linux-kernel - Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of running thread

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Date:	Tue, 5 Jan 2016 17:34:49 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Paul Turner <pjt@...gle.com>,
	Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH 1/3] getcpu_cache system call: cache CPU number of
 running thread

----- On Jan 5, 2016, at 12:31 PM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:

> ----- On Jan 5, 2016, at 7:04 AM, Will Deacon will.deacon@....com wrote:
> 
>> Hi Mathieu,
>> 
>> On Tue, Jan 05, 2016 at 02:01:58AM -0500, Mathieu Desnoyers wrote:
>>> Expose a new system call allowing threads to register userspace memory
>>> areas where to store the CPU number on which the calling thread is
>>> running. Scheduler migration sets the TIF_NOTIFY_RESUME flag on the
>>> current thread. Upon return to user-space, a notify-resume handler
>>> updates the current CPU value within each registered user-space memory
>>> area. User-space can then read the current CPU number directly from
>>> memory.
>> 
>> What guarantees do you provide if a thread other than the one which
>> registered the cache tries to access the value? Obviously, there's a
>> potential data race here with the kernel issuing a parallel update, but
>> are you intending to have single-copy atomicity semantics (like relaxed
>> atomics in C11) or is this simply going to give you junk?
>> 
>> I ask because, in the absence of alignment checks on the cache pointer,
>> we can't guarantee single-copy atomicity on ARM when the kernel writes
>> the current CPU value.
> 
> Hi Will,
> 
> This is an excellent question. My initial thinking was that only the
> thread registering the cache would read it, but now that you ask,
> there might be use-cases where other threads would be interested in
> reading each other's current CPU number.
> 
> For instance, an application could create a linked list or hash map
> of thread control structures, which could contain the current CPU
> number of each thread. A dispatch thread could then traverse or
> lookup this structure to see on which CPU each thread is running and
> do work queue dispatch or scheduling decisions accordingly.
> 
> This use-case would imply ensuring that reading the current CPU value
> from another CPU will never result in reading a garbage value.
> 
> If we indeed intend to enable this use-case, we should:
> 
> 1) Add an alignment check on the cpu_cache pointer. Should we
>   return -EINVAL if unaligned ?
> 2) Document this alignment requirement in the man page, and the
>   atomicity guarantees it provides,

Related question: if we check that cpu_cache pointer is aligned
on 4 bytes, does put_user() then guarantee single-copy atomicity
on all architectures ?

Thanks,

Mathieu

> 
> The tiny downside of having this alignment requirement is that
> it would not be possible to put the cpu_cache into a packed
> structure. I don't think anyone would care though.
> 
> Thanks!
> 
> Mathieu
> 
> 
>> 
>> Cheers,
>> 
>> Will
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
--
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