Re: [Xen-devel] [PATCH v3 2/2] x86/smp: use APIC ALLBUT destination shorthand when possible

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.01.2020 10:52, Roger Pau Monne wrote:
> If the IPI destination mask matches the mask of online CPUs use the
> APIC ALLBUT destination shorthand in order to send an IPI to all CPUs
> on the system except the current one. This can only be safely used
> when no CPU hotplug or unplug operations are taking place, no
> offline CPUs or those have been onlined and parked, all CPUs in the
> system have been accounted for (ie: the number of CPUs doesn't exceed
> NR_CPUS and APIC IDs are below MAX_APICS) and there's no possibility
> of CPU hotplug (ie: no disabled CPUs have been reported by the
> firmware tables).
> 
> This is specially beneficial when using the PV shim, since using the
> shorthand avoids performing an APIC register write (or multiple ones
> if using xAPIC mode) for each destination when doing a global TLB
> flush.
> 
> The lock time of flush_lock on a 32 vCPU guest using the shim in
> x2APIC mode without the shorthand is:
> 
> Global lock flush_lock: addr=ffff82d0804b21c0, lockval=f602f602, not locked
>   lock:228455938(79406065573135), block:205908580(556416605761539)
> 
> Average lock time: 347577ns
> 
> While the same guest using the shorthand:
> 
> Global lock flush_lock: addr=ffff82d0804b41c0, lockval=d9c4d9bc, cpu=12
>   lock:1890775(416719148054), block:1663958(2500161282949)
> 
> Average lock time: 220395ns
> 
> Approximately a 1/3 improvement in the lock time.
> 
> Note that this requires locking the CPU maps (get_cpu_maps) which uses
> a trylock. This is currently safe as all users of cpu_add_remove_lock
> do a trylock, but will need reevaluating if non-trylock users appear.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>


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