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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 10/09/2018 04:12 PM, Wei Liu wrote:
> 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?

Is it?  I thought the `stream` meant it was only called when doing
migrate / resume.

 -George

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