From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794414304; b=ER74SAaNRqw9/cO82L0n/fSv7gcXDnp2w6LYEZiLs95vTneqJ6xk6lbz8xLfTU8IbVW/ JLU8rp2m/807As6cPr4TIccL1YegfsaqtsmnSBvRHHNd94B1bftTntcg8qrlJP7EXeCaJ qGerRknf43/M/yB/zIkEm7tGcXoLNjsp76i/LCtIsSv4Km+rA28Tawk9gwoghugOACZCJ fd8tHp72ckCYoaBzVJWCOjsbesxK8Z+T68FLacqb/juGRn2Ylpi4O2a83/f6iOJrehnBI IQI6EhQugJfF+84th3bRBryef6k0b5lGp0xNKCfFmqc1ULrIB4RxgqEF6jrYUXuwCMw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794414304; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition: Content-Transfer-Encoding:In-Reply-To; bh=hyYB6LoS2U1xjzCyMtjFsNBPztOkeup14THpbIKKyRA=; b=XaLTbGyMUzsB0jdOzO+nPhcc8rUcuLOhxycXfjvCQsHBIInMFR1GqGOKbxTMwNRcbRKL R5sb1HP8V40nFg9RHXSvboPC3Z/9R5sIGYFrk81vX0SHwHIkdhp7Ns81LMC8qfZKAlR5W EjaCmqdE7dlJ+ntZ2A2aXF8W4cFh6YqVOJkEMSOha+nIjgmONTOGSmj+NiXmXf3YAdl6C maI+DW7VudM6nB1pbofD2rjFPKTXDbC98h6dmo1rj7RknSHjAnxp9h3cB6C2qBWhJY88A XJwLZPPprKYI1vE1m3AZphGG28y6XaxDCTqLzvZwc6sSG85ak5yfcfltuhXyC83tScA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=bbYIACwK; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CF59D606A321 for ; Wed, 11 Nov 2026 16:25:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=bbYIACwK Received: by smtp.kernel.org (Postfix) with ESMTPSA id 21EFB1F00A05; Wed, 11 Nov 2026 16:25:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794414304; bh=hyYB6LoS2U1xjzCyMtjFsNBPztOkeup14THpbIKKyRA=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=bbYIACwKteLPnI8/ivKQuJNBxFXYvfpzSZL0poH1id1ZiSygIGItH7QxlQklh/k+t SD0bmemJvlROmqQDDZkDF0yR1ZAqkW7YHbDw1Oiqy2qOGi32vjr4XBHY6om0VQXWG6 AT0wnJTQqwPrXCK9BpcZ3hoQxCMhUMxXo9gX7sxmJeoYw4WTRq3Z6QGKO/GuKbHOwG 6bBkTSzF7E5j/fUZHWfzsQqfEQggFQScRF419bUKjZoPyijJErCI/nlpoPxgkoi4QL zPu05t3+jYthTHf2kWbyd9/R4dPVFvSRoWU9HIM4ArTmgzvCTv3wAelFOOKGx83z2B hBoqrfQDF2LyA== Date: Wed, 11 Nov 2026 17:25:01 +0100 From: Frederic Weisbecker To: Marco Crivellari Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Michal Hocko , Aaron Tomlin , Marcelo Tosatti , "Paul E. McKenney" , Anna-Maria Behnsen , Thomas Gleixner Subject: Re: [PATCH v2 1/4] tick/nohz: Re-evaluate the tick on return to user space Message-ID: References: <20261111114406.346953-1-marco.crivellari@suse.com> <20261111114406.346953-2-marco.crivellari@suse.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20261111114406.346953-2-marco.crivellari@suse.com> Le Wed, Nov 11, 2026 at 12:43:47PM +0100, Marco Crivellari a écrit : > On nohz_full CPUs the tick is re-evaluated on IRQ exit and on idle exit. > > This is not enough on return to user space. If the tick is left running in > the kernel, for example because a tick dependency was cleared > (tick_nohz_dep_clear*() don't kick the CPU) or a timer that kept it > running was cancelled, it fires at least once in userspace before > tick_nohz_irq_exit() stops it. > > Stop it from context tracking right before resuming userspace. When the > tick is already stopped, the common case, this costs a few flag tests on > each return to userspace. > > Cc: Paul E. McKenney > Cc: Anna-Maria Behnsen > Cc: Thomas Gleixner > Suggested-by: Frederic Weisbecker > Signed-off-by: Marco Crivellari > --- > include/linux/tick.h | 8 ++++++++ > kernel/context_tracking.c | 8 ++++++++ > kernel/time/tick-sched.c | 24 ++++++++++++++++++++++++ > 3 files changed, 40 insertions(+) > > diff --git a/include/linux/tick.h b/include/linux/tick.h > index b121c5d53e30..8bfbb053f4fd 100644 > --- a/include/linux/tick.h > +++ b/include/linux/tick.h > @@ -269,6 +269,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal, > extern void tick_nohz_full_kick_cpu(int cpu); > extern void __tick_nohz_task_switch(void); > extern void __init tick_nohz_full_setup(cpumask_var_t cpumask); > +extern void __tick_nohz_user_enter(void); > #else > static inline bool tick_nohz_full_enabled(void) { return false; } > static inline bool tick_nohz_full_cpu(int cpu) { return false; } > @@ -294,6 +295,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal, > static inline void tick_nohz_full_kick_cpu(int cpu) { } > static inline void __tick_nohz_task_switch(void) { } > static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { } > +static inline void __tick_nohz_user_enter(void) { } > #endif > > static inline void tick_nohz_task_switch(void) > @@ -302,6 +304,12 @@ static inline void tick_nohz_task_switch(void) > __tick_nohz_task_switch(); > } > > +static inline void tick_nohz_user_enter(void) > +{ > + if (tick_nohz_full_enabled()) > + __tick_nohz_user_enter(); > +} > + > static inline void tick_nohz_user_enter_prepare(void) > { > if (tick_nohz_full_cpu(smp_processor_id())) > diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c > index a743e7ffa6c0..347f99e1880f 100644 > --- a/kernel/context_tracking.c > +++ b/kernel/context_tracking.c > @@ -24,6 +24,7 @@ > #include > #include > #include > +#include > > > DEFINE_PER_CPU(struct context_tracking, context_tracking) = { > @@ -466,6 +467,13 @@ void noinstr __ct_user_enter(enum ctx_state state) > */ > if (state == CT_STATE_USER) { > instrumentation_begin(); > + /* > + * On nohz_full CPUs the tick may have been left running in the > + * kernel, e.g. kept by a tick dependency that is now gone. > + * Stop it now rather than let it interrupt userspace once > + * more before the next IRQ exit notices. > + */ > + tick_nohz_user_enter(); You can move that a little bit further, outside "state == CT_STATE_USER", right before rcu_irq_work_resched(). This will need a new pair of instrumentation_begin()/end(). This way it also applies before switching to guest. > trace_user_enter(0); > vtime_user_enter(current); > instrumentation_end(); > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index c6d86e4da8a9..645dba919386 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -1071,6 +1071,30 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts) > __tick_nohz_full_update_tick(ts, ktime_get()); > } > > +#ifdef CONFIG_NO_HZ_FULL > +void __tick_nohz_user_enter(void) > +{ > + struct tick_sched *ts; > + int cpu; > + > + lockdep_assert_irqs_disabled(); > + > + cpu = smp_processor_id(); > + if (!tick_nohz_full_cpu(cpu)) > + return; > + > + ts = this_cpu_ptr(&tick_cpu_sched); > + if (!tick_sched_flag_test(ts, TS_FLAG_NOHZ)) > + return; > + > + if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) > + return; > + > + if (can_stop_full_tick(cpu, ts)) > + tick_nohz_full_stop_tick(ts, cpu); > +} > +#endif Looks like this can move right below the definition of tick_nohz_full_stop_tick() to avoid more ifdef blocks. Thanks. > + > /* > * A pending softirq outside an IRQ (or softirq disabled section) context > * should be waiting for ksoftirqd to handle it. Therefore we shouldn't > -- > 2.56.0 > -- Frederic Weisbecker SUSE Labs