Date: Sat, 12 Mar 2016 19:48:36 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Rich Felker <dalias@libc.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
Message-ID: <20160312184836.GA17707@gmail.com>
References: <20160310033446.GL9349@brightrain.aerifal.cx>
 <20160310111646.GA13102@gmail.com>
 <20160310164104.GM9349@brightrain.aerifal.cx>
 <20160310180331.GB15940@gmail.com>
 <20160310232819.GR9349@brightrain.aerifal.cx>
 <20160311093347.GA17749@gmail.com>
 <20160311113914.GD29662@port70.net>
 <CA+55aFxvMM3j1aWjN-kr5Hn8CUC_RSNw5hc+X8zFXMaMv+mGww@mail.gmail.com>
 <20160312170040.GA1108@gmail.com>
 <20160312180531.GD9349@brightrain.aerifal.cx>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160312180531.GD9349@brightrain.aerifal.cx>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1719
Lines: 47


* 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!

You are scaring me ... :-(

Thanks,

	Ingo