Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers

[Ingo Molnar <mingo@kernel.org>]

* 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