Re: [RFC PATCH 00/11] sched: CFS low-latency features

Mathieu Desnoyers
2010-08-27T16:09:50Z

* Peter Zijlstra (peterz@infradead.org) wrote:
> On Fri, 2010-08-27 at 11:21 -0400, Mathieu Desnoyers wrote:
> > SIGEV_THREAD
> > Upon timer expiration, invoke sigev_notify_function as if it
> > were the start function of a new thread. (Among the implementa‐
> > tion possibilities here are that each timer notification could
> > result in the creation of a new thread, or that a single thread
> > is created to receive all notifications.) The function is
> > invoked with sigev_value as its sole argument. If
> > sigev_notify_attributes is not NULL, it should point to a
> > pthread_attr_t structure that defines attributes for the new
> > thread (see pthread_attr_init(3).
> >
> > So basically, it's the glibc implementation that is broken, not the standard.
>
> The standard is broken too, what context will the new thread inherit?

Besides pthread_attr_t, thinking of the scheduler/cgroups/etc stuff, I'd think
it might be expected to inherit the state of the thread which calls
timer_create(). But this is not what glibc does right now, and it is not spelled
out clearly by the standard.

> The pthread_attr_t stuff tries to cover some of that, but pthread_attr_t
> doesn't cover all inherited task attributes, and allows for some very
> 'interesting' bugs [1].

(see below)

>
> The specification also doesn't cover the case where the handler takes
> more time to execute than the timer interval.

Why should it ? It seems valid for a workload to result in spawning many threads
bound to more than a single core on a multi-core system. So concurrency
management should be performed by the application.

>
> [1] - consider the case where pthread_attr_t includes the stack and we
> use a spawn thread on expire policy and then run into the situation
> where the handler is delayed past the next expiration.

Setting a thread stack and generating the signal more than once is taken into
account in the standard. It leads to unspecified result (IOW: don't do this):

http://www.opengroup.org/onlinepubs/009695399/functions/timer_create.html

"If evp->sigev_sigev_notify is SIGEV_THREAD and sev->sigev_notify_attributes is
not NULL, if the attribute pointed to by sev->sigev_notify_attributes has a
thread stack address specified by a call to pthread_attr_setstack() or
pthread_attr_setstackaddr(), the results are unspecified if the signal is
generated more than once."

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.