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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 Tue, 2025-09-02 at 14:45 +0200, Frederic Weisbecker wrote: > > On Mon, 2025-09-01 at 23:11 +0200, Frederic Weisbecker wrote: > > > Right, Thomas didn't like it, but the organization of the code > > > has changed a bit since then with the late initcall. If the best > > > we can do to workaround the situation is to make the CPU > > > unavailable regardless and then undo that right after with an > > > IPI, then it's a good sign that we should just simplify and > > > eventually check tick_nohz_cpu_hotpluggable() from > > > tmigr_is_isolated(). > > Here I'm a bit unsure how to proceed though. We want to fail any > > single isolated cpuset that includes the tick CPU under nohz_full. > > I can do it directly in isolcpus_nohz_conflict and that looks easy. > >=20 > > But is that going to be clear for the user? > > Can the user even know what the tick CPU is? Besides /assuming/ 0. >=20 > You're right the user can't know in advance which CPU is the tick. > I don't mind if we prevent or not cpuset from allowing to isolate > the timekeeper but either way, a pr_info() could be helpful to tell > that either: >=20 > * The isolated timekeeper will have limited isolation > or > * The timekeeper can't be isolated. >=20 As far as I understand, we are now decided to fail cpuset isolated assignments involving the tick CPU when nohz_full is active. What prevents us from doing the same in the isolcpus code? Or at least un-isolate the tick cpu like it happens for nohz_full? The patch currently avoids the tick CPU in the isolated tmigr logic but allows it anywhere else. I believe if we are changing this, cpusets and isolcpus should at least be consistent. I remember I went down this path because it was mentioned the tick CPU is allowed to change. Is this really a concern when nohz_full is active? If the tick CPU can change after boot, we would probably need to track this change either way (in tmigr or isolation/cpuset), but it doesn't seem the case to me. If we guarantee the tick cpu is never isolated, we can get rid of the tick_nohz_cpu_hotpluggable() check for good. Am I missing anything? Thanks, Gabriele