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[212.8.243.115]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c1c32a7986bsm315726666b.7.2026.07.24.00.20.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Jul 2026 00:20:46 -0700 (PDT) Message-ID: Subject: Re: [RFC PATCH] Documentation/rv: Explain epoll and aborted sleeps From: Gabriele Monaco To: Chao Liu , Nam Cao , Steven Rostedt , Jonathan Corbet Cc: lianux.mm@gmail.com, lianux.wang@processmission.com, Shuah Khan , linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 24 Jul 2026 09:20:44 +0200 In-Reply-To: <20260722045706.85633-1-chao.liu@processmission.com> References: <20260722045706.85633-1-chao.liu@processmission.com> Autocrypt: addr=gmonaco@redhat.com; prefer-encrypt=mutual; keydata=mDMEZuK5YxYJKwYBBAHaRw8BAQdAmJ3dM9Sz6/Hodu33Qrf8QH2bNeNbOikqYtxWFLVm0 1a0JEdhYnJpZWxlIE1vbmFjbyA8Z21vbmFjb0BrZXJuZWwub3JnPoiZBBMWCgBBFiEEysoR+AuB3R Zwp6j270psSVh4TfIFAmjKX2MCGwMFCQWjmoAFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AACgk Q70psSVh4TfIQuAD+JulczTN6l7oJjyroySU55Fbjdvo52xiYYlMjPG7dCTsBAMFI7dSL5zg98I+8 cXY1J7kyNsY6/dcipqBM4RMaxXsOtCRHYWJyaWVsZSBNb25hY28gPGdtb25hY29AcmVkaGF0LmNvb T6InAQTFgoARAIbAwUJBaOagAULCQgHAgIiAgYVCgkICwIEFgIDAQIeBwIXgBYhBMrKEfgLgd0WcK eo9u9KbElYeE3yBQJoymCyAhkBAAoJEO9KbElYeE3yjX4BAJ/ETNnlHn8OjZPT77xGmal9kbT1bC1 7DfrYVISWV2Y1AP9HdAMhWNAvtCtN2S1beYjNybuK6IzWYcFfeOV+OBWRDQ== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Wed, 2026-07-22 at 12:57 +0800, Chao Liu wrote: > The rtapp sleep monitor accepts epoll_wait() as a valid sleeping > reason, but the prose only discusses clock_nanosleep and futexes. Add > epoll_wait to the list of valid sleeping reasons. >=20 > ABORT_SLEEP represents a task restoring TASK_RUNNING before entering > the scheduler, but its meaning is not described. Explain why the > aborted sleep attempt is valid. >=20 > This RFC is based on Nam Cao's pending "rv: rtapp monitor update" v2 > series: >=20 > https://lore.kernel.org/r/cover.1781852967.git.namcao@linutronix.de >=20 > Signed-off-by: Chao Liu > --- Thanks for the contribution! I have a couple of comments, we want to be assertive in this docs: in general, let's not say something like "this is v= alid because the monitor thinks it's valid" but "this is valid because it is rt-= safe for reason X". > =C2=A0Documentation/trace/rv/monitor_rtapp.rst | 6 ++++++ > =C2=A01 file changed, 6 insertions(+) >=20 > diff --git a/Documentation/trace/rv/monitor_rtapp.rst > b/Documentation/trace/rv/monitor_rtapp.rst > index 238b59395ff5..234ed4d581ac 100644 > --- a/Documentation/trace/rv/monitor_rtapp.rst > +++ b/Documentation/trace/rv/monitor_rtapp.rst > @@ -67,6 +67,8 @@ thread to sleep for one of the following reasons: > =C2=A0=C2=A0=C2=A0=C2=A0 variables as safe for real-time. As an alternati= ve, the librtpi library > =C2=A0=C2=A0=C2=A0=C2=A0 exists to provide a conditional variable impleme= ntation that is correct > for > =C2=A0=C2=A0=C2=A0=C2=A0 real-time applications in Linux. > +=C2=A0 - Real-time thread waiting for events using `epoll_wait`, > + which the monitor accepts as a valid sleeping reason for real-time task= s. Not adding much value, try: which is a real-time-safe syscall for sleeping as it uses PI-aware locking= . > =C2=A0 > =C2=A0Beside the reason for sleeping, the eventual waker should also be > =C2=A0real-time-safe. Namely, one of: > @@ -114,6 +116,10 @@ The monitor's specification is:: > =C2=A0=C2=A0 ALLOWLIST =3D BLOCK_ON_RT_MUTEX > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 or FUT= EX_LOCK_PI > =C2=A0 > +`ABORT_SLEEP` represents a task restoring its state to `TASK_RUNNING` be= fore > +entering the scheduler. In this case, the task does not actually block, > + so the monitor treats the aborted sleep attempt as valid. This sentence is a bit vague. Indeed an /aborted/ sleep is a valid wakeup, = as if the task woke up itself without even sleeping, saying "the monitor treats t= he attempt as valid" adds no value. You could rephrase it to (full paragraph for clarity): `ABORT_SLEEP` represents a task restoring its state to `TASK_RUNNING` befor= e entering the scheduler. In this case, the task does not actually block, so = it is back to runnable without any wakeup sequence unsafe for real-time. What do you think? Thanks, Gabriele