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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: Florian Weimer <fweimer@redhat.com>, 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>
Subject: Re: Restartable Sequences system call merged into Linux
Date: Thu, 14 Jun 2018 09:38:16 -0400 (EDT)	[thread overview]
Message-ID: <956816108.13001.1528983496098.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20180614132557.GA15201@amd>

----- On Jun 14, 2018, at 9:25 AM, Pavel Machek pavel@ucw.cz wrote:

> 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.

Fair point! Do we have a standard benchmark that would stress this ?

If it ends up being noticeable overhead, I wonder whether we could extend clone() with a
new CLONE_RSEQ flag so glibc could pass a pointer to the rseq TLS area through an extra
argument to the clone system call rather than do an extra syscall on thread creation ?

Thanks,

Mathieu


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

  parent reply	other threads:[~2018-06-14 13:38 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-11 19:49 Mathieu Desnoyers
2018-06-11 19:55 ` Florian Weimer
2018-06-11 20:04   ` Mathieu Desnoyers
2018-06-12 13:11     ` Florian Weimer
2018-06-12 16:31       ` Mathieu Desnoyers
2018-06-13  8:21         ` Florian Weimer
2018-06-14 12:27         ` Pavel Machek
2018-06-14 13:01           ` Mathieu Desnoyers
2018-06-14 13:25             ` Pavel Machek
2018-06-14 13:32               ` Florian Weimer
2018-06-14 13:46                 ` Mathieu Desnoyers
2018-06-15  5:10                   ` Florian Weimer
2018-06-15 17:44                     ` Mathieu Desnoyers
2018-06-14 13:38               ` Mathieu Desnoyers [this message]
2018-06-14 13:49                 ` Pavel Machek
2018-06-14 14:00                   ` Florian Weimer
2018-06-14 14:36                     ` Mathieu Desnoyers
2018-06-14 14:41                       ` Florian Weimer
2018-06-14 15:09                         ` Mathieu Desnoyers
2018-06-15  5:09             ` Florian Weimer
2018-06-15 17:50               ` Mathieu Desnoyers
2018-06-15  5:07           ` Florian Weimer
2018-06-13 11:48 ` Heiko Carstens
2018-06-13 16:14   ` Mathieu Desnoyers
2018-06-13 19:53     ` Mathieu Desnoyers

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