Re: [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks with pv ticketlocks

[Stephan Diestelhorst <stephan.diestelhorst@amd.com>]

On Wednesday 21 March 2012, 13:49:28 Attilio Rao wrote:
> On 21/03/12 13:22, Stephan Diestelhorst wrote:
> > On Wednesday 21 March 2012, 13:04:25 Attilio Rao wrote:
> >    
> >> On 21/03/12 10:20, Raghavendra K T wrote:
> >>      
> >>> From: Jeremy Fitzhardinge<jeremy.fitzhardinge@citrix.com>
> >>>
> >>> Rather than outright replacing the entire spinlock implementation in
> >>> order to paravirtualize it, keep the ticket lock implementation but add
> >>> a couple of pvops hooks on the slow patch (long spin on lock, unlocking
> >>> a contended lock).
> >>>
> >>> Ticket locks have a number of nice properties, but they also have some
> >>> surprising behaviours in virtual environments.  They enforce a strict
> >>> FIFO ordering on cpus trying to take a lock; however, if the hypervisor
> >>> scheduler does not schedule the cpus in the correct order, the system can
> >>> waste a huge amount of time spinning until the next cpu can take the lock.
> >>>
> >>> (See Thomas Friebel's talk "Prevent Guests from Spinning Around"
> >>> http://www.xen.org/files/xensummitboston08/LHP.pdf  for more details.)
> >>>
> >>> To address this, we add two hooks:
> >>>    - __ticket_spin_lock which is called after the cpu has been
> >>>      spinning on the lock for a significant number of iterations but has
> >>>      failed to take the lock (presumably because the cpu holding the lock
> >>>      has been descheduled).  The lock_spinning pvop is expected to block
> >>>      the cpu until it has been kicked by the current lock holder.
> >>>    - __ticket_spin_unlock, which on releasing a contended lock
> >>>      (there are more cpus with tail tickets), it looks to see if the next
> >>>      cpu is blocked and wakes it if so.
> >>>
> >>> When compiled with CONFIG_PARAVIRT_SPINLOCKS disabled, a set of stub
> >>> functions causes all the extra code to go away.
> >>>
> >>>        
> >> I've made some real world benchmarks based on this serie of patches
> >> applied on top of a vanilla Linux-3.3-rc6 (commit
> >> 4704fe65e55fb088fbcb1dc0b15ff7cc8bff3685), with both
> >> CONFIG_PARAVIRT_SPINLOCK=y and n, which means essentially 4 versions
> >> compared:
> >> * vanilla - CONFIG_PARAVIRT_SPINLOCK - patch
> >> * vanilla + CONFIG_PARAVIRT_SPINLOCK - patch
> >> * vanilla - CONFIG_PARAVIRT_SPINLOCK + patch
> >> * vanilla + CONFIG_PARAVIRT_SPINLOCK + patch
> >>
> >>      
> > [...]
> >    
> >> == Results
> >> This test points in the direction that Jeremy's rebased patches don't
> >> introduce a peformance penalty at all, but also that we could likely
> >> consider CONFIG_PARAVIRT_SPINLOCK option removal, or turn it on by
> >> default and suggest disabling just on very old CPUs (assuming a
> >> performance regression can be proven there).
> >>      
> > Very interesting results, in particular knowing that in the one guest
> > case things do not get (significantly) slower due to the added logic
> > and LOCKed RMW in the unlock path.
> >
> > AFAICR, the problem really became apparent when running multiple guests
> > time sharing the physical CPUs, i.e., two guests with eight vCPUs each
> > on an eight core machine.  Did you look at this setup with your tests?
> >
> >    
> 
> Please note that my tests are made on native Linux, without XEN involvement.
> 
> You maybe meant that the spinlock paravirtualization became generally 
> useful in the case you mentioned? (2 guests, 8vpcu + 8vcpu)?

Yes, that is what I meant.  Just to clarify why you do not see any
speed-ups, and were wondering why.  If the whole point of the exercise
was to see that there are no perforamnce regressions, fine.  In that
case I misunderstood.

Stephan
-- 
Stephan Diestelhorst, AMD Operating System Research Center
stephan.diestelhorst@amd.com
Tel. +49 (0)351 448 356 719

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