From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Torsten Duwe <duwe@lst.de>
Cc: Tom Musta <tommusta@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
Anton Blanchard <anton@samba.org>,
Scott Wood <scottwood@freescale.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
linuxppc-dev@lists.ozlabs.org, Ingo Molnar <mingo@kernel.org>,
Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH v2] powerpc ticket locks
Date: Tue, 11 Feb 2014 13:44:20 +1100 [thread overview]
Message-ID: <1392086660.3996.50.camel@pasglop> (raw)
In-Reply-To: <20140210155217.GF2107@lst.de>
(Linus, Al, a question for you down there about lockref "ref" size)
On Mon, 2014-02-10 at 16:52 +0100, Torsten Duwe wrote:
> What if I squeeze the bits a little?
> 4k vCPUs, and 256 physical, as a limit to stay within 32 bits?
> At the cost that unlock may become an ll/sc operation again.
> I could think about a trick against that.
> But alas, hw_cpu_id is 16 bit, which makes a lookup table neccessary :-/
>
> Doing another round of yields for lockrefs now doesn't
> sound so bad any more.
So, the ticketpair has to be 16:16 so we can avoid the atomic on unlock
That leaves us with 32 bits to put the ref and the owner. The question
is how big the ref really has to be and can we have a reasonable failure
mode if it overflows ?
If we limit ourselves to, for example, 16-bit for the ref in lockref,
then we can have the second 32-bit split between the owner and the ref.
If we limit ourselves to 4k CPUs, then we get 4 more bits of ref ...
So the question is, is it reasonable to have the ref smaller than
32-bit...
Cheers,
Ben.
next prev parent reply other threads:[~2014-02-11 2:45 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-07 16:58 [PATCH v2] powerpc ticket locks Torsten Duwe
2014-02-07 17:12 ` Peter Zijlstra
2014-02-07 17:55 ` Torsten Duwe
2014-02-10 3:10 ` Benjamin Herrenschmidt
2014-02-10 15:52 ` Torsten Duwe
2014-02-10 17:53 ` Peter Zijlstra
2014-02-11 2:44 ` Benjamin Herrenschmidt [this message]
2014-02-11 2:56 ` Al Viro
2014-02-11 3:38 ` Benjamin Herrenschmidt
2014-02-11 9:53 ` Raghavendra KT
2014-02-11 10:40 ` Torsten Duwe
2014-02-11 18:30 ` Scott Wood
2014-02-11 19:34 ` Benjamin Herrenschmidt
2014-02-11 9:39 ` Raghavendra KT
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