This is the mail archive of the
mailing list for the glibc project.
Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)
- From: Rich Felker <dalias at libc dot org>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- Cc: Linus Torvalds <torvalds at linux-foundation dot org>, Andy Lutomirski <luto at amacapital dot net>, Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>, Ben Maurer <bmaurer at fb dot com>, Paul Turner <pjt at google dot com>, Andrew Hunter <ahh at google dot com>, Peter Zijlstra <peterz at infradead dot org>, Ingo Molnar <mingo at redhat dot com>, rostedt <rostedt at goodmis dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Josh Triplett <josh at joshtriplett dot org>, Lai Jiangshan <laijs at cn dot fujitsu dot com>, Andrew Morton <akpm at linux-foundation dot org>, linux-api <linux-api at vger dot kernel dot org>, libc-alpha <libc-alpha at sourceware dot org>
- Date: Sat, 18 Jul 2015 03:34:33 -0400
- 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> <5CDDBDF2D36D9F43B9F5E99003F6A0D48D5F39C6 at PRN-MBX02-1 dot TheFacebook dot com> <587954201 dot 31 dot 1436808992876 dot JavaMail dot zimbra at efficios dot com> <5CDDBDF2D36D9F43B9F5E99003F6A0D48D5F5DA0 at PRN-MBX02-1 dot TheFacebook dot com> <549319255 dot 383 dot 1437070088597 dot JavaMail dot zimbra at efficios dot com> <CALCETrWEKE=mow3vVh7C4r8CuGy_d5VOEz7KkpijuR5cpBfFtg at mail dot gmail dot com> <CA+55aFz-VBnEKh0SPKgu8xV5=Zb+=6odybVUDoOYOknshbcFJA at mail dot gmail dot com> <20150717232836 dot GA13604 at domone>
On Sat, Jul 18, 2015 at 01:28:36AM +0200, OndÅej BÃlka wrote:
> On Fri, Jul 17, 2015 at 11:48:14AM -0700, Linus Torvalds wrote:
> > On Thu, Jul 16, 2015 at 12:27 PM, Andy Lutomirski <email@example.com> wrote:
> > >
> > > If we actually bit the bullet and implemented per-cpu mappings
> > That's not ever going to happen.
> > The Linux VM model of "one page table per VM" is the right one.
> > Anything else sucks, and makes threading a disaster.
> > So you can try to prove me wrong, but seriously, I doubt you'll succeed.
> > On x86, if you want per-cpu memory areas, you should basically plan on
> > using segment registers instead (although other odd state has been
> > used - there's been the people who use segment limits etc rather than
> > the *pointer* itself, preferring to use "lsl" to get percpu data. You
> > could also imaging hiding things in the vector state somewhere if you
> > control your environment well enough).
> Thats correct, problem is that you need some sort of hack like this on
> archs that otherwise would need syscall to get tid/access tls variable.
> On x64 and archs that have register for tls this could be implemented
> relatively easily.
> Kernel needs to allocate
> int running_cpu_for_tid;
This does not scale. You're assuming the default task ("pid") number
limit, but this can be raised up to 512k (beyond that is impossible
because of PI/robust futex ABI).
> On context switch it atomically writes to this table
> running_cpu_for_tid[tid] = cpu;
> This table is read-only accessible from userspace as mmaped file.
There is a much simpler solution: use a per-cpu (rather than per-task)
page that contains the right value for the cpu. I believe vdso already
does something like this, no?