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" > 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. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
