From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ben Maurer <bmaurer@fb.com>, Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Russell King <linux@arm.linux.org.uk>,
linux-api <linux-api@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Michael Kerrisk <mtk.manpages@gmail.com>,
Dave Watson <davejwatson@fb.com>, rostedt <rostedt@goodmis.org>,
Andy Lutomirski <luto@amacapital.net>,
Will Deacon <will.deacon@arm.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Chris Lameter <cl@linux.com>, Andi Kleen <andi@firstfloor.org>,
Josh Triplett <josh@joshtriplett.org>,
Paul Turner <pjt@google.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Andrew Hunter <ahh@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread
Date: Sun, 28 Feb 2016 14:32:28 +0000 (UTC) [thread overview]
Message-ID: <1538518747.10504.1456669948568.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CA+55aFwZY1BR5NdjAP-rytd29hE2EZEXgcaBKjepWFFj+oR1SA@mail.gmail.com>
----- On Feb 27, 2016, at 7:57 PM, Linus Torvalds torvalds@linux-foundation.org wrote:
> On Sat, Feb 27, 2016 at 4:39 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>>
>> I'm particularly interested to know what are the best practices to
>> deal with an extensible bitfield (the features mask). cpu_set_t
>> and sigmask each seem to do their own thing.
>
> Quite frankly, why would the kernel ever touch anything else?
>
> And if the kernel doesn't touch anything else, why make it part of the ABI?
>
> I don't see why the kernel would ever want to have a more complex
> interface. Explain.
The part of ABI I'm trying to express here is for discoverability
of available features by user-space. For instance, a kernel
could be configured with "CONFIG_RSEQ=n", and userspace should
not rely on the rseq fields of the thread-local ABI in that case.
The initial idea I had was to populate a mask of available features
(hence my question above), but now that I think about it, we could
perhaps have a "query" system call receiving a "feature number", no
mask needed then. E.g.:
enum thread_local_abi_features {
THREAD_LOCAL_FEATURE_CPU_ID = 0,
THREAD_LOCAL_FEATURE_RSEQ = 1,
/* Add future features here. */
};
int thread_local_abi_feature(uint64_t feature);
Another option would be to rely on specific "uninitialized"
values for each feature in struct thread_local_abi (e.g. -1
for cpu_id). We may need to reserve extra space for
"feature enabled" booleans in cases where the uninitialized
value is also used when initialized (e.g. a sequence counteR).
The advantage of using the uninitialized value and/or the
"boolean" within the struct thread_local_abi is that testing
whether the feature is active can be done by reading from
the same cache-line as when using the feature (in user-space).
Not sure what would be the best option here.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2016-02-28 14:32 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-23 23:28 [PATCH v4 0/5] getcpu_cache system call for 4.6 Mathieu Desnoyers
2016-02-23 23:28 ` [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread Mathieu Desnoyers
2016-02-24 11:11 ` Thomas Gleixner
2016-02-24 17:17 ` Mathieu Desnoyers
2016-02-25 23:32 ` Rasmus Villemoes
2016-02-26 17:47 ` Mathieu Desnoyers
2016-02-25 9:56 ` Peter Zijlstra
2016-02-25 16:55 ` Mathieu Desnoyers
2016-02-25 17:04 ` Peter Zijlstra
2016-02-25 17:17 ` Mathieu Desnoyers
2016-02-26 11:33 ` Peter Zijlstra
2016-02-26 16:29 ` Thomas Gleixner
2016-02-26 17:20 ` Mathieu Desnoyers
2016-02-26 18:01 ` Thomas Gleixner
2016-02-26 20:24 ` Mathieu Desnoyers
2016-02-26 23:04 ` H. Peter Anvin
2016-02-27 0:40 ` Mathieu Desnoyers
2016-02-27 6:24 ` H. Peter Anvin
2016-02-27 14:15 ` Mathieu Desnoyers
2016-02-27 14:58 ` Peter Zijlstra
2016-02-27 18:35 ` Linus Torvalds
2016-02-27 19:01 ` H. Peter Anvin
2016-02-27 23:53 ` Mathieu Desnoyers
[not found] ` <CA+55aFwcgwRxvVBz5kk_3O8dESXAGJ4KHBkf=pSXjiS7Xh4NwA@mail.gmail.com>
[not found] ` <1082926946.10326.1456619994590.JavaMail.zimbra@efficios.com>
2016-02-28 0:57 ` Linus Torvalds
2016-02-28 14:32 ` Mathieu Desnoyers [this message]
2016-02-29 10:35 ` Peter Zijlstra
2016-03-01 20:23 ` Mathieu Desnoyers
2016-03-01 21:32 ` Peter Zijlstra
2016-03-01 21:36 ` Peter Zijlstra
2016-03-01 21:47 ` H. Peter Anvin
2016-03-02 10:34 ` Peter Zijlstra
2016-02-29 10:32 ` Peter Zijlstra
2016-02-29 10:39 ` Arnd Bergmann
2016-02-29 12:41 ` Mathieu Desnoyers
2016-02-29 13:08 ` Arnd Bergmann
2016-02-29 18:19 ` H. Peter Anvin
2016-03-02 10:44 ` Geert Uytterhoeven
2016-03-01 18:25 ` H. Peter Anvin
2016-03-01 18:40 ` Mathieu Desnoyers
2016-02-28 13:07 ` Geert Uytterhoeven
2016-02-28 16:21 ` Linus Torvalds
2016-02-29 10:01 ` Peter Zijlstra
2016-02-27 15:04 ` H. Peter Anvin
2016-02-23 23:28 ` [PATCH v4 2/5] getcpu_cache: ARM resume notifier Mathieu Desnoyers
2016-02-23 23:28 ` [PATCH v4 3/5] getcpu_cache: wire up ARM system call Mathieu Desnoyers
2016-02-24 0:54 ` kbuild test robot
2016-02-24 1:05 ` [PATCH v4 (updated)] " Mathieu Desnoyers
2016-02-24 5:28 ` kbuild test robot
2016-02-24 6:54 ` kbuild test robot
2016-02-23 23:28 ` [PATCH v4 4/5] getcpu_cache: x86 32/64 resume notifier Mathieu Desnoyers
2016-02-23 23:28 ` [PATCH v4 5/5] getcpu_cache: wire up x86 32/64 system call Mathieu Desnoyers
2016-02-24 1:36 ` [PATCH v4 0/5] getcpu_cache system call for 4.6 H. Peter Anvin
2016-02-24 4:09 ` Mathieu Desnoyers
2016-02-24 20:07 ` H. Peter Anvin
2016-02-24 22:38 ` Mathieu Desnoyers
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