Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks
From: Avi Kivity
Date: Mon Mar 26 2012 - 10:32:29 EST
On 03/21/2012 12:20 PM, Raghavendra K T wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> 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"
> 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.
error compiling committee.c: too many arguments to function
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