From: Tom Zanussi <zanussi@kernel.org>
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
Steven Rostedt <rostedt@goodmis.org>,
linux-kernel@vger.kernel.org,
linux-rt-users <linux-rt-users@vger.kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Carsten Emde <C.Emde@osadl.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
John Kacur <jkacur@redhat.com>, Daniel Wagner <wagi@monom.org>,
Julien Grall <julien.grall@arm.com>
Subject: Re: [PATCH RT 10/30] hrtimer: Prevent using hrtimer_grab_expiry_lock() on migration_base
Date: Tue, 28 Apr 2020 07:59:26 -0500 [thread overview]
Message-ID: <7fce15e17cd3bef1216473386718e3cfd67349a4.camel@kernel.org> (raw)
In-Reply-To: <1a5e52a6-39e4-ac9a-e11a-5df261e83068@prevas.dk>
On Tue, 2020-04-28 at 09:03 +0200, Rasmus Villemoes wrote:
> On 23/01/2020 21.39, Steven Rostedt wrote:
> > 4.19.94-rt39-rc2 stable review patch.
> > If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Julien Grall <julien.grall@arm.com>
> >
> > [ Upstream commit cef1b87f98823af923a386f3f69149acb212d4a1 ]
> >
> > As tglx puts it:
> > > If base == migration_base then there is no point to lock
> > > soft_expiry_lock
> > > simply because the timer is not executing the callback in soft
> > > irq context
> > > and the whole lock/unlock dance can be avoided.
>
> Hold on a second. This patch (hrtimer: Prevent using
> hrtimer_grab_expiry_lock() on migration_base) indeed seems to
> implement
> the optimization implied by the above, namely avoid the lock/unlock
> in
> case base == migration_base:
>
> > - if (timer->is_soft && base && base->cpu_base) {
> > + if (timer->is_soft && base != &migration_base) {
>
> But the followup patch (hrtimer: Add a missing bracket and hide
> `migration_base on !SMP) to fix the build on !SMP [the missing
> bracket
> part seems to have been fixed when backporting the above to 4.19-rt]
> replaces that logic by
>
> +static inline bool is_migration_base(struct hrtimer_clock_base
> *base)
> +{
> + return base == &migration_base;
> +}
> +
> ...
> - if (timer->is_soft && base != &migration_base) {
> + if (timer->is_soft && is_migration_base(base)) {
>
> in the SMP case, i.e. the exact opposite condition. One of these
> can't
> be correct.
>
> Assuming the followup patch was wrong and the condition should have
> read
>
> timer->is_soft && !is_migration_base(base)
>
> while keeping is_migration_base() false on !SMP might explain the
> problem I see. But I'd like someone who knows this code to chime in.
>
I don't know this code, but I think you're correct - the followup patch
reversed the condition by forgetting the !.
So, does your problem go away when you make that change?
Tom
> Thanks,
> Rasmus
next prev parent reply other threads:[~2020-04-28 12:59 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-23 20:39 [PATCH RT 00/30] Linux 4.19.94-rt39-rc2 Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 01/30] i2c: exynos5: Remove IRQF_ONESHOT Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 02/30] i2c: hix5hd2: " Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 03/30] sched/deadline: Ensure inactive_timer runs in hardirq context Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 04/30] thermal/x86_pkg_temp: make pkg_temp_lock a raw spinlock Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 05/30] dma-buf: Use seqlock_t instread disabling preemption Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 06/30] KVM: arm/arm64: Let the timer expire in hardirq context on RT Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 07/30] x86: preempt: Check preemption level before looking at lazy-preempt Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 08/30] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock() Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 09/30] hrtimer: Dont grab the expiry lock for non-soft hrtimer Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 10/30] hrtimer: Prevent using hrtimer_grab_expiry_lock() on migration_base Steven Rostedt
2020-04-27 13:10 ` Rasmus Villemoes
2020-04-27 19:06 ` Steven Rostedt
2020-04-27 19:26 ` Tom Zanussi
2020-04-28 6:51 ` Rasmus Villemoes
2020-04-28 7:03 ` Rasmus Villemoes
2020-04-28 12:59 ` Tom Zanussi [this message]
2020-04-28 13:07 ` Rasmus Villemoes
2020-04-28 13:43 ` Tom Zanussi
2020-01-23 20:39 ` [PATCH RT 11/30] hrtimer: Add a missing bracket and hide `migration_base on !SMP Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 12/30] posix-timers: Unlock expiry lock in the early return Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 14/30] sched: __set_cpus_allowed_ptr: Check cpus_mask, not cpus_ptr Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 15/30] sched: Remove dead __migrate_disabled() check Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 16/30] sched: migrate disable: Protect cpus_ptr with lock Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 17/30] lib/smp_processor_id: Dont use cpumask_equal() Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 18/30] futex: Make the futex_hash_bucket spinlock_t again and bring back its old state Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 19/30] locking/rtmutex: Clean ->pi_blocked_on in the error case Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 20/30] lib/ubsan: Dont seralize UBSAN report Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 21/30] kmemleak: Change the lock of kmemleak_object to raw_spinlock_t Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 22/30] sched: migrate_enable: Use select_fallback_rq() Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 23/30] sched: Lazy migrate_disable processing Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 24/30] sched: migrate_enable: Use stop_one_cpu_nowait() Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 25/30] Revert "ARM: Initialize split page table locks for vector page" Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 26/30] locking: Make spinlock_t and rwlock_t a RCU section on RT Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 27/30] sched/core: migrate_enable() must access takedown_cpu_task on !HOTPLUG_CPU Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 28/30] lib/smp_processor_id: Adjust check_preemption_disabled() Steven Rostedt
2020-01-23 20:39 ` [PATCH RT 29/30] sched: migrate_enable: Busy loop until the migration request is completed Steven Rostedt
2020-01-23 20:40 ` [PATCH RT 30/30] Linux 4.19.94-rt39-rc2 Steven Rostedt
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