Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on sched_attr::sched_period
From: Peter Zijlstra
Date: Mon Sep 02 2019 - 08:31:34 EST
On Mon, Sep 02, 2019 at 11:16:23AM +0200, Peter Zijlstra wrote:
> On Sat, Aug 31, 2019 at 03:41:17PM +0100, Alessio Balsini wrote:
> > Right!
> > Verified that sysctl_sched_dl_period_max and sysctl_sched_dl_period_min values
> > are now always consistent.
> > I spent some time in trying to figure out if not having any mutex in
> > __checkparam_dl() is safe. There can surely happen that "max < min", e.g.:
> > Sharing my thoughts, a "BUG_ON(max < min)" in __checkparam_dl() is then a
> > guaranteed source of explosions, but the good news is that "if (period < min ||
> > period > max" in __checkparam_dl() surely fails if "max < min". Also the fact
> > that, when we are writing the new sysctl_sched_dl_* values, only one is
> > actually changed at a time, that surely helps to preserve the consistency.
> > But is that enough?
> Strictly speaking, no, I suppose it is not. We can have two changes in
> between the two READ_ONCE()s and then we'd be able to observe a
> The easy way to fix that is do something like:
> + synchronize_rcu();
> in sched_dl_period_handler(). And do:
> + preempt_disable();
> max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC;
> min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC;
> + preempt_enable();
> in __checkparam_dl().
> That would prohibit we see two changes, and seeing only the single
> change is safe.
I pushed out a new version; and added patch to sched_rt_handler() on
Please have a look at:
I'll move these two patches to sched/core if everything looks good.