On Mon, Dec 16, 2013 at 04:45:39PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 16, 2013 at 07:32:48AM -0800, Paul E. McKenney wrote:
> > On Mon, Dec 16, 2013 at 04:26:36PM +0100, Peter Zijlstra wrote:
> > > On Mon, Dec 16, 2013 at 07:19:22AM -0800, tip-bot for Paul E. McKenney
> > > wrote:
> > > > The underlying problem is that perf is invoking call_rcu() with the
> > > > scheduler locks held, but in NOCB mode, call_rcu() will with high
> > > > probability invoke the scheduler -- which just might want to use its
> > > > locks. The reason that call_rcu() needs to invoke the scheduler is
> > > > to wake up the corresponding rcuo callback-offload kthread, which
> > > > does the job of starting up a grace period and invoking the callbacks
> > > > afterwards.
> > > >
> > > > One solution (championed on a related problem by Lai Jiangshan) is to
> > > > simply defer the wakeup to some point where scheduler locks are no
> > > > longer
> > > > held. Since we don't want to unnecessarily incur the cost of such
> > > > deferral, the task before us is threefold:
> > > >
> > > > 1. Determine when it is likely that a relevant scheduler lock is
> > > > held.
> > > >
> > > > 2. Defer the wakeup in such cases.
> > > >
> > > > 3. Ensure that all deferred wakeups eventually happen, preferably
> > > > sooner rather than later.
> > > >
> > > > We use irqs_disabled_flags() as a proxy for relevant scheduler locks
> > > > being held. This works because the relevant locks are always acquired
> > > > with interrupts disabled. We may defer more often than needed, but that
> > > > is at least safe.
> > >
> > > This would also allow us to do away with things like the below patch,
> > > right?
> >
> > It takes care of one problem, but there are others, including
> > rcu_read_unlock() inovking the scheduler to deboost itself. So for the
> > moment, we still need the below patch.
>
> Oh right, see I knew I was forgetting something... :-)
I am hoping to make your patch unnecessary, but it ain't trivial. ;-)
We will get there! Especially if I can find Lai Jiangshan's old patch
that reworked deboosting. :-/
Thanx, Paul
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