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[110.175.65.7]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1f44c967acbsm58346825ad.149.2024.05.27.04.01.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 May 2024 04:01:57 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Mon, 27 May 2024 21:01:51 +1000 Message-Id: From: "Nicholas Piggin" To: "Oleg Nesterov" , "Thomas Gleixner" Cc: "Frederic Weisbecker" , "Ingo Molnar" , "Peter Zijlstra" , "Phil Auld" , "Chris von Recklinghausen" , Subject: Re: sched/isolation: tick_take_do_timer_from_boot() calls smp_call_function_single() with irqs disabled X-Mailer: aerc 0.17.0 References: <20240522151742.GA10400@redhat.com> <20240523132358.GA1965@redhat.com> <87h6eneeu7.ffs@tglx> <20240524183700.GA17065@redhat.com> <87v832dfw1.ffs@tglx> <20240526192758.GA21193@redhat.com> In-Reply-To: <20240526192758.GA21193@redhat.com> On Mon May 27, 2024 at 5:27 AM AEST, Oleg Nesterov wrote: > The more I grep the more I confused. > > On 05/25, Thomas Gleixner wrote: > > > > Right. It does not happen because the kernel starts with jiffies as > > clocksource except on S390. The jiffies clocksource is not qualified to > > switch over to NOHZ mode for obvious reasons. > > Not obvious for those who never looked at this code ;) > > OK, clocksource_jiffies doesn't have CLOCK_SOURCE_VALID_FOR_HRES, jiffies clocksource requires a timer to run it I suppose. [snip] > > So up to this point the tick is never stopped neither on housekeeping > > nor on NOHZ FULL CPUs: > > > > tick_nohz_full_update_tick() > > if (!tick_sched_flag_test(ts, TS_FLAG_NOHZ)) > > return; > > OK... But tick_nohz_idle_update_tick() doesn't check TS_FLAG_NOHZ and > the tick_nohz_full_cpu() check can't help at boot time. > > And I still don't understand why we can rely on can_stop_idle_tick() even > in tick_nohz_idle_stop_tick(). AFAIKS timer_expires_base would be 0 unless tick_nohz_next_event() were called, but that is only called from places that checked can_stop_idle_tick() or is already a tick_nohz_full() CPU. And timer tick CPU can't be unplugged with nohz full. Thanks, Nick