Re: Restartable Sequences system call merged into Linux

[Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>]

----- On Jun 14, 2018, at 9:25 AM, Pavel Machek pavel@ucw.cz wrote:

> Hi!
> 
>> >> >>>> It should be noted that there can be only one rseq TLS area registered per
>> >> >>>> thread,
>> >> >>>> which can then be used by many libraries and by the executable, so this is a
>> >> >>>> process-wide (per-thread) resource that we need to manage carefully.
>> >> >>>
>> >> >>> Is it possible to resize the area after thread creation, perhaps even
>> >> >>> from other threads?
>> >> >> 
>> >> >> I'm not sure why we would want to resize it. The per-thread area is fixed-size.
>> >> >> Its layout is here: include/uapi/linux/rseq.h: struct rseq
>> >> > 
>> >> > Looks I was mistaken and this is very similar to the robust mutex list.
>> >> > 
>> >> > Should we treat it the same way?  Always allocate it for each new thread
>> >> > and register it with the kernel?
>> >> 
>> >> That would be an efficient way to do it, indeed. There is very little
>> >> performance overhead to have rseq registered for all threads, whether or
>> >> not they intend to run rseq critical sections.
>> > 
>> > People with slow / low memory machines would prefer not to see
>> > overhead they don't need...
>> 
>> In terms of memory usage, if people don't want the extra few bytes of memory
>> used by rseq in the kernel, they should use CONFIG_RSEQ=n.
>> 
>> In terms of overhead, let's have a closer look at what it means: when a thread
>> is registered to rseq, but does not enter rseq critical sections, only this
>> extra work is done by the kernel:
>> 
>> - rseq_preempt(): on preemption, the scheduler sets the TIF_NOTIFY_RESUME thread
>>   flag, so rseq_handle_notify_resume() can check whether it's in a rseq critical
>>   section when returning to user-space,
>> - rseq_signal_deliver(): on signal delivery, rseq_handle_notify_resume() checks
>>   whether it's in a rseq critical section,
>> - rseq_migrate: on migration, the scheduler sets TIF_NOTIFY_RESUME as well,
> 
> Yes, this is not likely to be noticeable.
> 
> But the proposal wanted to add a syscall to thread creation, right?
> And I believe that may be noticeable.

Fair point! Do we have a standard benchmark that would stress this ?

If it ends up being noticeable overhead, I wonder whether we could extend clone() with a
new CLONE_RSEQ flag so glibc could pass a pointer to the rseq TLS area through an extra
argument to the clone system call rather than do an extra syscall on thread creation ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com