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

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Message-ID: <20180614132557.GA15201@amd>
Date:   Thu, 14 Jun 2018 15:25:58 +0200
From:   Pavel Machek <pavel@....cz>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Florian Weimer <fweimer@...hat.com>, 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

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.
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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