This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- 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 12:58:13 +0000 (UTC)
- Subject: Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)
- Authentication-results: sourceware.org; auth=none
- References: <1436724386-30909-1-git-send-email-mathieu dot desnoyers at efficios dot com> <55AD14A4 dot 6030101 at redhat dot com> <CALCETrUx6wFxmz+9TyW5bNgaMN0q180G8y9YOyq_D41sdhFaRQ at mail dot gmail dot com> <CA+55aFzMJkzydXb7uVv1iSUnp=539d43ghQaonGdzMoF7QLZBA at mail dot gmail dot com> <CALCETrUZ8vB30rdmeoV4JKPUsRnVPvoxXRJ47CEFud2aSF2=Ew at mail dot gmail dot com> <CA+55aFwLZLeeN7UN82dyt=emQcNBc8qZPJAw5iqtAbBwFA7FPQ at mail dot gmail dot com> <2010227315 dot 699 dot 1437438300542 dot JavaMail dot zimbra at efficios dot com> <20150721073053 dot GA14716 at domone>
----- On Jul 21, 2015, at 3:30 AM, OndÅej BÃlka neleai@seznam.cz wrote:
> 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.
I don't see how this would be acceptable on memory-constrained embedded
systems. They have multiple cores, and performance requirements, so
having a fast getcpu would be useful there (e.g. telecom industry),
but they clearly cannot afford a 512kB table per process just for that.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com