linux-kernel - Re: [PATCH V2 2/3] sched/numa: Enhance vma scanning logic

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Date:   Fri, 3 Feb 2023 12:27:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Raghavendra K T <raghavendra.kt@....com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, rppt@...nel.org,
        Bharata B Rao <bharata@....com>,
        Disha Talreja <dishaa.talreja@....com>
Subject: Re: [PATCH V2 2/3] sched/numa: Enhance vma scanning logic

On Fri, Feb 03, 2023 at 12:15:48PM +0100, Peter Zijlstra wrote:

> > +static inline void vma_set_active_pid_bit(struct vm_area_struct *vma)
> > +{
> > +	unsigned int active_pid_bit;
> > +
> > +	if (vma->numab) {
> > +		active_pid_bit = current->pid % BITS_PER_LONG;
> > +		vma->numab->accessing_pids |= 1UL << active_pid_bit;
> > +	}
> > +}
> 
> Perhaps:
> 
> 	if (vma->numab)
> 		__set_bit(current->pid % BITS_PER_LONG, &vma->numab->pids);
> 
> ?
> 
> Or maybe even:
> 
> 	bit = current->pid % BITS_PER_LONG;
> 	if (vma->numab && !__test_bit(bit, &vma->numab->pids))
> 		__set_bit(bit, &vma->numab->pids);
> 

The alternative to just taking the low n bits is to use:

  hash_32(current->pid, BITS_PER_LONG)

That mixes things up a bit.