Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

[Ingo Molnar]

* pageexec@xxxxxxxxxxx <pageexec@xxxxxxxxxxx> wrote:

> On 6 Jun 2011 at 17:26, Ingo Molnar wrote:
> 
> > 
> > * pageexec@xxxxxxxxxxx <pageexec@xxxxxxxxxxx> wrote:
> > 
> > > > > a page fault is never a fast path, after all the cpu has just 
> > > > > taken an exception (vs. the syscall/sysenter style actually 
> > > > > fast user->kernel transition) and is about to make page table 
> > > > > changes (and possibly TLB flushes).
> > > >
> > > > Sure it is.  It's a path that's optimized carefully and needs to 
> > > > be as fast as possible.  Just because it's annoyingly slow 
> > > > doesn't mean we get to make it even slower.
> > > 
> > > sorry, but stating that the pf handler is a fast path doesn't 
> > > make it so ;) [...]
> > 
> > Are you talking about the Linux kernel?
> 
> yes, what else? ;)

Dunno, Windows perhaps? You were talking about a page fault handler 
that was a slowpath, you cannot possibly have meant Linux with that.

> > FYI, incredible amount of work has gone into making pagefaults as 
> > fast and scalable as possible.
> 
> i wasn't talking about scalability (it's irrelevant anyway here), 
> only speed. [...]

Which part of "fast and scalable" did you not understand?

Just a couple of days ago i noticed a single cycle inefficiency in 
the pagefault fastpath, introduced in the 3.0 merge window. I 
requested (and got) an urgent fix for that:

 b80ef10e84d8: x86: Move do_page_fault()'s error path under unlikely()

 | Ingo suggested SIGKILL check should be moved into slowpath
 | function. This will reduce the page fault fastpath impact
 | of this recent commit:
 |
 |   37b23e0525d3: x86,mm: make pagefault killable

I treated it as a performance regression.

So i ask you again, what is your basis for calling the #PF path on 
Linux a 'slowpath'? Is Linus's and my word and 5 years of Git history 
showing that it's optimized as a fastpath not enough proof for you?

Thanks,

	Ingo
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