On Tue, Sep 15, 2015 at 06:42:19PM +0200, Paolo Bonzini wrote:
>
>
> On 15/09/2015 15:36, Christian Borntraeger wrote:
> > I am wondering why the old code behaved in such fatal ways. Is there
> > some interaction between waiting for a reschedule in the
> > synchronize_sched writer and some fork code actually waiting for the
> > read side to get the lock together with some rescheduling going on
> > waiting for a lock that fork holds? lockdep does not give me an hints
> > so I have no clue :-(
>
> It may just be consuming too much CPU usage. kernel/rcu/tree.c warns
> about it:
>
> * if you are using synchronize_sched_expedited() in a loop, please
> * restructure your code to batch your updates, and then use a single
> * synchronize_sched() instead.
>
> and you may remember that in KVM we switched from RCU to SRCU exactly to
> avoid userspace-controlled synchronize_rcu_expedited().
>
> In fact, I would say that any userspace-controlled call to *_expedited()
> is a bug waiting to happen and a bad idea---because userspace can, with
> little effort, end up calling it in a loop.
Excellent points!
Other options in such situations include the following:
o Rework so that the code uses call_rcu*() instead of *_expedited().
o Maintain a per-task or per-CPU counter so that every so many
*_expedited() invocations instead uses the non-expedited
counterpart. (For example, synchronize_rcu instead of
synchronize_rcu_expedited().)
Note that synchronize_srcu_expedited() is less troublesome than are the
other *_expedited() functions, because synchronize_srcu_expedited() does
not inflict OS jitter on other CPUs. This situation is being improved,
so that the other *_expedited() functions inflict less OS jitter and
(mostly) avoid inflicting OS jitter on nohz_full CPUs and idle CPUs (the
latter being important for battery-powered systems). In addition, the
*_expedited() functions avoid hammering CPUs with N-squared OS jitter
in response to concurrent invocation from all CPUs because multiple
concurrent *_expedited() calls will be satisfied by a single expedited
grace-period operation. Nevertheless, as Paolo points out, it is still
necessary to exercise caution when exposing synchronous grace periods
to userspace control.
Thanx, Paul
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