linux-kernel - Re: Restartable Sequences system call merged into Linux

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Message-ID: <21eae301-50ff-e95a-f3c7-dedcf2f66842@redhat.com>
Date:   Thu, 14 Jun 2018 16:41:46 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Pavel Machek <pavel@....cz>, carlos <carlos@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        libc-alpha <libc-alpha@...rceware.org>
Subject: Re: Restartable Sequences system call merged into Linux

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

Thanks,
Florian