On Thu, Jul 11, 2024 at 02:52:35PM -0700, Yichen Wang wrote:
> v5
> * Rebase on top of 39a032cea23e522268519d89bb738974bc43b6f6.
> * Rename struct definitions with typedef and CamelCase names;
> * Add build and runtime checks about DSA accelerator;
> * Address all comments from v4 reviews about typos, licenses, comments,
> error reporting, etc.
>
> v4
> * Rebase on top of 85b597413d4370cb168f711192eaef2eb70535ac.
> * A separate "multifd zero page checking" patchset was split from this
> patchset's v3 and got merged into master. v4 re-applied the rest of all
> commits on top of that patchset, re-factored and re-tested.
> https://lore.kernel.org/all/20240311180015.3359271-1-hao.xi...@linux.dev/
> * There are some feedback from v3 I likely overlooked.
>
> v3
> * Rebase on top of 7425b6277f12e82952cede1f531bfc689bf77fb1.
> * Fix error/warning from checkpatch.pl
> * Fix use-after-free bug when multifd-dsa-accel option is not set.
> * Handle error from dsa_init and correctly propogate the error.
> * Remove unnecessary call to dsa_stop.
> * Detect availability of DSA feature at compile time.
> * Implement a generic batch_task structure and a DSA specific one
> dsa_batch_task.
> * Remove all exit() calls and propagate errors correctly.
> * Use bytes instead of page count to configure multifd-packet-size option.
>
> v2
> * Rebase on top of 3e01f1147a16ca566694b97eafc941d62fa1e8d8.
> * Leave Juan's changes in their original form instead of squashing them.
> * Add a new commit to refactor the multifd_send_thread function to prepare
> for introducing the DSA offload functionality.
> * Use page count to configure multifd-packet-size option.
> * Don't use the FLAKY flag in DSA tests.
> * Test if DSA integration test is setup correctly and skip the test if
> * not.
> * Fixed broken link in the previous patch cover.
>
> * Background:
The DSA interface here is extremely low level: it would mean we add a
ton of complex fragile code for any new accelerator.
Please add something high level and simple on top of this.
Off the top of my head:
void start_memcmp(void *a, void *b, int cnt, void *opaque,
void (*callback)(int result, void *a, void *b, int cnt, void *opaque)
);
Do all the batching hacks internally.
> I posted an RFC about DSA offloading in QEMU:
> https://patchew.org/QEMU/20230529182001.2232069-1-hao.xi...@bytedance.com/
>
> This patchset implements the DSA offloading on zero page checking in
> multifd live migration code path.
>
> * Overview:
>
> Intel Data Streaming Accelerator(DSA) is introduced in Intel's 4th generation
> Xeon server, aka Sapphire Rapids.
> https://cdrdv2-public.intel.com/671116/341204-intel-data-streaming-accelerator-spec.pdf
> https://www.intel.com/content/www/us/en/content-details/759709/intel-data-streaming-accelerator-user-guide.html
> One of the things DSA can do is to offload memory comparison workload from
> CPU to DSA accelerator hardware. This patchset implements a solution to
> offload
> QEMU's zero page checking from CPU to DSA accelerator hardware. We gain
> two benefits from this change:
> 1. Reduces CPU usage in multifd live migration workflow across all use
> cases.
> 2. Reduces migration total time in some use cases.
>
> * Design:
>
> These are the logical steps to perform DSA offloading:
> 1. Configure DSA accelerators and create user space openable DSA work
> queues via the idxd driver.
> 2. Map DSA's work queue into a user space address space.
> 3. Fill an in-memory task descriptor to describe the memory operation.
> 4. Use dedicated CPU instruction _enqcmd to queue a task descriptor to
> the work queue.
> 5. Pull the task descriptor's completion status field until the task
> completes.
> 6. Check return status.
>
> The memory operation is now totally done by the accelerator hardware but
> the new workflow introduces overheads. The overhead is the extra cost CPU
> prepares and submits the task descriptors and the extra cost CPU pulls for
> completion. The design is around minimizing these two overheads.
>
> 1. In order to reduce the overhead on task preparation and submission,
> we use batch descriptors. A batch descriptor will contain N individual
> zero page checking tasks where the default N is 128 (default packet size
> / page size) and we can increase N by setting the packet size via a new
> migration option.
> 2. The multifd sender threads prepares and submits batch tasks to DSA
> hardware and it waits on a synchronization object for task completion.
> Whenever a DSA task is submitted, the task structure is added to a
> thread safe queue. It's safe to have multiple multifd sender threads to
> submit tasks concurrently.
> 3. Multiple DSA hardware devices can be used. During multifd initialization,
> every sender thread will be assigned a DSA device to work with. We
> use a round-robin scheme to evenly distribute the work across all used
> DSA devices.
> 4. Use a dedicated thread dsa_completion to perform busy pulling for all
> DSA task completions. The thread keeps dequeuing DSA tasks from the
> thread safe queue. The thread blocks when there is no outstanding DSA
> task. When pulling for completion of a DSA task, the thread uses CPU
> instruction _mm_pause between the iterations of a busy loop to save some
> CPU power as well as optimizing core resources for the other hypercore.
> 5. DSA accelerator can encounter errors. The most popular error is a
> page fault. We have tested using devices to handle page faults but
> performance is bad. Right now, if DSA hits a page fault, we fallback to
> use CPU to complete the rest of the work. The CPU fallback is done in
> the multifd sender thread.
> 6. Added a new migration option multifd-dsa-accel to set the DSA device
> path. If set, the multifd workflow will leverage the DSA devices for
> offloading.
> 7. Added a new migration option multifd-normal-page-ratio to make
> multifd live migration easier to test. Setting a normal page ratio will
> make live migration recognize a zero page as a normal page and send
> the entire payload over the network. If we want to send a large network
> payload and analyze throughput, this option is useful.
> 8. Added a new migration option multifd-packet-size. This can increase
> the number of pages being zero page checked and sent over the network.
> The extra synchronization between the sender threads and the dsa
> completion thread is an overhead. Using a large packet size can reduce
> that overhead.
>
> * Performance:
>
> We use two Intel 4th generation Xeon servers for testing.
>
> Architecture: x86_64
> CPU(s): 192
> Thread(s) per core: 2
> Core(s) per socket: 48
> Socket(s): 2
> NUMA node(s): 2
> Vendor ID: GenuineIntel
> CPU family: 6
> Model: 143
> Model name: Intel(R) Xeon(R) Platinum 8457C
> Stepping: 8
> CPU MHz: 2538.624
> CPU max MHz: 3800.0000
> CPU min MHz: 800.0000
>
> We perform multifd live migration with below setup:
> 1. VM has 100GB memory.
> 2. Use the new migration option multifd-set-normal-page-ratio to control the
> total
> size of the payload sent over the network.
> 3. Use 8 multifd channels.
> 4. Use tcp for live migration.
> 4. Use CPU to perform zero page checking as the baseline.
> 5. Use one DSA device to offload zero page checking to compare with the
> baseline.
> 6. Use "perf sched record" and "perf sched timehist" to analyze CPU usage.
>
> A) Scenario 1: 50% (50GB) normal pages on an 100GB vm.
>
> CPU usage
>
> |---------------|---------------|---------------|---------------|
> | |comm |runtime(msec) |totaltime(msec)|
> |---------------|---------------|---------------|---------------|
> |Baseline |live_migration |5657.58 | |
> | |multifdsend_0 |3931.563 | |
> | |multifdsend_1 |4405.273 | |
> | |multifdsend_2 |3941.968 | |
> | |multifdsend_3 |5032.975 | |
> | |multifdsend_4 |4533.865 | |
> | |multifdsend_5 |4530.461 | |
> | |multifdsend_6 |5171.916 | |
> | |multifdsend_7 |4722.769 |41922 |
> |---------------|---------------|---------------|---------------|
> |DSA |live_migration |6129.168 | |
> | |multifdsend_0 |2954.717 | |
> | |multifdsend_1 |2766.359 | |
> | |multifdsend_2 |2853.519 | |
> | |multifdsend_3 |2740.717 | |
> | |multifdsend_4 |2824.169 | |
> | |multifdsend_5 |2966.908 | |
> | |multifdsend_6 |2611.137 | |
> | |multifdsend_7 |3114.732 | |
> | |dsa_completion |3612.564 |32568 |
> |---------------|---------------|---------------|---------------|
>
> Baseline total runtime is calculated by adding up all multifdsend_X
> and live_migration threads runtime. DSA offloading total runtime is
> calculated by adding up all multifdsend_X, live_migration and
> dsa_completion threads runtime. 41922 msec VS 32568 msec runtime and
> that is 23% total CPU usage savings.
>
> Latency
>
> |---------------|---------------|---------------|---------------|---------------|---------------|
> | |total time |down time |throughput
> |transferred-ram|total-ram |
>
> |---------------|---------------|---------------|---------------|---------------|---------------|
>
> |Baseline |10343 ms |161 ms |41007.00 mbps
> |51583797 kb |102400520 kb |
>
> |---------------|---------------|---------------|---------------|-------------------------------|
> |DSA offload |9535 ms |135 ms |46554.40 mbps
> |53947545 kb |102400520 kb |
>
> |---------------|---------------|---------------|---------------|---------------|---------------|
>
> Total time is 8% faster and down time is 16% faster.
>
> B) Scenario 2: 100% (100GB) zero pages on an 100GB vm.
>
> CPU usage
> |---------------|---------------|---------------|---------------|
> | |comm |runtime(msec) |totaltime(msec)|
> |---------------|---------------|---------------|---------------|
> |Baseline |live_migration |4860.718 | |
> | |multifdsend_0 |748.875 | |
> | |multifdsend_1 |898.498 | |
> | |multifdsend_2 |787.456 | |
> | |multifdsend_3 |764.537 | |
> | |multifdsend_4 |785.687 | |
> | |multifdsend_5 |756.941 | |
> | |multifdsend_6 |774.084 | |
> | |multifdsend_7 |782.900 |11154 |
> |---------------|---------------|-------------------------------|
> |DSA offloading |live_migration |3846.976 | |
> | |multifdsend_0 |191.880 | |
> | |multifdsend_1 |166.331 | |
> | |multifdsend_2 |168.528 | |
> | |multifdsend_3 |197.831 | |
> | |multifdsend_4 |169.580 | |
> | |multifdsend_5 |167.984 | |
> | |multifdsend_6 |198.042 | |
> | |multifdsend_7 |170.624 | |
> | |dsa_completion |3428.669 |8700 |
> |---------------|---------------|---------------|---------------|
>
> Baseline total runtime is 11154 msec and DSA offloading total runtime is
> 8700 msec. That is 22% CPU savings.
>
> Latency
>
> |--------------------------------------------------------------------------------------------|
> | |total time |down time |throughput
> |transferred-ram|total-ram |
>
> |---------------|---------------|---------------|---------------|---------------|------------|
>
> |Baseline |4867 ms |20 ms |1.51 mbps |565 kb
> |102400520 kb|
>
> |---------------|---------------|---------------|---------------|----------------------------|
> |DSA offload |3888 ms |18 ms |1.89 mbps |565 kb
> |102400520 kb|
>
> |---------------|---------------|---------------|---------------|---------------|------------|
>
> Total time 20% faster and down time 10% faster.
>
> * Testing:
>
> 1. Added unit tests for cover the added code path in dsa.c
> 2. Added integration tests to cover multifd live migration using DSA
> offloading.
>
> Hao Xiang (12):
> meson: Introduce new instruction set enqcmd to the build system.
> util/dsa: Implement DSA device start and stop logic.
> util/dsa: Implement DSA task enqueue and dequeue.
> util/dsa: Implement DSA task asynchronous completion thread model.
> util/dsa: Implement zero page checking in DSA task.
> util/dsa: Implement DSA task asynchronous submission and wait for
> completion.
> migration/multifd: Add new migration option for multifd DSA
> offloading.
> migration/multifd: Prepare to introduce DSA acceleration on the
> multifd path.
> migration/multifd: Enable DSA offloading in multifd sender path.
> migration/multifd: Add migration option set packet size.
> util/dsa: Add unit test coverage for Intel DSA task submission and
> completion.
> migration/multifd: Add integration tests for multifd with Intel DSA
> offloading.
>
> Yichen Wang (1):
> util/dsa: Add idxd into linux header copy list.
>
> include/qemu/dsa.h | 176 +++++
> meson.build | 14 +
> meson_options.txt | 2 +
> migration/migration-hmp-cmds.c | 22 +-
> migration/migration.c | 2 +-
> migration/multifd-zero-page.c | 100 ++-
> migration/multifd-zlib.c | 6 +-
> migration/multifd-zstd.c | 6 +-
> migration/multifd.c | 53 +-
> migration/multifd.h | 8 +-
> migration/options.c | 85 +++
> migration/options.h | 2 +
> qapi/migration.json | 49 +-
> scripts/meson-buildoptions.sh | 3 +
> scripts/update-linux-headers.sh | 2 +-
> tests/qtest/migration-test.c | 80 ++-
> tests/unit/meson.build | 6 +
> tests/unit/test-dsa.c | 503 ++++++++++++++
> util/dsa.c | 1082 +++++++++++++++++++++++++++++++
> util/meson.build | 3 +
> 20 files changed, 2177 insertions(+), 27 deletions(-)
> create mode 100644 include/qemu/dsa.h
> create mode 100644 tests/unit/test-dsa.c
> create mode 100644 util/dsa.c
>
> --
> Yichen Wang
