Re: Restartable Sequences system call merged into Linux

From: Florian Weimer
Date: Thu Jun 14 2018 - 10:41:54 EST

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:

- 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.