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Thu, 04 Dec 2025 01:37:16 -0800 (PST) X-Google-Smtp-Source: AGHT+IE0SY25UtrYt6eRQNsdWR6CCkzLqjAbaEqbaqRyMFSgqWbPz7KoJ7eSS4UdOxdcOm7BbVU54Q== X-Received: by 2002:a05:6000:1a8d:b0:429:c709:7b58 with SMTP id ffacd0b85a97d-42f7985c6afmr1914016f8f.50.1764841035945; Thu, 04 Dec 2025 01:37:15 -0800 (PST) Received: from [192.168.0.5] ([47.64.112.197]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42f7cbfeb38sm2158849f8f.12.2025.12.04.01.37.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 04 Dec 2025 01:37:15 -0800 (PST) Message-ID: <01700ae1-b5ef-43f2-af2b-2eb648c5a147@redhat.com> Date: Thu, 4 Dec 2025 10:37:13 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] target/s390x: Fix infinite loop during replay To: Ilya Leoshkevich , =?UTF-8?Q?Alex_Benn=C3=A9e?= Cc: qemu-s390x@nongnu.org, qemu-devel@nongnu.org, Pavel Dovgalyuk References: <20251201215514.1751994-1-iii@linux.ibm.com> From: Thomas Huth Content-Language: en-US Autocrypt: addr=thuth@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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 On 01/12/2025 22.49, Ilya Leoshkevich wrote: > Hi, > > Here is my attempt to fix [1] based on the discussion in [2]. > > I'm sending this as an RFC, because I have definitely misunderstood a > thing or two about record-replay, missed some timer bookkeeping > intricacies, and haven't split arch-dependent and independent parts > into different patches. > > This survives "make check" and "make check-tcg" with the test from [2], > both with and without extra load in background. > > Please let me know what you think about the approach. > > Best regards, > Ilya > > [1] https://lore.kernel.org/qemu-devel/a0accce9-6042-4a7b-a7c7-218212818891@redhat.com/ > [2] https://lore.kernel.org/qemu-devel/20251128133949.181828-1-thuth@redhat.com/ > > --- > > Replaying even trivial s390x kernels hangs, because: > > - cpu_post_load() fires the TOD timer immediately. > > - s390_tod_load() schedules work for firing the TOD timer. > > - If rr loop sees work and then timer, we get one timer expiration. > > - If rr loop sees timer and then work, we get two timer expirations. > > - Record and replay may diverge due to this race. > > - In this particular case divergence makes replay loop spin: it sees that > TOD timer has expired, but cannot invoke its callback, because there > is no recorded CHECKPOINT_CLOCK_VIRTUAL. > > - The order in which rr loop sees work and timer depends on whether > and when rr loop wakes up during load_snapshot(). > > - rr loop may wake up after the main thread kicks the CPU and drops > the BQL, which may happen if it calls, e.g., qemu_cond_wait_bql(). > > Firing TOD timer twice is duplicate work, but it was introduced > intentionally in commit 7c12f710bad6 ("s390x/tcg: rearm the CKC timer > during migration") in order to avoid dependency on migration order. > > The key culprits here are timers that are armed ready expired. They > break the ordering between timers and CPU work, because they are not > constrained by instruction execution, thus introducing non-determinism > and record-replay divergence. > > Fix by converting such timer callbacks to CPU work. Also add TOD clock > updates to the save path, mirroring the load path, in order to have the > same CHECKPOINT_CLOCK_VIRTUAL during recording and replaying. > > Signed-off-by: Ilya Leoshkevich > --- > hw/s390x/tod.c | 5 +++++ > stubs/async-run-on-cpu.c | 7 +++++++ > stubs/cpus-queue.c | 4 ++++ > stubs/meson.build | 2 ++ > target/s390x/machine.c | 4 ++++ > util/qemu-timer.c | 30 ++++++++++++++++++++++++++++++ > 6 files changed, 52 insertions(+) > create mode 100644 stubs/async-run-on-cpu.c > create mode 100644 stubs/cpus-queue.c Thanks, this indeed fixes the test for me, so: Tested-by: Thomas Huth