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

[Rich Felker]

On Sat, Mar 12, 2016 at 07:48:36PM +0100, Ingo Molnar wrote:
> 
> * Rich Felker <dalias@xxxxxxxx> wrote:
> 
> > On Sat, Mar 12, 2016 at 06:00:40PM +0100, Ingo Molnar wrote:
> > > 
> > > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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