Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

[Avi Kivity <avi@redhat.com>]

On 03/21/2012 12:20 PM, Raghavendra K T wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
>
> Changes since last posting: (Raghavendra K T)
> [
>  - Rebased to linux-3.3-rc6.
>  - used function+enum in place of macro (better type checking) 
>  - use cmpxchg while resetting zero status for possible race
> 	[suggested by Dave Hansen for KVM patches ]
> ]
>
> This series replaces the existing paravirtualized spinlock mechanism
> with a paravirtualized ticketlock mechanism.
>
> Ticket locks have an inherent problem in a virtualized case, because
> the vCPUs are scheduled rather than running concurrently (ignoring
> gang scheduled vCPUs).  This can result in catastrophic performance
> collapses when the vCPU scheduler doesn't schedule the correct "next"
> vCPU, and ends up scheduling a vCPU which burns its entire timeslice
> spinning.  (Note that this is not the same problem as lock-holder
> preemption, which this series also addresses; that's also a problem,
> but not catastrophic).
>
> (See Thomas Friebel's talk "Prevent Guests from Spinning Around"
> http://www.xen.org/files/xensummitboston08/LHP.pdf for more details.)
>
> Currently we deal with this by having PV spinlocks, which adds a layer
> of indirection in front of all the spinlock functions, and defining a
> completely new implementation for Xen (and for other pvops users, but
> there are none at present).
>
> PV ticketlocks keeps the existing ticketlock implemenentation
> (fastpath) as-is, but adds a couple of pvops for the slow paths:
>
> - If a CPU has been waiting for a spinlock for SPIN_THRESHOLD
>   iterations, then call out to the __ticket_lock_spinning() pvop,
>   which allows a backend to block the vCPU rather than spinning.  This
>   pvop can set the lock into "slowpath state".
>
> - When releasing a lock, if it is in "slowpath state", the call
>   __ticket_unlock_kick() to kick the next vCPU in line awake.  If the
>   lock is no longer in contention, it also clears the slowpath flag.
>
> The "slowpath state" is stored in the LSB of the within the lock tail
> ticket.  This has the effect of reducing the max number of CPUs by
> half (so, a "small ticket" can deal with 128 CPUs, and "large ticket"
> 32768).
>
> This series provides a Xen implementation, but it should be
> straightforward to add a KVM implementation as well.
>

Looks like a good baseline on which to build the KVM implementation.  We
might need some handshake to prevent interference on the host side with
the PLE code.

-- 
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