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

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Date:   Mon, 9 Jul 2018 12:44:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...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, 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.

> Of course, we can explicitly put a check into the KVM loop, but that
> brings me back to my original concern — why is it OK to do it there as
> a special case and not for the general case construct of
> if (need_resched) { drop_local_locks(); cond_resched(); get_local_locks(); }

I'm not proposing anything that would differentiate between KVM and
anything else.

I just want to keep my preemption state sane-ish, and adding random
conditions to part of it just doesn't look attractive.