Re: Restartable Sequences system call merged into Linux

[Florian Weimer]

On 06/14/2018 04:36 PM, Mathieu Desnoyers wrote:
> ----- On Jun 14, 2018, at 10:00 AM, Florian Weimer fweimer@xxxxxxxxxx wrote:
>
> > On 06/14/2018 03:49 PM, Pavel Machek wrote:
> > > Hi!
> > >
> > > > > > - 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 ?
> > >
> > > Web server performance benchmarks basically test clone() performance
> > > in many cases.
> >
> > Isn't that fork?  I expect that the rseq arena is inherited on fork and
> > fork-type clone, otherwise it's going to be painful.
>
> On fork or clone creating a new process, the rseq tls area is inherited
> from the thread that does the fork syscall.
>
> On creation of a new thread with clone, there is no such inheritance.

Makes sense.  So fork-based (web) servers will not be impacted by the 
additional system call, and thread-based servers likely use a thread 
pool anyway.  I'm not really concerned about the additional system call 
here.

Thanks,
Florian