linux-kernel - Re: [patch V2 10/20] sched/mmcid: Convert mm CID mask to a bitmap

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Message-ID: <aPpb-lAkNACezbkh@yury>
Date: Thu, 23 Oct 2025 12:46:50 -0400
From: Yury Norov <yury.norov@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Jeanson <mjeanson@...icios.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Florian Weimer <fweimer@...hat.com>,
	Tim Chen <tim.c.chen@...el.com>
Subject: Re: [patch V2 10/20] sched/mmcid: Convert mm CID mask to a bitmap

On Wed, Oct 22, 2025 at 02:55:32PM +0200, Thomas Gleixner wrote:
> This is truly a bitmap and just conveniently uses a cpumask because the
> maximum size of the bitmap is nr_cpu_ids.
> 
> But that prevents to do searches for a zero bit in a limited range, which
> is helpful to provide an efficient mechanism to consolidate the CID space
> when the number of users decreases.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Acked-by: Yury Norov (NVIDIA) <yury.norov@...il.com>

> ---
>  include/linux/mm_types.h |    6 +++---
>  kernel/sched/core.c      |    2 +-
>  kernel/sched/sched.h     |    6 +++---
>  3 files changed, 7 insertions(+), 7 deletions(-)
> 
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1342,13 +1342,13 @@ static inline cpumask_t *mm_cpus_allowed
>  }
>  
>  /* Accessor for struct mm_struct's cidmask. */
> -static inline cpumask_t *mm_cidmask(struct mm_struct *mm)
> +static inline unsigned long *mm_cidmask(struct mm_struct *mm)
>  {
>  	unsigned long cid_bitmap = (unsigned long)mm_cpus_allowed(mm);
>  
>  	/* Skip mm_cpus_allowed */
>  	cid_bitmap += cpumask_size();
> -	return (struct cpumask *)cid_bitmap;
> +	return (unsigned long *)cid_bitmap;
>  }
>  
>  static inline void mm_init_cid(struct mm_struct *mm, struct task_struct *p)
> @@ -1363,7 +1363,7 @@ static inline void mm_init_cid(struct mm
>  	mm->mm_cid.nr_cpus_allowed = p->nr_cpus_allowed;
>  	raw_spin_lock_init(&mm->mm_cid.lock);
>  	cpumask_copy(mm_cpus_allowed(mm), &p->cpus_mask);
> -	cpumask_clear(mm_cidmask(mm));
> +	bitmap_zero(mm_cidmask(mm), cpumask_size());
>  }
>  
>  static inline int mm_alloc_cid_noprof(struct mm_struct *mm, struct task_struct *p)
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10399,7 +10399,7 @@ void sched_mm_cid_exit_signals(struct ta
>  	guard(preempt)();
>  	t->mm_cid.active = 0;
>  	if (t->mm_cid.cid != MM_CID_UNSET) {
> -		cpumask_clear_cpu(t->mm_cid.cid, mm_cidmask(mm));
> +		clear_bit(t->mm_cid.cid, mm_cidmask(mm));
>  		t->mm_cid.cid = MM_CID_UNSET;
>  	}
>  }
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3558,7 +3558,7 @@ static inline bool __mm_cid_get(struct t
>  
>  	if (cid >= max_cids)
>  		return false;
> -	if (cpumask_test_and_set_cpu(cid, mm_cidmask(mm)))
> +	if (test_and_set_bit(cid, mm_cidmask(mm)))
>  		return false;
>  	t->mm_cid.cid = t->mm_cid.last_cid = cid;
>  	__this_cpu_write(mm->mm_cid.pcpu->cid, cid);
> @@ -3581,7 +3581,7 @@ static inline bool mm_cid_get(struct tas
>  		return true;
>  
>  	/* Try the first zero bit in the cidmask. */
> -	return __mm_cid_get(t, cpumask_first_zero(mm_cidmask(mm)), max_cids);
> +	return __mm_cid_get(t, find_first_zero_bit(mm_cidmask(mm), num_possible_cpus()), max_cids);
>  }
>  
>  static inline void mm_cid_select(struct task_struct *t)
> @@ -3602,7 +3602,7 @@ static inline void switch_mm_cid(struct
>  {
>  	if (prev->mm_cid.active) {
>  		if (prev->mm_cid.cid != MM_CID_UNSET)
> -			cpumask_clear_cpu(prev->mm_cid.cid, mm_cidmask(prev->mm));
> +			clear_bit(prev->mm_cid.cid, mm_cidmask(prev->mm));
>  		prev->mm_cid.cid = MM_CID_UNSET;
>  	}
>