Re: [Xen-devel] [PATCH 4/4] xen: sched: simplify (and speedup) checking soft-affinity

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 09/15/2017 06:35 PM, Dario Faggioli wrote:
> The fact of whether or not a vCPU has a soft-affinity
> which is effective, i.e., with the power of actually
> affecting the scheduling of the vCPU itself rarely
> changes. Very, very rarely, as compared to how often
> we need to check for the same thing (basically, at
> every scheduling decision!).
> 
> That can be improved by storing in a (per-vCPU) flag
> (it's actually a boolean field in struct vcpu) whether
> or not, considering how hard-affinity and soft-affinity
> look like, soft-affinity should or not be taken into
> account during scheduling decisions.
> 
> This saves some cpumask manipulations, which is nice,
> considering how frequently they were being done. Note
> that we can't get rid of 100% of the cpumask operations
> involved in the check, because soft-affinity being
> effective or not, not only depends on the relationship
> between the hard and soft-affinity masks of a vCPU, but
> also of the online pCPUs and/or of what pCPUs are part
> of the cpupool where the vCPU lives, and that's rather
> impractical to store in a per-vCPU flag. Still the
> overhead is reduced to "just" one cpumask_subset() (and
> only if the newly introduced flag is 'true')!
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

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