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

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Date:   Mon, 09 Jul 2018 11:56:41 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        mhillenb@...zon.de, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Mon, 2018-07-09 at 12:44 +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 10:18:55AM +0100, David Woodhouse wrote:
> > 
> > > 
> > > Which seems like an entirely reasonable amount of time to kick a task.
> > > Not scheduling for a second is like an eternity.
> >
> > If that is our only "fix" for KVM, then wouldn't that mean that things
> > like expand_fdtable() would be *expected* to take "an eternity" when
> > another CPU happens to be in the guest? Because vcpu_run() would still
> > loop until the task gets kicked after a second?
>
> But either proposal is exactly the same in this respect. The whole
> rcu_urgent_qs thing won't be set any earlier either.

Er.... Marius, our latencies in expand_fdtable() definitely went from
~10s to well below one second when we just added the rcu_all_qs() into
the loop, didn't they? And that does nothing if !rcu_urgent_qs.

Doesn't rcu_implicit_dynticks_qs() set it after jiffies_till_sched_qs
(default HZ/10) has expired?

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