On Wed, Jan 08, 2025 at 10:52:30AM -0300, Fabiano Rosas wrote:
> 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},
> ...
> ]}
Yes something like this would work, thanks. We could even omit most of the
struct details but only show the important ones:
{"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",
... }, "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.
I think it will work no matter how it's ordered after your patch? IOW I'd
hope it'll keep working if the 1st is a nullptr:
{"name": "css", "type": "uint8", "size": 1},
{"name": "css", "type": "struct", "struct": {"vmsd_name": "s390_css_img",
... }, "size": 768},
{"name": "css", "array_len": 254, "type": "uint8", "size": 1},
Because IIUC the python script will parse each of the lines above into a
VMSD field.
>
> >> 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},
Correct.
I think that means for each of the break-down entries there'll be an
"index" if it's an array. What you changed above is the case where "index"
is not available, which is processing the non-array entry. Why does that
need change? What happens if you run this without the python part you
changed in this patch?
--
Peter Xu
