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Wed, 8 Jan 2025 17:15:22 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 0+P6B6qyfmcLAgAAD6G6ig (envelope-from ); Wed, 08 Jan 2025 17:15:22 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Thomas Huth Subject: Re: [PATCH 6/7] migration: Fix arrays of pointers in JSON writer In-Reply-To: References: <20250107195025.9951-1-farosas@suse.de> <20250107195025.9951-7-farosas@suse.de> <87cygx4dlt.fsf@suse.de> Date: Wed, 08 Jan 2025 14:15:19 -0300 Message-ID: <875xmp447s.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[3]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,suse.de:mid] Received-SPF: pass client-ip=195.135.223.130; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu writes: > On Wed, Jan 08, 2025 at 10:52:30AM -0300, Fabiano Rosas wrote: >> Peter Xu 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 >> >> --- >> >> 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. Yes, but all fields go into self.data of the VMSDFieldStruct, so self.data["css"] will increase beyond the size of the array. >> >> >> 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? It needs to append to, not overwrite the previous self.data[name] > What happens if you run this without the python part you > changed in this patch? The last nullptr overwrites everything else: "s390_css (14)": { "pending_crws": "00", "sei_pending": false, "do_crw_mchk": true, "crws_lost": false, "max_cssid": "0x00", "max_ssid": "0x00", "chnmon_active": false, "chnmon_area": "0x0000000000000000", --> "css": "nullptr", "default_cssid": "0xfe" },