Peter Xu <pet...@redhat.com> writes:
> On Tue, Jan 07, 2025 at 04:50:24PM -0300, Fabiano Rosas wrote:
>> Currently, if an array of pointers contains a NULL pointer, that
>> pointer will be encoded as '0' in the stream. Since the JSON writer
>> doesn't define a "pointer" type, that '0' will now be an uint64, which
>> is different from the original type being pointed to, e.g. struct.
>>
>> That mixed-type array shouldn't be compressed, otherwise data is lost
>> as the code currently makes the whole array have the type of the first
>> element.
>>
>> While we could disable the array compression when a NULL pointer is
>> found, the JSON part of the stream still makes part of downtime, so we
>> should avoid writing unecessary bytes to it.
>>
>> Keep the array compression in place, but break the array into several
>> type-contiguous pieces if NULL and non-NULL pointers are mixed.
>
> Could I request for a sample JSON dump for an example array in the commit
> log? This whole solution looks working but is tricky. A sample could help
> people understand (e.g. showing the same "name" being dumped multiple
> times..).
{"name": "s390_css", "instance_id": 0, "vmsd_name": "s390_css",
"version": 1, "fields": [
...,
{"name": "css", "array_len": 254, "type": "uint8", "size": 1},
{"name": "css", "type": "struct", "struct": {
"vmsd_name": "s390_css_img", "version": 1, "fields": [{"name":
"chpids", "array_len": 256, "type": "struct", "struct": {"vmsd_name":
"s390_chp_info", "version": 1, "fields": [{"name": "in_use", "type":
"uint8", "size": 1}, {"name": "type", "type": "uint8", "size": 1},
{"name": "is_virtual", "type": "uint8", "size": 1}]}, "size": 3}]},
"size": 768},
{"name": "css", "type": "uint8", "size": 1},
...
]}
>
> Side note: I tried to dump a very basic VM's JSON out to disk, it scares me
> on the size:
>
> $ ls -lhS JSON.out
> -rw-r--r--. 1 peterx peterx 106K Jan 7 17:18 JSON.out
>
> That's a simplest VM with all default stuff, mostly nothing complex.. I may
> really need to measure how the JSON debug strings affect migration function
> or perf at some point..
>
Agreed.
>>
>> Signed-off-by: Fabiano Rosas <faro...@suse.de>
>> ---
>> migration/vmstate.c | 33 ++++++++++++++++++++++++++++++++-
>> scripts/analyze-migration.py | 9 ++++++++-
>> 2 files changed, 40 insertions(+), 2 deletions(-)
>>
>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>> index 52704c822c..a79ccf3875 100644
>> --- a/migration/vmstate.c
>> +++ b/migration/vmstate.c
>> @@ -425,15 +425,19 @@ int vmstate_save_state_v(QEMUFile *f, const
>> VMStateDescription *vmsd,
>> int size = vmstate_size(opaque, field);
>> uint64_t old_offset, written_bytes;
>> JSONWriter *vmdesc_loop = vmdesc;
>> + bool is_prev_null = false;
>>
>> trace_vmstate_save_state_loop(vmsd->name, field->name, n_elems);
>> if (field->flags & VMS_POINTER) {
>> first_elem = *(void **)first_elem;
>> assert(first_elem || !n_elems || !size);
>> }
>> +
>> for (i = 0; i < n_elems; i++) {
>> void *curr_elem = first_elem + size * i;
>> const VMStateField *inner_field;
>> + bool is_null;
>> + int max_elems = n_elems - i;
>>
>> old_offset = qemu_file_transferred(f);
>> if (field->flags & VMS_ARRAY_OF_POINTER) {
>> @@ -448,12 +452,39 @@ int vmstate_save_state_v(QEMUFile *f, const
>> VMStateDescription *vmsd,
>> * not follow.
>> */
>> inner_field = vmsd_create_fake_nullptr_field(field);
>> + is_null = true;
>> } else {
>> inner_field = field;
>> + is_null = false;
>> + }
>> +
>> + /*
>> + * Due to the fake nullptr handling above, if there's mixed
>> + * null/non-null data, it doesn't make sense to emit a
>> + * compressed array representation spanning the entire array
>> + * because the field types will be different (e.g. struct
>> + * vs. uint64_t). Search ahead for the next null/non-null
>> + * element and start a new compressed array if found.
>> + */
>> + if (field->flags & VMS_ARRAY_OF_POINTER &&
>> + is_null != is_prev_null) {
>> +
>> + is_prev_null = is_null;
>> + vmdesc_loop = vmdesc;
>> +
>> + for (int j = i + 1; j < n_elems; j++) {
>> + void *elem = *(void **)(first_elem + size * j);
>> + bool elem_is_null = !elem && size;
>> +
>> + if (is_null != elem_is_null) {
>> + max_elems = j - i;
>> + break;
>> + }
>> + }
>> }
>>
>> vmsd_desc_field_start(vmsd, vmdesc_loop, inner_field,
>> - i, n_elems);
>> + i, max_elems);
>>
>> if (inner_field->flags & VMS_STRUCT) {
>> ret = vmstate_save_state(f, inner_field->vmsd,
>> diff --git a/scripts/analyze-migration.py b/scripts/analyze-migration.py
>> index 4836920ddc..9138e91a11 100755
>> --- a/scripts/analyze-migration.py
>> +++ b/scripts/analyze-migration.py
>> @@ -497,7 +497,14 @@ def read(self):
>> raise Exception("internal index of data field unmatched
>> (%d/%d)" % (len(a), int(field['index'])))
>> a.append(field['data'])
There's actually a bug here, the code above does:
if len(a) != int(field['index']):
raise Exception()
Which only works with this patch because the compressed array happens to
come first.
>> else:
>> - self.data[field['name']] = field['data']
>> + # There could be multiple entries for the same field
>> + # name, e.g. when a compressed array was broken in
>> + # more than one piece.
>> + if (field['name'] in self.data and
>> + type(self.data[field['name']]) == list):
>> + self.data[field['name']].append(field['data'])
>> + else:
>> + self.data[field['name']] = field['data']
>
> Do we realy need these script changes? I thought VMSDFieldStruct always
> breaks array_len field into "index" based anyway?
>
> new_fields = []
> for field in self.desc['struct']['fields']:
> if not 'array_len' in field:
> new_fields.append(field)
> continue
> array_len = field.pop('array_len')
> field['index'] = 0
> new_fields.append(field)
> for i in range(1, array_len):
> c = field.copy()
> c['index'] = i
> new_fields.append(c)
>
> self.desc['struct']['fields'] = new_fields
This code is about decompressing the array, it doesn't handle multiple
entries with the same name. See the JSON I posted up there.
This makes the single:
{"name": "css", "array_len": 254, "type": "uint8", "size": 1},
become multiple:
{"name": "css", "index": 0, "type": "uint8", "size": 1},
{"name": "css", "index": 1, "type": "uint8", "size": 1},
...
{"name": "css", "index": 253, "type": "uint8", "size": 1},
