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Mon, 21 Jul 2025 08:23:15 -0700 (PDT) X-Google-Smtp-Source: AGHT+IH4jpDBvB8ticrivjLowJPy7FbEU+blN23cr0WoEPnxY8sSdfnn3cHYyANAFCoWuFvzaB4h7w== X-Received: by 2002:a5d:6f0e:0:b0:3a6:d92d:9f7c with SMTP id ffacd0b85a97d-3b60e4c5212mr18160886f8f.9.1753111394637; Mon, 21 Jul 2025 08:23:14 -0700 (PDT) Received: from gmonaco-thinkpadt14gen3.rmtit.csb ([185.107.56.42]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b61ca2548fsm10797741f8f.7.2025.07.21.08.23.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 21 Jul 2025 08:23:14 -0700 (PDT) Message-ID: <78c60d531c8ec862a0fe5e6ef4a40c1f65fe6544.camel@redhat.com> Subject: Re: [PATCH v4 10/14] rv: Retry when da monitor detects race conditions From: Gabriele Monaco To: Nam Cao Cc: linux-kernel@vger.kernel.org, Steven Rostedt , Masami Hiramatsu , linux-trace-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Tomas Glozar , Juri Lelli , Clark Williams , John Kacur Date: Mon, 21 Jul 2025 17:23:12 +0200 In-Reply-To: <20250721150143.sl2MIKSi@linutronix.de> References: <20250721082325.71554-1-gmonaco@redhat.com> <20250721082325.71554-11-gmonaco@redhat.com> <20250721150143.sl2MIKSi@linutronix.de> Autocrypt: addr=gmonaco@redhat.com; prefer-encrypt=mutual; keydata=mDMEZuK5YxYJKwYBBAHaRw8BAQdAmJ3dM9Sz6/Hodu33Qrf8QH2bNeNbOikqYtxWFLVm0 1a0JEdhYnJpZWxlIE1vbmFjbyA8Z21vbmFjb0ByZWRoYXQuY29tPoiZBBMWCgBBFiEEysoR+AuB3R Zwp6j270psSVh4TfIFAmbiuWMCGwMFCQWjmoAFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AACgk Q70psSVh4TfJzZgD/TXjnqCyqaZH/Y2w+YVbvm93WX2eqBqiVZ6VEjTuGNs8A/iPrKbzdWC7AicnK xyhmqeUWOzFx5P43S1E1dhsrLWgP Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-1.fc42) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2025-07-21 at 17:01 +0200, Nam Cao wrote: > On Mon, Jul 21, 2025 at 10:23:20AM +0200, Gabriele Monaco wrote: > > DA monitor can be accessed from multiple cores simultaneously, this > > is > > likely, for instance when dealing with per-task monitors reacting > > on > > events that do not always occur on the CPU where the task is > > running. > > This can cause race conditions where two events change the next > > state > > and we see inconsistent values. E.g.: > >=20 > > =C2=A0 [62] event_srs: 27: sleepable x sched_wakeup -> running (final) > > =C2=A0 [63] event_srs: 27: sleepable x sched_set_state_sleepable -> > > sleepable > > =C2=A0 [63] error_srs: 27: event sched_switch_suspend not expected in > > the state running > >=20 > > In this case the monitor fails because the event on CPU 62 wins > > against > > the one on CPU 63, although the correct state should have been > > sleepable, since the task get suspended. > >=20 > > Detect if the current state was modified by using try_cmpxchg while > > storing the next value. If it was, try again reading the current > > state. > > After a maximum number of failed retries, react by calling a > > special > > tracepoint, print on the console and reset the monitor. > >=20 > > Remove the functions da_monitor_curr_state() and > > da_monitor_set_state() > > as they only hide the underlying implementation in this case. > >=20 > > Monitors where this type of condition can occur must be able to > > account > > for racing events in any possible order, as we cannot know the > > winner. > >=20 > > Cc: Ingo Molnar > > Cc: Peter Zijlstra > > Signed-off-by: Gabriele Monaco > > --- > >=20 > > =C2=A0static inline > > bool \ > > =C2=A0da_event_##name(struct da_monitor *da_mon, enum events_##name > > event) \ > > =C2=A0{ =09 > > \ > > - type curr_state =3D > > da_monitor_curr_state_##name(da_mon); \ > > - type next_state =3D model_get_next_state_##name(curr_state, > > event); \ > > - > > \ > > - if (next_state !=3D INVALID_STATE) > > { \ > > - da_monitor_set_state_##name(da_mon, > > next_state); \ > > - > > \ > > - > > trace_event_##name(model_get_state_name_##name(curr_state), \ > > - =C2=A0=C2=A0 > > model_get_event_name_##name(event), \ > > - =C2=A0=C2=A0 > > model_get_state_name_##name(next_state), \ > > - =C2=A0=C2=A0 > > model_is_final_state_##name(next_state)); \ > > - > > \ > > - return > > true; \ > > + enum states_##name curr_state, > > next_state; \ > > + =09 > > \ > > + curr_state =3D READ_ONCE(da_mon- > > >curr_state); \ > > + for (int i =3D 0; i < MAX_DA_RETRY_RACING_EVENTS; i++) > > { \ > > + next_state =3D > > model_get_next_state_##name(curr_state, event); \ > > + if (next_state =3D=3D INVALID_STATE) > > { \ > > + cond_react_##name(curr_state, > > event); \ > > + trace_error_##name(model_get_state_name_## > > name(curr_state), \ > > + =C2=A0=C2=A0 > > model_get_event_name_##name(event)); \ > > + return > > false; \ > > + } =09 > > \ > > + if (likely(try_cmpxchg(&da_mon->curr_state, > > &curr_state, next_state))) { \ > > + trace_event_##name(model_get_state_name_## > > name(curr_state), \ > > + =C2=A0=C2=A0 > > model_get_event_name_##name(event), \ > > + =C2=A0=C2=A0 > > model_get_state_name_##name(next_state), \ > > + =C2=A0=C2=A0 > > model_is_final_state_##name(next_state)); \ > > + return > > true; \ > > + } =09 > > \ > > =C2=A0 } =09 > > \ > > =C2=A0 =09 > > \ > > - cond_react_##name(curr_state, > > event); \ > > - > > \ > > - > > trace_error_##name(model_get_state_name_##name(curr_state), \ > > - =C2=A0=C2=A0 > > model_get_event_name_##name(event)); \ > > - > > \ > > + trace_rv_retries_error(#name, > > smp_processor_id()); \ > > + pr_warn("rv: " > > __stringify(MAX_DA_RETRY_RACING_EVENTS) \ > > + " retries reached, resetting monitor %s", > > #name); \ >=20 > smp_processor_id() requires preemption to be disabled. >=20 > At the moment, trace point handler is called with preemption > disabled, so > we are fine. But there is plan to change that: > https://lore.kernel.org/lkml/20241206120709.736f943e@gandalf.local.home/T= /#u >=20 > Perhaps use get_cpu() and put_cpu() instead? Mmh, then I'd need to execute them only if the tracepoint is enabled, I'm not sure it's worth the effort.. I wanted to avoid creating two different tracepoints (implicit and id), but I might have to. The CPU is rarely needed there since (for now) per-cpu monitors assume event cpu and monitor cpu are the same. I'll have a thought about it, thanks for pointing it out! Gabriele