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Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- Cc: Linus Torvalds <torvalds at linux-foundation dot org>, Andy Lutomirski <luto at amacapital dot net>, Ben Maurer <bmaurer at fb dot com>, Ingo Molnar <mingo at redhat dot com>, libc-alpha <libc-alpha at sourceware dot org>, Andrew Morton <akpm at linux-foundation dot org>, linux-api <linux-api at vger dot kernel dot org>, rostedt <rostedt at goodmis dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Florian Weimer <fweimer at redhat dot com>, Josh Triplett <josh at joshtriplett dot org>, Lai Jiangshan <laijs at cn dot fujitsu dot com>, Paul Turner <pjt at google dot com>, Andrew Hunter <ahh at google dot com>, Peter Zijlstra <peterz at infradead dot org>
- Date: Tue, 21 Jul 2015 09:30:53 +0200
- Subject: Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)
- Authentication-results: sourceware.org; auth=none
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On Tue, Jul 21, 2015 at 12:25:00AM +0000, Mathieu Desnoyers wrote:
> >> Does it solve the Wine problem? If Wine uses gs for something and
> >> calls a function that does this, Wine still goes boom, right?
> >
> > So the advantage of just making a global segment descriptor available
> > is that it's not *that* expensive to just save/restore segments. So
> > either wine could do it, or any library users would do it.
> >
> > But anyway, I'm not sure this is a good idea. The advantage of it is
> > that the kernel support really is _very_ minimal.
>
> Considering that we'd at least also want this feature on ARM and
> PowerPC 32/64, and that the gs segment selector approach clashes with
> existing apps (wine), I'm not sure that implementing a gs segment
> selector based approach to cpu number caching would lead to an overall
> decrease in complexity if it leads to performance similar to those of
> portable approaches.
>
> I'm perfectly fine with architecture-specific tweaks that lead to
> fast-path speedups, but if we have to bite the bullet and implement
> an approach based on TLS and registering a memory area at thread start
> through a system call on other architectures anyway, it might end up
> being less complex to add a new system call on x86 too, especially if
> fast path overhead is similar.
>
> But I'm inclined to think that some aspect of the question eludes me,
> especially given the amount of interest generated by the gs-segment
> selector approach. What am I missing ?
>
As I wrote before you don't have to bite bullet as I said before. It
suffices to create 128k element array with cpu for each tid, make that
mmapable file and userspace could get cpu with nearly same performance
without hacks.