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Date: Sat, 12 Mar 2016 14:08:59 -0500
From: Rich Felker <dalias@...c.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>,
"musl@...ts.openwall.com" <musl@...ts.openwall.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation
helpers
On Sat, Mar 12, 2016 at 07:48:36PM +0100, Ingo Molnar wrote:
>
> * Rich Felker <dalias@...c.org> wrote:
>
> > On Sat, Mar 12, 2016 at 06:00:40PM +0100, Ingo Molnar wrote:
> > >
> > > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > >
> > > > [...]
> > > >
> > > > Because if that's the case, I wonder if what you really want is not "sticky
> > > > signals" as much as "synchronous signals" - ie the ability to say that a signal
> > > > shouldn't ever interrupt in random places, but only at well-defined points
> > > > (where a system call would be one such point - are there others?)
> > >
> > > Yes, I had similar 'deferred signal delivery' thoughts after having written up the
> > > sticky signals approach, I just couldn't map all details of the semantics: see the
> > > 'internal libc functions' problem below.
> > >
> > > If we can do this approach then there's another advantage as well: this way the C
> > > library does not even have to poll for cancellation at syscall boundaries: i.e.
> > > the regular system call fast path gets faster by 2-3 instructions as well.
> >
> > That is not a measurable benefit. You're talking about 2-3 cycles out of 10k or
> > more cycles (these are heavy blocking syscalls not light things like SYS_time or
> > SYS_getpid).
>
> Huh? The list of 'must be' cancellable system calls includes key system calls
> like:
>
> open()
> close()
> read() variants
> write() variants
> poll()
> select()
>
> which can be and often are very lightweight. The list of 'may be cancellable'
> system calls includes even more lightweight system calls.
>
> I think you are confusing 'might block' with 'will block'. Most IO operations on a
> modern kernel with modern hardware will not block!
No, I just mean syscalls that may block are generally heavy
operations. There may be a few exceptions (especially close in the
case where it's not the last fd for an open file) but I think you'd be
hard pressed to find a case where 2-3 cycles is even 0.2% of the
syscall time. But my point was not to get derailed on an argument
about the exact performance (non-)benefits of "saving 2-3 cycles",
just to say this is not an interesting argument for one approach vs
another and that it's a distraction from other much-more-important
issues.
> You are scaring me ... :-(
I'm not sure how to interpret this, but if you really feel what I'm
writing is scary/hostile I'll try to convey my ideas differently.
Rich
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