From: Rich Felker <dalias@libc.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Borislav Petkov <bp@alien8.de>,
"musl@lists.openwall.com" <musl@lists.openwall.com>,
Andrew Morton <akpm@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers
Date: Sat, 12 Mar 2016 14:08:59 -0500 [thread overview]
Message-ID: <20160312190859.GF9349@brightrain.aerifal.cx> (raw)
In-Reply-To: <20160312184836.GA17707@gmail.com>
On Sat, Mar 12, 2016 at 07:48:36PM +0100, Ingo Molnar wrote:
>
> * Rich Felker <dalias@libc.org> wrote:
>
> > On Sat, Mar 12, 2016 at 06:00:40PM +0100, Ingo Molnar wrote:
> > >
> > > * Linus Torvalds <torvalds@linux-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
next prev parent reply other threads:[~2016-03-12 19:09 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-09 1:24 [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski
2016-03-09 8:56 ` Ingo Molnar
2016-03-09 11:34 ` [musl] " Szabolcs Nagy
2016-03-09 11:40 ` Szabolcs Nagy
2016-03-09 19:47 ` Linus Torvalds
2016-03-09 20:57 ` Andy Lutomirski
2016-03-09 21:26 ` Linus Torvalds
2016-03-10 10:57 ` Ingo Molnar
2016-03-10 3:34 ` Rich Felker
2016-03-10 11:16 ` Ingo Molnar
2016-03-10 16:41 ` Rich Felker
2016-03-10 18:03 ` Ingo Molnar
2016-03-10 23:28 ` Rich Felker
2016-03-11 0:18 ` Szabolcs Nagy
2016-03-11 0:48 ` Rich Felker
2016-03-11 1:14 ` Andy Lutomirski
2016-03-11 1:39 ` Szabolcs Nagy
2016-03-11 1:49 ` Szabolcs Nagy
2016-03-11 1:55 ` Rich Felker
2016-03-11 9:33 ` Ingo Molnar
2016-03-11 11:39 ` Szabolcs Nagy
2016-03-11 19:27 ` Linus Torvalds
2016-03-11 19:30 ` Andy Lutomirski
2016-03-11 19:39 ` Linus Torvalds
2016-03-11 19:44 ` Linus Torvalds
2016-03-12 17:05 ` Ingo Molnar
2016-03-12 18:10 ` Rich Felker
2016-03-12 17:00 ` Ingo Molnar
2016-03-12 18:05 ` Rich Felker
2016-03-12 18:48 ` Ingo Molnar
2016-03-12 19:08 ` Rich Felker [this message]
2016-03-12 17:08 ` Ingo Molnar
2016-03-09 17:58 ` Andy Lutomirski
2016-03-09 21:19 ` Andy Lutomirski
2016-03-12 18:13 ` Andy Lutomirski
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