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Date:   Thu, 14 Jun 2018 11:09:42 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Pavel Machek <pavel@ucw.cz>, carlos <carlos@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        libc-alpha <libc-alpha@sourceware.org>
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Subject: Re: Restartable Sequences system call merged into Linux
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Thread-Topic: Restartable Sequences system call merged into Linux
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----- On Jun 14, 2018, at 10:41 AM, Florian Weimer fweimer@redhat.com wrote:

> On 06/14/2018 04:36 PM, Mathieu Desnoyers wrote:
>> ----- On Jun 14, 2018, at 10:00 AM, Florian Weimer fweimer@redhat.com 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.

Just for the sake of completeness, there is (of course) no inheritance
on exec(). So glibc would also have to register the rseq TLS in its
constructors.

Thanks,

Mathieu

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