From: Rich Felker <dalias@libc.org>
To: Jeff Law <law@redhat.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>,
Florian Weimer <fweimer@redhat.com>,
James Greenhalgh <james.greenhalgh@arm.com>,
nd@arm.com, GNU C Library <libc-alpha@sourceware.org>,
Richard Earnshaw <Richard.Earnshaw@arm.com>,
Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Subject: Re: [RFC] nptl: change default stack guard size of threads
Date: Wed, 20 Dec 2017 04:49:00 -0000 [thread overview]
Message-ID: <20171220044744.GV1627@brightrain.aerifal.cx> (raw)
In-Reply-To: <5194e219-78fd-80a0-e91a-9af7ac86dbe0@redhat.com>
On Tue, Dec 19, 2017 at 09:41:03PM -0700, Jeff Law wrote:
> On 12/19/2017 01:34 PM, Rich Felker wrote:
> > On Tue, Dec 19, 2017 at 06:21:32PM +0000, Szabolcs Nagy wrote:
> >> On 19/12/17 13:06, Florian Weimer wrote:
> >>> On 12/19/2017 01:34 PM, James Greenhalgh wrote:
> >>>
> >>>> Option 1: 64k guard pages for LP64 on AArch64.
> >>>
> >>>> Option 2: 4k guard pages for LP64 for AArch64
> >>>
> >>>> Our proposal then, having spoken things through with the Arm engineers
> >>>> here, and taken in to consideration the opinions on this thread, is that
> >>>> we move to two "blessed" configurations of the GCC support for AArch64.
> >>>
> >>> Are there any Arm engineers who prefer Option 2, or is that just there to accommodate feedback on libc-alpha?
> >>>
> >>> My main concern was the variance in configurations with Option 1 (compared to Option 2). To some extent, the
> >>> variance with Option 1 is temporary. If both Option 1 and 2 are offered, we have permanent variance. From my
> >>> point of view, that's worth that just going with Option 1.
> >>>
> >>> So if this is some sort of consensus proposal, as opposed to actual technical requirements which favor Option 2
> >>> in some deployments, I think that's not a good idea, and we should go with Option 1 instead.
> >>>
> >>
> >> well glibc can pretend that only Option 1 is available,
> >> my latest patch assumes 64k probe interval:
> >> https://sourceware.org/ml/libc-alpha/2017-12/msg00451.html
> >>
> >> however Option 1 requires generic code to be changed
> >> for aarch64 only (in the libc and elsewhere) and we
> >> cannot easily do that on all (non-glibc) systems.
> >>
> >> it seems to me if there are systems where Option 1
> >> may not provide guaranteed trap on stack overflow
> >> then gcc should have Option 2 for those systems.
> >
> > For what it's worth, I would prefer having the assumed minimum guard
> > size be 4k for musl targets. Even if we do increase the default guard
> > to 64k for 64-bit archs (seems likely), applications that manually set
> > it lower for whatever reason should still be handled safely.
> >
> > I'm utterly unconvinced that there's any practical measurable
> > performance difference either way, unless someone can demonstrate an
> > existing real-world program (not artificial benchmark) where the
> > difference is measurable.
> I've believed all along that stack clash probing as implemented by GCC
> is pretty damn cheap -- cheap enough that we ought to just turn it on by
> default and move on to other more useful work items. And I hold that
> position regardless of whether or not the guard is 4k or 64k.
>
> 64k is marginally better simply because there's less probing,
> particularly in a functions that allocate something like char array of
> MAXPATHLEN entries. Based on what I've looked at on a distro-wide
> basis, MAXPATHLEN arrays on the stack are the single biggest reasons we
> have to probe in prologues.
They're also almost certainly indicative of the function making
syscalls, in which case <10 instructions for a probe are completely
dominated by the syscall time (hundreds if not thousands of times
larger).
Rich
next prev parent reply other threads:[~2017-12-20 4:49 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-29 14:59 Szabolcs Nagy
2017-11-29 15:18 ` Florian Weimer
2017-11-29 18:17 ` Carlos O'Donell
2017-11-29 18:29 ` Rich Felker
2017-11-29 20:33 ` Florian Weimer
2017-11-29 18:40 ` Szabolcs Nagy
2017-11-29 20:44 ` Florian Weimer
2017-11-29 20:52 ` Rich Felker
2017-11-29 21:02 ` Florian Weimer
2017-11-29 23:13 ` Szabolcs Nagy
2017-12-05 10:55 ` James Greenhalgh
2017-12-06 12:51 ` Florian Weimer
2017-12-11 23:49 ` Jeff Law
2017-12-12 11:43 ` Szabolcs Nagy
2017-12-12 16:36 ` Rich Felker
2017-12-12 18:07 ` Szabolcs Nagy
2017-12-12 19:30 ` Florian Weimer
2017-12-13 11:58 ` Szabolcs Nagy
2017-12-19 12:35 ` James Greenhalgh
2017-12-19 13:06 ` Florian Weimer
2017-12-19 18:21 ` Szabolcs Nagy
2017-12-19 20:34 ` Rich Felker
2017-12-20 4:42 ` Jeff Law
2017-12-20 4:49 ` Rich Felker [this message]
2017-12-27 13:08 ` Wilco Dijkstra
2017-12-20 4:45 ` Jeff Law
2017-11-29 22:28 ` Wilco Dijkstra
2017-11-29 22:38 ` Carlos O'Donell
2017-12-06 12:53 ` Florian Weimer
2017-12-06 13:10 ` Wilco Dijkstra
2017-12-06 13:13 ` Florian Weimer
2017-11-29 23:02 ` Rich Felker
2017-12-06 13:16 ` Florian Weimer
2017-12-06 13:40 ` Joseph Myers
2017-12-06 13:51 ` Florian Weimer
2017-12-06 14:44 ` Jeff Law
2017-12-06 14:27 ` Wilco Dijkstra
2017-12-06 20:41 ` Szabolcs Nagy
2017-12-06 21:24 ` Adhemerval Zanella
2017-12-06 22:08 ` Rich Felker
2017-12-08 18:28 ` Szabolcs Nagy
2017-11-29 22:45 ` Szabolcs Nagy
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