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 13:02:00 -0000 [thread overview]
Message-ID: <894222691.12973.1528981314012.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20180614122759.GB8798@amd>
----- On Jun 14, 2018, at 8:27 AM, Pavel Machek pavel@ucw.cz wrote:
> On Tue 2018-06-12 12:31:24, Mathieu Desnoyers wrote:
>> ----- On Jun 12, 2018, at 9:11 AM, Florian Weimer fweimer@redhat.com wrote:
>>
>> > On 06/11/2018 10:04 PM, Mathieu Desnoyers wrote:
>> >> ----- On Jun 11, 2018, at 3:55 PM, Florian Weimer fweimer@redhat.com wrote:
>> >>
>> >>> On 06/11/2018 09:49 PM, Mathieu Desnoyers wrote:
>> >>>> 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,
>
>> I have a few possible approaches in mind (feel free to suggest other
>> options):
>>
>> A) glibc exposes a strong __rseq_abi TLS symbol:
>>
>> - should ideally *not* be global-dynamic for performance reasons,
>> - registration to kernel can either be handled explicitly by requiring
>> application or libraries to call an API, or implicitly at thread
>> creation,
>
> ...so I'd prefer explicit API call.
I have use-cases where a library wants to link against librseq and have rseq
critical sections, without requiring the application to explicitly add rseq
registration calls on thread creation/destruction. Is there a way to register
callbacks to glibc which could be invoked on thread creation/destruction ?
Then if we include dynamic loading of libraries (dlopen/dlclose) in the
picture, this gets even worse, as we'd need to be able to iterate on all
existing threads to invoke registration/unregistration callbacks.
One alternative approach would be to let the user library lazily register rseq
when needed, and use a pthread_key for unregistration. However, this does not
allow dlclose of the user library without figuring a way to iterate on all
threads.
Another alternative would be to somehow let glibc handle the registration,
perhaps only doing it for applications expressing their interest for rseq.
Thoughts ?
Thanks,
Mathieu
>
>> B) librseq.so exposes a strong __rseq_abi symbol:
>
> Works for me.
> Pavel
>
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures)
> http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2018-06-14 13:02 UTC|newest]
Thread overview: 35+ 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 8:23 ` Florian Weimer
2018-06-14 12:28 ` Pavel Machek
2018-06-14 13:02 ` Mathieu Desnoyers [this message]
2018-06-14 13:26 ` 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
2018-06-14 13:50 ` Pavel Machek
2018-06-14 14:01 ` 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
2018-07-11 16:03 ` Stefan Liebler
2018-07-11 16:23 ` Adhemerval Zanella
2018-07-11 17:19 ` Florian Weimer
2018-07-11 17:42 ` Adhemerval Zanella
2018-07-11 17:45 ` Florian Weimer
2018-07-11 17:59 ` Adhemerval Zanella
2018-07-12 8:08 ` Stefan Liebler
2018-07-11 19:05 ` Carlos O'Donell
2018-07-12 8:10 ` Szabolcs Nagy
2018-07-13 13:22 ` Stefan Liebler
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