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Tsirkin" , peterx@redhat.com, Alex Williamson , Jason Wang , Bandan Das , Prasad Pandit , Fabiano Rosas Subject: [PATCH 08/10] docs/migration: Organize "Postcopy" page Date: Tue, 9 Jan 2024 14:46:26 +0800 Message-ID: <20240109064628.595453-9-peterx@redhat.com> In-Reply-To: <20240109064628.595453-1-peterx@redhat.com> References: <20240109064628.595453-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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, DKIMWL_WL_HIGH=-2.243, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1704782905147100007 Content-Type: text/plain; charset="utf-8" From: Peter Xu Reorganize the page, moving things around, and add a few headlines ("Postcopy internals", "Postcopy features") to cover sub-areas. Signed-off-by: Peter Xu Reviewed-by: C=C3=A9dric Le Goater --- docs/devel/migration/postcopy.rst | 159 ++++++++++++++++-------------- 1 file changed, 84 insertions(+), 75 deletions(-) diff --git a/docs/devel/migration/postcopy.rst b/docs/devel/migration/postc= opy.rst index d60eec06ab..6c51e96d79 100644 --- a/docs/devel/migration/postcopy.rst +++ b/docs/devel/migration/postcopy.rst @@ -1,6 +1,9 @@ +=3D=3D=3D=3D=3D=3D=3D=3D Postcopy =3D=3D=3D=3D=3D=3D=3D=3D =20 +.. contents:: + 'Postcopy' migration is a way to deal with migrations that refuse to conve= rge (or take too long to converge) its plus side is that there is an upper bou= nd on the amount of migration traffic and time it takes, the down side is that d= uring @@ -14,7 +17,7 @@ Postcopy can be combined with precopy (i.e. normal migrat= ion) so that if precopy doesn't finish in a given time the switch is made to postcopy. =20 Enabling postcopy ------------------ +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =20 To enable postcopy, issue this command on the monitor (both source and destination) prior to the start of migration: @@ -49,8 +52,71 @@ time per vCPU. ``migrate_set_parameter`` is ignored (to avoid delaying requested pages = that the destination is waiting for). =20 -Postcopy device transfer ------------------------- +Postcopy internals +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +State machine +------------- + +Postcopy moves through a series of states (see postcopy_state) from +ADVISE->DISCARD->LISTEN->RUNNING->END + + - Advise + + Set at the start of migration if postcopy is enabled, even + if it hasn't had the start command; here the destination + checks that its OS has the support needed for postcopy, and performs + setup to ensure the RAM mappings are suitable for later postcopy. + The destination will fail early in migration at this point if the + required OS support is not present. + (Triggered by reception of POSTCOPY_ADVISE command) + + - Discard + + Entered on receipt of the first 'discard' command; prior to + the first Discard being performed, hugepages are switched off + (using madvise) to ensure that no new huge pages are created + during the postcopy phase, and to cause any huge pages that + have discards on them to be broken. + + - Listen + + The first command in the package, POSTCOPY_LISTEN, switches + the destination state to Listen, and starts a new thread + (the 'listen thread') which takes over the job of receiving + pages off the migration stream, while the main thread carries + on processing the blob. With this thread able to process page + reception, the destination now 'sensitises' the RAM to detect + any access to missing pages (on Linux using the 'userfault' + system). + + - Running + + POSTCOPY_RUN causes the destination to synchronise all + state and start the CPUs and IO devices running. The main + thread now finishes processing the migration package and + now carries on as it would for normal precopy migration + (although it can't do the cleanup it would do as it + finishes a normal migration). + + - Paused + + Postcopy can run into a paused state (normally on both sides when + happens), where all threads will be temporarily halted mostly due to + network errors. When reaching paused state, migration will make sure + the qemu binary on both sides maintain the data without corrupting + the VM. To continue the migration, the admin needs to fix the + migration channel using the QMP command 'migrate-recover' on the + destination node, then resume the migration using QMP command 'migrate' + again on source node, with resume=3Dtrue flag set. + + - End + + The listen thread can now quit, and perform the cleanup of migration + state, the migration is now complete. + +Device transfer +--------------- =20 Loading of device data may cause the device emulation to access guest RAM that may trigger faults that have to be resolved by the source, as such @@ -130,7 +196,20 @@ processing. is no longer used by migration, while the listen thread carries on serv= icing page data until the end of migration. =20 -Postcopy Recovery +Source side page bitmap +----------------------- + +The 'migration bitmap' in postcopy is basically the same as in the precopy, +where each of the bit to indicate that page is 'dirty' - i.e. needs +sending. During the precopy phase this is updated as the CPU dirties +pages, however during postcopy the CPUs are stopped and nothing should +dirty anything any more. Instead, dirty bits are cleared when the relevant +pages are sent during postcopy. + +Postcopy features +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Postcopy recovery ----------------- =20 Comparing to precopy, postcopy is special on error handlings. When any @@ -166,76 +245,6 @@ configurations of the guest. For example, when with a= sync page fault enabled, logically the guest can proactively schedule out the threads accessing missing pages. =20 -Postcopy states ---------------- - -Postcopy moves through a series of states (see postcopy_state) from -ADVISE->DISCARD->LISTEN->RUNNING->END - - - Advise - - Set at the start of migration if postcopy is enabled, even - if it hasn't had the start command; here the destination - checks that its OS has the support needed for postcopy, and performs - setup to ensure the RAM mappings are suitable for later postcopy. - The destination will fail early in migration at this point if the - required OS support is not present. - (Triggered by reception of POSTCOPY_ADVISE command) - - - Discard - - Entered on receipt of the first 'discard' command; prior to - the first Discard being performed, hugepages are switched off - (using madvise) to ensure that no new huge pages are created - during the postcopy phase, and to cause any huge pages that - have discards on them to be broken. - - - Listen - - The first command in the package, POSTCOPY_LISTEN, switches - the destination state to Listen, and starts a new thread - (the 'listen thread') which takes over the job of receiving - pages off the migration stream, while the main thread carries - on processing the blob. With this thread able to process page - reception, the destination now 'sensitises' the RAM to detect - any access to missing pages (on Linux using the 'userfault' - system). - - - Running - - POSTCOPY_RUN causes the destination to synchronise all - state and start the CPUs and IO devices running. The main - thread now finishes processing the migration package and - now carries on as it would for normal precopy migration - (although it can't do the cleanup it would do as it - finishes a normal migration). - - - Paused - - Postcopy can run into a paused state (normally on both sides when - happens), where all threads will be temporarily halted mostly due to - network errors. When reaching paused state, migration will make sure - the qemu binary on both sides maintain the data without corrupting - the VM. To continue the migration, the admin needs to fix the - migration channel using the QMP command 'migrate-recover' on the - destination node, then resume the migration using QMP command 'migrate' - again on source node, with resume=3Dtrue flag set. - - - End - - The listen thread can now quit, and perform the cleanup of migration - state, the migration is now complete. - -Source side page map --------------------- - -The 'migration bitmap' in postcopy is basically the same as in the precopy, -where each of the bit to indicate that page is 'dirty' - i.e. needs -sending. During the precopy phase this is updated as the CPU dirties -pages, however during postcopy the CPUs are stopped and nothing should -dirty anything any more. Instead, dirty bits are cleared when the relevant -pages are sent during postcopy. - Postcopy with hugepages ----------------------- =20 @@ -293,7 +302,7 @@ Retro-fitting postcopy to existing clients is possible: guest memory access is made while holding a lock then all other threads waiting for that lock will also be blocked. =20 -Postcopy Preemption Mode +Postcopy preemption mode ------------------------ =20 Postcopy preempt is a new capability introduced in 8.0 QEMU release, it --=20 2.41.0