linux-kernel - Re: [PATCH v16 08/14] pvqspinlock: Implement simple paravirt support for the qspinlock

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Message-ID: <5547A948.5050406@hp.com>
Date:	Mon, 04 May 2015 13:15:52 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v16 08/14] pvqspinlock: Implement simple paravirt support
 for the qspinlock

On 05/04/2015 10:20 AM, Peter Zijlstra wrote:
> I changed it to the below; I've not gotten around to compiling or even
> running it yet :-(
>
> The biggest change is the pv_hash/pv_unhash functions, which I've
> rewritten to hopefully be clearer (and also hopefully not wrecked them).
> I took out the cacheline sized structure which takes out that double
> loop and simplifies things. I've also added some comments which
> hopefully explain how/why we ended up with this exact scheme.
>
> I've also moved the __pv_queue_spin_unlock() function to the tail, such
> that we keep the 'wait'/'kick' order for both node and head.
>
> In any case, like I just wrote on the other email, I've stuck some
> things in my queue (up to and including patch 11) and if it all works
> out we can continue from there.
>
> ---
> Subject: pvqspinlock: Implement simple paravirt support for the qspinlock
> From: Waiman Long<Waiman.Long@...com>
> Date: Fri, 24 Apr 2015 14:56:37 -0400
>
> Provide a separate (second) version of the spin_lock_slowpath for
> paravirt along with a special unlock path.
>
> The second slowpath is generated by adding a few pv hooks to the
> normal slowpath, but where those will compile away for the native
> case, they expand into special wait/wake code for the pv version.
>
> The actual MCS queue can use extra storage in the mcs_nodes[] array to
> keep track of state and therefore uses directed wakeups.
>
> The head contender has no such storage directly visible to the
> unlocker.  So the unlocker searches a hash table with open addressing
> using a simple binary Galois linear feedback shift register.

I am fine with the change as it makes it simpler. BTW, I just saw a 
build error mail. That should be fixed easily with some minor edit.

Cheers,
Longman

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