Re: [PATCH rfc 1/4] percpu: implement partial chunk depopulation

Dennis Zhou
2021-03-29T17:21:59Z

On Wed, Mar 24, 2021 at 12:06:23PM -0700, Roman Gushchin wrote:
> This patch implements partial depopulation of percpu chunks.
>
> As now, a chunk can be depopulated only as a part of the final
> destruction, when there are no more outstanding allocations. However
> to minimize a memory waste, it might be useful to depopulate a
> partially filed chunk, if a small number of outstanding allocations
> prevents the chunk from being reclaimed.
>
> This patch implements the following depopulation process: it scans
> over the chunk pages, looks for a range of empty and populated pages
> and performs the depopulation. To avoid races with new allocations,
> the chunk is previously isolated. After the depopulation the chunk is
> returned to the original slot (but is appended to the tail of the list
> to minimize the chances of population).
>
> Because the pcpu_lock is dropped while calling pcpu_depopulate_chunk(),
> the chunk can be concurrently moved to a different slot. So we need
> to isolate it again on each step. pcpu_alloc_mutex is held, so the
> chunk can't be populated/depopulated asynchronously.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
> mm/percpu.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 90 insertions(+)
>
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 6596a0a4286e..78c55c73fa28 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -2055,6 +2055,96 @@ static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
> mutex_unlock(&pcpu_alloc_mutex);
> }
>
> +/**
> + * pcpu_shrink_populated - scan chunks and release unused pages to the system
> + * @type: chunk type
> + *
> + * Scan over all chunks, find those marked with the depopulate flag and
> + * try to release unused pages to the system. On every attempt clear the
> + * chunk's depopulate flag to avoid wasting CPU by scanning the same
> + * chunk again and again.
> + */
> +static void pcpu_shrink_populated(enum pcpu_chunk_type type)
> +{
> + struct list_head *pcpu_slot = pcpu_chunk_list(type);
> + struct pcpu_chunk *chunk;
> + int slot, i, off, start;
> +
> + spin_lock_irq(&pcpu_lock);
> + for (slot = pcpu_nr_slots - 1; slot >= 0; slot--) {
> +restart:
> + list_for_each_entry(chunk, &pcpu_slot[slot], list) {
> + bool isolated = false;
> +
> + if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH)
> + break;
> +

Deallocation makes me a little worried for the atomic case as now we
could in theory pathologically scan deallocated chunks before finding a
populated one.

I wonder if we should do something like once a chunk gets depopulated,
it gets deprioritized and then only once we exhaust looking through
allocated chunks we then find a depopulated chunk and add it back into
the rotation. Possibly just add another set of slots? I guess it adds a
few dimensions to pcpu_slots after the memcg change.

> + for (i = 0, start = -1; i < chunk->nr_pages; i++) {
> + if (!chunk->nr_empty_pop_pages)
> + break;
> +
> + /*
> + * If the page is empty and populated, start or
> + * extend the [start, i) range.
> + */
> + if (test_bit(i, chunk->populated)) {
> + off = find_first_bit(
> + pcpu_index_alloc_map(chunk, i),
> + PCPU_BITMAP_BLOCK_BITS);
> + if (off >= PCPU_BITMAP_BLOCK_BITS) {
> + if (start == -1)
> + start = i;
> + continue;
> + }

Here instead of looking at the alloc_map, you can look at the
pcpu_block_md and look for a fully free contig_hint.

> + }
> +
> + /*
> + * Otherwise check if there is an active range,
> + * and if yes, depopulate it.
> + */
> + if (start == -1)
> + continue;
> +
> + /*
> + * Isolate the chunk, so new allocations
> + * wouldn't be served using this chunk.
> + * Async releases can still happen.
> + */
> + if (!list_empty(&chunk->list)) {
> + list_del_init(&chunk->list);
> + isolated = true;

Maybe when freeing a chunk, we should consider just isolating it period
and preventing pcpu_free_area() from being able to add the chunk back
to a pcpu_slot.

> + }
> +
> + spin_unlock_irq(&pcpu_lock);
> + pcpu_depopulate_chunk(chunk, start, i);
> + cond_resched();
> + spin_lock_irq(&pcpu_lock);
> +
> + pcpu_chunk_depopulated(chunk, start, i);
> +
> + /*
> + * Reset the range and continue.
> + */
> + start = -1;
> + }
> +
> + if (isolated) {
> + /*
> + * The chunk could have been moved while
> + * pcpu_lock wasn't held. Make sure we put
> + * the chunk back into the slot and restart
> + * the scanning.
> + */
> + if (list_empty(&chunk->list))
> + list_add_tail(&chunk->list,
> + &pcpu_slot[slot]);
> + goto restart;
> + }
> + }
> + }
> + spin_unlock_irq(&pcpu_lock);
> +}
> +
> /**
> * pcpu_balance_workfn - manage the amount of free chunks and populated pages
> * @work: unused
> --
> 2.30.2
>