Re: [PATCH rfc 2/4] percpu: split __pcpu_balance_workfn()

Roman Gushchin
2021-03-29T18:21:37Z

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@fb.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@kernel.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!