LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
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 09:01:54 -0400 (EDT)	[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

  reply	other threads:[~2018-06-14 13:02 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 [this message]
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
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=894222691.12973.1528981314012.JavaMail.zimbra@efficios.com \
    --to=mathieu.desnoyers@efficios.com \
    --cc=boqun.feng@gmail.com \
    --cc=carlos@redhat.com \
    --cc=fweimer@redhat.com \
    --cc=libc-alpha@sourceware.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=pavel@ucw.cz \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --subject='Re: Restartable Sequences system call merged into Linux' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).