Re: [Xen-devel] [PATCH v3 2/3] Introduce migration precopy policy

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Sep 25, 2017 at 07:55:34PM +0100, Jennifer Herbert wrote:
> This Patch allows a migration precopy policy to be specified.
> 
> The precopy phase of the xc_domain_save() live migration algorithm has
> historically been implemented to run until either a) (almost) no pages
> are dirty or b) some fixed, hard-coded maximum number of precopy
> iterations has been exceeded.  This policy and its implementation are
> less than ideal for a few reasons:
> - the logic of the policy is intertwined with the control flow of the
>   mechanism of the precopy stage
> - it can't take into account facts external to the immediate
>   migration context, such external state transfer state, interactive
>   user input, or the passage of wall-clock time.
> - it does not permit the user to change their mind, over time, about
>   what to do at the end of the precopy (they get an unconditional
>   transition into the stop-and-copy phase of the migration)
> 
> To permit callers to implement arbitrary higher-level policies governing
> when the live migration precopy phase should end, and what should be
> done next:
> - add a precopy_policy() callback to the xc_domain_save() user-supplied
>   callbacks
> - during the precopy phase of live migrations, consult this policy after
>   each batch of pages transmitted and take the dictated action, which
>   may be to a) abort the migration entirely, b) continue with the
>   precopy, or c) proceed to the stop-and-copy phase.
> - provide an implementation of the old policy, used when
>   precopy_policy callback  is not provided.
> 
> Signed-off-by: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>
> Signed-off-by: Joshua Otto <jtotto@xxxxxxxxxxxx>
> Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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