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

[Dennis Zhou <dennis@kernel.org>]

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
>