> > David is working with v4.15.  Is this maybe something that has changed
> > since then?
> 
> To clarify: in 4.15 without CONFIG_PREEMPT and without NO_HZ_FULL I was
> seeing RCU stalls because a thread in vcpu_run() was *never* seen to go
> through a quiescent state. Hence the change to need_resched() in the
> first patch in this thread, which fixed the problem at hand and seemed
> to address the general case.
> 
> It then seemed by *inspection* that the NO_HZ_FULL case was probably
> broken, because we'd failed to spot the rcu_user_* calls. But
> rcu_user_enter() does nothing in the !NO_HZ_FULL case, so wouldn't have
> helped in the testing that we were doing anyway.

Oh ok, so the optimization you saw is likely unrelated to the rcu_user* things.