Re: [Xen-devel] [PATCH] libxl: Restore scheduling parameters after migrate in best-effort fashion

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Oct 02, 2018 at 04:49:26PM +0100, George Dunlap wrote:
> Commit 3b4adba ("tools/libxl: include scheduler parameters in the
> output of xl list -l") added scheduling parameters to the set of
> information collected by libxl_retrieve_domain_configuration(), in
> order to report that information in `xl list -l`.
> 
> Unfortunately, libxl_retrieve_domain_configuration() is also called by
> the migration / save code, and the results passed to the restore /
> receive code.  This meant scheduler parameters were inadvertently
> added to the migration stream, without proper consideration for how to
> handle corner cases.  The result was that if migrating from a host
> running one scheduler to a host running a different scheduler, the
> migration would fail with an error like the following:
> 
> libxl: error: libxl_sched.c:232:sched_credit_domain_set: Domain 1:Getting 
> domain sched credit: Invalid argument
> libxl: error: libxl_create.c:1275:domcreate_rebuild_done: Domain 1:cannot 
> (re-)build domain: -3
> 
> Luckily there's a fairly straightforward way to set parameters in a
> "best-effort" fashion.  libxl provides a single struct containing the
> parameters of all schedulers, as well as a parameter specifying which
> scheduler.  Parameters not used by a given scheduler are ignored.
> Additionally, the struct contains a parameter to specify the
> scheduler.  If you specify a specific scheduler,
> libxl_domain_sched_params_set() will fail if there's a different
> scheduler.  However, if you pass LIBXL_SCHEDULER_UNKNOWN, it will use
> the value of the current scheduler for that domain.
> 
> In domcreate_stream_done(), before calling libxl__build_post(), set
> the scheduler to LIBXL_SCHEDULER_UNKNOWN.  This will propagate
> scheduler parameters from the previous instantiation on a best-effort
> basis.
> 
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> ---
> CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Dario Faggioli <dfaggioli@xxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> ---
>  tools/libxl/libxl_create.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> index dcfde7787e..caf79d4620 100644
> --- a/tools/libxl/libxl_create.c
> +++ b/tools/libxl/libxl_create.c
> @@ -1237,6 +1237,15 @@ static void domcreate_stream_done(libxl__egc *egc,
>          ret = ERROR_INVAL;
>          goto out;
>      }
> +
> +    /* 
> +     * The scheduler on the sending domain may be different than the
> +     * scheduler running here.  Setting the scheduler to UNKNOWN will
> +     * cause the code to take to take whatever parameters are
> +     * available in that scheduler, while discarding the rest.
> +     */
> +    info->sched_params.sched = LIBXL_SCHEDULER_UNKNOWN;

But this function is called by normal domain creation as well. What if
libxl user does set the parameter?

Wei.

> +    
>      ret = libxl__build_post(gc, domid, info, state, vments, localents);
>      if (ret)
>          goto out;
> -- 
> 2.19.0
> 

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