From: Andi Kleen <andi@firstfloor.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org, Paul Turner <pjt@google.com>,
Andrew Hunter <ahh@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Andy Lutomirski <luto@amacapital.net>,
Andi Kleen <andi@firstfloor.org>,
Dave Watson <davejwatson@fb.com>, Chris Lameter <cl@linux.com>,
Ingo Molnar <mingo@redhat.com>, Ben Maurer <bmaurer@fb.com>,
Steven Rostedt <rostedt@goodmis.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Josh Triplett <josh@joshtriplett.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
linux-api@vger.kernel.org
Subject: Re: [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86)
Date: Sun, 13 Dec 2015 19:15:27 +0100 [thread overview]
Message-ID: <20151213181527.GV15533@two.firstfloor.org> (raw)
In-Reply-To: <1449761990-23525-1-git-send-email-mathieu.desnoyers@efficios.com>
> This getcpu cache is an alternative to the sched_getcpu() vdso which has
> a few benefits:
Note the first version of getcpu() I proposed had a cache. But it was
rejected.
> - It is faster to do a memory read that to call a vDSO,
> - This cached value can be read from within an inline assembly, which
> makes it a useful building block for restartable sequences.
On x86 we already have the de-facto ABI of using LSL with the magic
segment directly. While that is a few cycles slower than a memory load
I question the difference is big enough to justify a new system call,
and risk slow page fault in context switches.
BTW the vdso could be also optimized I think. For example glibc today
does some stupid (slow) things with it, like doing double iindirect
jumps.
-Andi
next prev parent reply other threads:[~2015-12-13 18:15 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-10 15:39 [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86) Mathieu Desnoyers
[not found] ` <1449761990-23525-1-git-send-email-mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2015-12-10 15:39 ` [RFC PATCH 2/2] thread_local_abi: wire up ARM system call Mathieu Desnoyers
2015-12-10 16:27 ` Russell King - ARM Linux
[not found] ` <20151210162723.GN8644-l+eeeJia6m9vn6HldHNs0ANdhmdF6hFW@public.gmane.org>
2015-12-10 16:59 ` Mathieu Desnoyers
2015-12-11 18:56 ` [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86) Michael Kerrisk (man-pages)
2015-12-12 12:40 ` Mathieu Desnoyers
[not found] ` <1734518724.233994.1449924035932.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2015-12-13 8:04 ` Michael Kerrisk (man-pages)
2015-12-13 18:15 ` Andi Kleen [this message]
[not found] ` <20151213181527.GV15533-1g7Xle2YJi4/4alezvVtWx2eb7JE58TQ@public.gmane.org>
2015-12-13 19:58 ` Mathieu Desnoyers
[not found] ` <450134747.239045.1450036728930.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2015-12-13 20:18 ` Andi Kleen
[not found] ` <20151213201841.GW15533-1g7Xle2YJi4/4alezvVtWx2eb7JE58TQ@public.gmane.org>
2015-12-13 20:26 ` Andy Lutomirski
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