linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFzg0WN1_SzRagwwEDGUkKou4t+-gBNMUd-W3KM7tC+-AA@mail.gmail.com>
Date:	Thu, 18 Dec 2014 20:48:24 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Suresh Siddha <sbsiddha@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Anvin <hpa@...ux.intel.com>
Subject: Re: frequent lockups in 3.18rc4

On Thu, Dec 18, 2014 at 8:03 PM, Dave Jones <davej@...hat.com> wrote:
>
> So the only thing that was on that could cause spinlock overhead
> was DEBUG_SPINLOCK (and LOCK_STAT, though iirc that's not huge either)

So DEBUG_SPINLOCK does have one big downside if I recall correctly -
the debugging spinlocks are very much not fair. So they don't work
like the real ticket spinlocks. That might have serious effects on the
contention case, with some thread not making any progress due to just
the implementation of the debug spinlocks.

Peter, Ingo, maybe I'm full of crap on the debug spinlock thing., but
a quick look tells me thay are all built on top of the "trylock"
primitive that does indeed not queue anything, and is thus not fair.

I'm not sure why the debug spinlocks couldn't just be ticket locks
instead. But there you are - once more, the debug infrastructure is
actually much weaker and inferior to the "real" code.

                   Linus
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