Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems
From: Will Deacon <will@kernel.org>
Date: 2021-08-18 10:42:38
Also in:
linux-arm-kernel, lkml
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:quoted
@@ -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; }quoted
+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:
quoted hunk ↗ jump to hunk
--- 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