From: Will Deacon <will@kernel.org> To: Peter Zijlstra <peterz@infradead.org> Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>, Marc Zyngier <maz@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Morten Rasmussen <morten.rasmussen@arm.com>, Qais Yousef <qais.yousef@arm.com>, Suren Baghdasaryan <surenb@google.com>, Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>, Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>, Vincent Guittot <vincent.guittot@linaro.org>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Dietmar Eggemann <dietmar.eggemann@arm.com>, Daniel Bristot de Oliveira <bristot@redhat.com>, Valentin Schneider <valentin.schneider@arm.com>, Mark Rutland <mark.rutland@arm.com>, kernel-team@android.com Subject: Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems Date: Wed, 18 Aug 2021 11:42:28 +0100 [thread overview] Message-ID: <20210818104227.GA13828@willie-the-truck> (raw) In-Reply-To: <YRvRfZ/NnuNyIu3s@hirez.programming.kicks-ass.net> Hi Peter, On Tue, Aug 17, 2021 at 05:10:53PM +0200, Peter Zijlstra wrote: > On Fri, Jul 30, 2021 at 12:24:35PM +0100, Will Deacon wrote: > > @@ -2783,20 +2778,173 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, > > > > __do_set_cpus_allowed(p, new_mask, flags); > > > > - return affine_move_task(rq, p, &rf, dest_cpu, flags); > > + if (flags & SCA_USER) > > + release_user_cpus_ptr(p); > > + > > + return affine_move_task(rq, p, rf, dest_cpu, flags); > > > > out: > > - task_rq_unlock(rq, p, &rf); > > + task_rq_unlock(rq, p, rf); > > > > return ret; > > } > > > +void relax_compatible_cpus_allowed_ptr(struct task_struct *p) > > +{ > > + unsigned long flags; > > + struct cpumask *mask = p->user_cpus_ptr; > > + > > + /* > > + * Try to restore the old affinity mask. If this fails, then > > + * we free the mask explicitly to avoid it being inherited across > > + * a subsequent fork(). > > + */ > > + if (!mask || !__sched_setaffinity(p, mask)) > > + return; > > + > > + raw_spin_lock_irqsave(&p->pi_lock, flags); > > + release_user_cpus_ptr(p); > > + raw_spin_unlock_irqrestore(&p->pi_lock, flags); > > +} > > Both these are a problem on RT. Ah, sorry. I didn't realise you couldn't _free_ with a raw lock held in RT. Is there somewhere I can read up on that? > The easiest recourse is simply never freeing the CPU mask (except on > exit). The alternative is something like the below I suppose.. > > I'm leaning towards the former option, wdyt? Defering the freeing until exit feels like a little fiddly, as we still want to clear ->user_cpus_ptr on affinity changes when SCA_USER is set so we'd have to keep track of the mask somewhere and reuse it instead of allocating a new one if we need it later on. Do-able, but feels a bit nasty, particular across fork(). As for your other suggestion: > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2733,6 +2733,7 @@ static int __set_cpus_allowed_ptr_locked > const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p); > const struct cpumask *cpu_valid_mask = cpu_active_mask; > bool kthread = p->flags & PF_KTHREAD; > + struct cpumask *user_mask = NULL; > unsigned int dest_cpu; > int ret = 0; > > @@ -2792,9 +2793,13 @@ static int __set_cpus_allowed_ptr_locked > __do_set_cpus_allowed(p, new_mask, flags); > > if (flags & SCA_USER) > - release_user_cpus_ptr(p); > + swap(user_mask, p->user_cpus_ptr); > + > + ret = affine_move_task(rq, p, rf, dest_cpu, flags); > + > + kfree(user_mask); > > - return affine_move_task(rq, p, rf, dest_cpu, flags); > + return ret; > > out: > task_rq_unlock(rq, p, rf); > @@ -2954,8 +2959,10 @@ void relax_compatible_cpus_allowed_ptr(s > return; > > raw_spin_lock_irqsave(&p->pi_lock, flags); > - release_user_cpus_ptr(p); > + p->user_cpus_ptr = NULL; > raw_spin_unlock_irqrestore(&p->pi_lock, flags); > + > + kfree(mask); I think the idea looks good, but perhaps we could wrap things up a bit: /* Comment about why this is useful with RT */ static cpumask_t *clear_user_cpus_ptr(struct task_struct *p) { struct cpumask *user_mask = NULL; swap(user_mask, p->user_cpus_ptr); return user_mask; } void release_user_cpus_ptr(struct task_struct *p) { kfree(clear_user_cpus_ptr(p)); } Then just use clear_user_cpus_ptr() in sched/core.c where we know what we're doing (well, at least one of us does!). Will
next prev parent reply other threads:[~2021-08-18 10:42 UTC|newest] Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-30 11:24 [PATCH v11 00/16] Add support for 32-bit tasks on asymmetric AArch32 systems Will Deacon 2021-07-30 11:24 ` [PATCH v11 01/16] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 02/16] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 03/16] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus() Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 04/16] cpuset: Cleanup cpuset_cpus_allowed_fallback() use in select_fallback_rq() Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 05/16] sched: Reject CPU affinity changes based on task_cpu_possible_mask() Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 06/16] sched: Introduce task_struct::user_cpus_ptr to track requested affinity Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 07/16] sched: Split the guts of sched_setaffinity() into a helper function Will Deacon 2021-08-17 15:40 ` Peter Zijlstra 2021-08-18 10:50 ` Will Deacon 2021-08-18 10:56 ` Peter Zijlstra 2021-08-18 11:11 ` Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems Will Deacon 2021-08-17 15:10 ` Peter Zijlstra 2021-08-18 10:42 ` Will Deacon [this message] 2021-08-18 10:56 ` Peter Zijlstra 2021-08-18 11:53 ` Peter Zijlstra 2021-08-18 12:19 ` Will Deacon 2021-08-18 11:06 ` Peter Zijlstra 2021-08-17 15:41 ` Peter Zijlstra 2021-08-18 10:43 ` Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 09/16] sched: Introduce dl_task_check_affinity() to check proposed affinity Will Deacon 2021-08-23 9:26 ` [tip: sched/core] " tip-bot2 for Will Deacon 2021-07-30 11:24 ` [PATCH v11 10/16] arm64: Implement task_cpu_possible_mask() Will Deacon 2021-07-30 11:24 ` [PATCH v11 11/16] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Will Deacon 2021-07-30 11:24 ` [PATCH v11 12/16] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system Will Deacon 2021-07-30 11:24 ` [PATCH v11 13/16] arm64: Advertise CPUs capable of running 32-bit applications in sysfs Will Deacon 2021-07-30 11:24 ` [PATCH v11 14/16] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0 Will Deacon 2021-07-30 11:24 ` [PATCH v11 15/16] arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores Will Deacon 2021-07-30 11:24 ` [PATCH v11 16/16] Documentation: arm64: describe asymmetric 32-bit support Will Deacon
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