linux-kernel - Re: [PATCH rfc 2/4] percpu: split __pcpu_balance

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Date:   Mon, 29 Mar 2021 11:20:34 -0700
From:   Roman Gushchin <guro@...com>
To:     Dennis Zhou <dennis@...nel.org>
CC:     Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH rfc 2/4] percpu: split __pcpu_balance_workfn()

On Mon, Mar 29, 2021 at 05:28:23PM +0000, Dennis Zhou wrote:
> On Wed, Mar 24, 2021 at 12:06:24PM -0700, Roman Gushchin wrote:
> > __pcpu_balance_workfn() became fairly big and hard to follow, but in
> > fact it consists of two fully independent parts, responsible for
> > the destruction of excessive free chunks and population of necessarily
> > amount of free pages.
> > 
> > In order to simplify the code and prepare for adding of a new
> > functionality, split it in two functions:
> > 
> >   1) pcpu_balance_free,
> >   2) pcpu_balance_populated.
> > 
> > Move the taking/releasing of the pcpu_alloc_mutex to an upper level
> > to keep the current synchronization in place.
> > 
> > Signed-off-by: Roman Gushchin <guro@...com>
> > ---
> >  mm/percpu.c | 46 +++++++++++++++++++++++++++++-----------------
> >  1 file changed, 29 insertions(+), 17 deletions(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 78c55c73fa28..015d076893f5 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -1930,31 +1930,22 @@ void __percpu *__alloc_reserved_percpu(size_t size, size_t align)
> >  }
> >  
> >  /**
> > - * __pcpu_balance_workfn - manage the amount of free chunks and populated pages
> > + * pcpu_balance_free - manage the amount of free chunks
> >   * @type: chunk type
> >   *
> > - * Reclaim all fully free chunks except for the first one.  This is also
> > - * responsible for maintaining the pool of empty populated pages.  However,
> > - * it is possible that this is called when physical memory is scarce causing
> > - * OOM killer to be triggered.  We should avoid doing so until an actual
> > - * allocation causes the failure as it is possible that requests can be
> > - * serviced from already backed regions.
> > + * Reclaim all fully free chunks except for the first one.
> >   */
> > -static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
> > +static void pcpu_balance_free(enum pcpu_chunk_type type)
> >  {
> > -	/* gfp flags passed to underlying allocators */
> > -	const gfp_t gfp = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
> >  	LIST_HEAD(to_free);
> >  	struct list_head *pcpu_slot = pcpu_chunk_list(type);
> >  	struct list_head *free_head = &pcpu_slot[pcpu_nr_slots - 1];
> >  	struct pcpu_chunk *chunk, *next;
> > -	int slot, nr_to_pop, ret;
> >  
> >  	/*
> >  	 * There's no reason to keep around multiple unused chunks and VM
> >  	 * areas can be scarce.  Destroy all free chunks except for one.
> >  	 */
> > -	mutex_lock(&pcpu_alloc_mutex);
> >  	spin_lock_irq(&pcpu_lock);
> >  
> >  	list_for_each_entry_safe(chunk, next, free_head, list) {
> > @@ -1982,6 +1973,25 @@ static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
> >  		pcpu_destroy_chunk(chunk);
> >  		cond_resched();
> >  	}
> > +}
> > +
> > +/**
> > + * pcpu_balance_populated - manage the amount of populated pages
> > + * @type: chunk type
> > + *
> > + * Maintain a certain amount of populated pages to satisfy atomic allocations.
> > + * It is possible that this is called when physical memory is scarce causing
> > + * OOM killer to be triggered.  We should avoid doing so until an actual
> > + * allocation causes the failure as it is possible that requests can be
> > + * serviced from already backed regions.
> > + */
> > +static void pcpu_balance_populated(enum pcpu_chunk_type type)
> > +{
> > +	/* gfp flags passed to underlying allocators */
> > +	const gfp_t gfp = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
> > +	struct list_head *pcpu_slot = pcpu_chunk_list(type);
> > +	struct pcpu_chunk *chunk;
> > +	int slot, nr_to_pop, ret;
> >  
> >  	/*
> >  	 * Ensure there are certain number of free populated pages for
> > @@ -2051,8 +2061,6 @@ static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
> >  			goto retry_pop;
> >  		}
> >  	}
> > -
> > -	mutex_unlock(&pcpu_alloc_mutex);
> >  }
> >  
> >  /**
> > @@ -2149,14 +2157,18 @@ static void pcpu_shrink_populated(enum pcpu_chunk_type type)
> >   * pcpu_balance_workfn - manage the amount of free chunks and populated pages
> >   * @work: unused
> >   *
> > - * Call __pcpu_balance_workfn() for each chunk type.
> > + * Call pcpu_balance_free() and pcpu_balance_populated() for each chunk type.
> >   */
> >  static void pcpu_balance_workfn(struct work_struct *work)
> >  {
> >  	enum pcpu_chunk_type type;
> >  
> > -	for (type = 0; type < PCPU_NR_CHUNK_TYPES; type++)
> > -		__pcpu_balance_workfn(type);
> > +	for (type = 0; type < PCPU_NR_CHUNK_TYPES; type++) {
> > +		mutex_lock(&pcpu_alloc_mutex);
> > +		pcpu_balance_free(type);
> > +		pcpu_balance_populated(type);
> > +		mutex_unlock(&pcpu_alloc_mutex);
> > +	}
> >  }
> >  
> >  /**
> > -- 
> > 2.30.2
> > 
> 
> Reviewed-by: Dennis Zhou <dennis@...nel.org>
> 
> This makes sense. If you want me to pick this and the last patch up
> first I can. Otherwise, do you mind moving this to the front of the
> stack because it is a clean up?

It's up to you :)
Sure, I can move it to the front, will do in the next version.

Thank you for taking a look!